The serum anion gap in the evaluation of acid-base disorders: what are its limitations and can its effectiveness be improved?

Association between anion gap and the 30-day mortality of patients with ventilator-associated pneumonia: a study of the MIMIC-III database

Background

Serum anion gap (AG) can potentially be applied to the diagnosis of various metabolic acidosis, and a recent study has reported the association of AG with the mortality of patients with coronavirus disease 2019 (COVID-19). However, the relationship of AG with the short-term mortality of patients with ventilator-associated pneumonia (VAP) is still unclear. Herein, we aimed to investigate the association between AG and the 30-day mortality of VAP patients, and construct and assess a multivariate predictive model for the 30-day mortality risk of VAP.

Methods

This retrospective cohort study extracted data of 477 patients with VAP from the Medical Information Mart for Intensive Care III (MIMIC-III) database. Data of patients were divided into a training set and a testing set with a ratio of 7:3. In the training set, variables significantly associated with the 30-day mortality of VAP patients were included in the multivariate predictive model through univariate Cox regression and stepwise regression analyses. Then, the predictive performance of the multivariate predictive model was assessed in both training set and testing set, and compared with the single AG and other scoring systems including the Sequential Organ Failure Assessment (SOFA) score, the confusion, urea, respiratory rate (RR), blood pressure, and age (≥65 years old) (CURB-65) score, and the blood urea nitrogen (BUN), altered mental status, pulse, and age (>65 years old) (BAP-65) score. In addition, the association of AG with the 30-day mortality of VAP patients was explored in subgroups of gender, age, and infection status. The evaluation indexes were hazard ratios (HRs), C-index, and 95% confidence intervals (CIs).

Results

A total of 70 patients died within 30 days. The multivariate predictive model consisted of AG (HR =1.052, 95% CI: 1.008-1.098), age (HR =1.037, 95% CI: 1.019-1.055), duration of mechanical ventilation (HR =0.998, 95% CI: 0.996-0.999), and vasopressors use (HR =1.795, 95% CI: 1.066-3.023). In both training set (C-index =0.725, 95% CI: 0.670-0.780) and testing set (C-index =0.717, 95% CI: 0.637-0.797), the multivariate model had a relatively superior predictive performance to the single AG value. Moreover, the association of AG with the 30-day mortality was also found in patients who were male (HR =1.088, 95% CI: 1.029-1.150), and whatever the pathogens they infected (bacterial infection: HR =1.059, 95% CI: 1.011-1.109; fungal infection: HR =1.057, 95% CI: 1.002-1.115).

Conclusions

The AG-related multivariate model had a potential predictive value for the 30-day mortality of patients with VAP. These findings may provide some references for further exploration on simple and robust predictors of the short-term mortality risk of VAP, which may further help clinicians to identify patients with high risk of mortality in an early stage in the intensive care units (ICUs).

Association between albumin corrected anion gap and all-cause mortality in critically ill patients with acute kidney injury: a retrospective study based on MIMIC-IV database

BACKGROUND

The early identification of patients at high risk for acute kidney injury (AKI) with a poor prognosis is crucial to prevent complications and minimize mortality. This study sought to investigate the association between albumin-corrected anion gap (ACAG) and all-cause mortality among critically ill patients with AKI.

METHODS

All eligible AKI patients from the Medical Information Mart for Intensive Care IV (MIMIC-IV version 2.0) database were considered for participation in this study. We employed Kaplan-Meier curves to assess the 30-d and 360-d cumulative survival rates among various groups. Flexibly visualizing the connection between ACAG and mortality, we utilize restricted cubic splines (RCS) and multivariate Cox regression models. Result robustness underwent assessment through subgroup analyses and sensitivity analyses. Receiver-operating characteristic (ROC) curves were generated to evaluate the predictive performance of ACAG.

RESULTS

The study included 9625 AKI participants, of whom 58.60% were male, and the 360-d all-cause mortality rate was 39.89%. According to Kaplan-Meier analysis, the 30-d and 360-d cumulative survival rates for AKI patients were significantly lower in the high ACAG group than in the normal ACAG group. RCS analysis indicated that ACAG levels had a non-linear correlation with the risk of 30-d and 360-d mortality for AKI patients. Cox regression analysis demonstrated that ACAG is an independent risk indicator for 30-d and 360-d prognosis in AKI patients in the ICU.

CONCLUSIONS

Elevated ACAG levels (> 20 mmol/L) at ICU admission were associated with 30-d and 360-d all-cause mortality in critically ill patients with AKI.

The association between serum anion gap and acute kidney injury after coronary artery bypass grafting in patients with acute coronary syndrome

BACKGROUND

The purpose of this study was to explore the association between serum anion gap (SAG) and acute kidney injury (AKI) after coronary artery bypass grafting (CABG) in patients with acute coronary syndrome (ACS) in the Intensive Care Unit (ICU).

METHODS

We retrospectively analyzed the clinical data of 2,428 ACS patients who underwent CABG in the Medical Information Mart for Intensive Care IV (Mimic-IV) database. The endpoint of this study was AKI after CABG. The baseline data of the two groups (non-AKI group vs. AKI group) was compared, and the restricted cubic spline (RCS) plot, multivariable logistic regression model, and subgroup analysis were used to explore the relationship between SAG and the risk of AKI after CABG.

RESULTS

In the adjusted multivariate logistic regression model, SAG was an independent predictor of AKI after CABG (OR = 1.12, 95% CI: 1.02-1.23, P = 0.015). The RCS revealed that the relationship between SAG levels and risk of AKI was J-shaped. When the SAG was ≥ 11.58 mmol/L, the risk of AKI increased by 26% for each unit increase in SAG. Additionally, we further divided the SAG into quartiles. In the fully adjusted model, compared with the first quartile of SAG, the odds ratios (ORs) and 95% confidence intervals (CIs) for AKI risk across the SAG quartiles were 0.729 (0.311, 1.600), 1.308 (0.688-2.478), and 2.221 (1.072, 4.576).

CONCLUSIONS

The SAG level was associated with the risk of AKI after CABG in a J-shaped curve in the ICU. However, the underlying causes of the problem need to be investigated.

A comparison of random survival forest and Cox regression for prediction of mortality in patients with hemorrhagic stroke

OBJECTIVE

To evaluate RSF and Cox models for mortality prediction of hemorrhagic stroke (HS) patients in intensive care unit (ICU).

METHODS

In the training set, the optimal models were selected using five-fold cross-validation and grid search method. In the test set, the bootstrap method was used to validate. The area under the curve(AUC) was used for discrimination, Brier Score (BS) was used for calibration, positive predictive value(PPV), negative predictive value(NPV), and F1 score were combined to compare.

RESULTS

A total of 2,990 HS patients were included. For predicting the 7-day mortality, the mean AUCs for RSF and Cox regression were 0.875 and 0.761, while the mean BS were 0.083 and 0.108. For predicting the 28-day mortality, the mean AUCs for RSF and Cox regression were 0.794 and 0.649, while the mean BS were 0.129 and 0.174. The mean AUCs of RSF and Cox versus conventional scores for predicting patients' 7-day mortality were 0.875 (RSF), 0.761 (COX), 0.736 (SAPS II), 0.723 (OASIS), 0.632 (SIRS), and 0.596 (SOFA), respectively.

CONCLUSIONS

RSF provided a better clinical reference than Cox. Creatine, temperature, anion gap and sodium were important variables in both models.

A clinical nomogram for predicting small bowel obstruction after extubation after radical resection of esophageal cancer and jejunostomy

BACKGROUND

Small bowel obstruction after extubation is among the most serious complications of radical esophageal cancer and jejunostomy resection. This study aimed to explore the risk factors and treatment methods for small bowel obstruction after extubation and construct a predictive model to guide its clinical management.

METHODS

Clinical data for 514 patients who underwent esophagectomy with jejunostomy for esophageal cancer were collected. A nomogram was constructed using the independent risk factors for small bowel obstruction after extubation determined on multivariable logistic regression analysis, and a subgroup analysis was performed of the treatment methods for the 61 patients with small bowel obstruction after extubation.

RESULTS

The nomogram incorporated the independent risk factors for small bowel obstruction after extubation (gastrointestinal function recovery [P < .001], postoperative albumin reduction ratio [P = .009], and serious postoperative complications [P < .001]) in the multivariable logistic regression analysis. The final model had an area under the curve of 0.829 (95% confidence interval, 0.775-0.883). The calibration plots demonstrated high concordance between the predicted and actual probabilities. The model demonstrated excellent discriminatory power for internal and time validation, with adjusted C-statistics of 0.821 and 0.810 (95% confidence interval, 0.686-0.933), respectively. In the subgroup analysis, an abnormal anion gap (P = .016) and low serum albumin level (P = .005) were associated with recurrent small bowel obstruction. The model's area under the curve was 0.815 (95% confidence interval, 0.683-0.948). The probability of recurrence among patients with small bowel obstruction after extubation was 78.3% when the 2 risk factors were present.

CONCLUSION

The clinical nomogram based on small bowel obstruction after extubation predictors recommends aggressive surgical intervention for patients with small bowel obstruction after extubation and an abnormal anion gap and low serum albumin level at admission.

Low bone mineralization in phenylketonuria may be due to undiagnosed metabolic acidosis

Background

Dietary intervention is to date the mainstay treatment to prevent toxic phenylalanine (Phe) accumulation in PKU patients. Despite success preventing central nervous system damage, there is increasing evidence of possible other unfavorable outcomes affecting other systems, e.g. kidney and bone; underlying mechanisms are yet to be fully elucidated.

Methods

This observational, cross-sectional and descriptive study investigated 20 adult with PKU evaluating biochemical parameters, BMD measurements and extrapolating data from 3-days food records and protein substitutes (PS) and special low protein foods (SLPF) composition.

Results

Blood gas venous analysis (VBG) indices were indicative of metabolic acidosis in 60% of PKU patients and VBG pH significantly correlated with BMD's Z-score (p-value = 0.022) even if its overall mean was in range (-1.29). Low bone mineral density for chronological age (Z-score < - 2.0) was found in 4 patients (20%). Indices of kidney function were not impaired. All used PS had a moderate excess of acidity, while SLPF were alkalizing and type/variety of consumed vegetables did not determine significant changes in acid-base equilibrium. Total intakes of potassium and magnesium were lower than expected.

Discussion

PKU patients seem to be at risk of metabolic acidosis, directly linked to possible low bone mineralization. This may be related to the acidic composition of PS, potentially capable of acidifying the entire diet. Reported low intakes of potassium and magnesium may be relevant to these observations. Further studies are needed to better address these topics.

Association between anion gap and all-cause mortality of critically ill surgical patients: a retrospective cohort study

BACKGROUND

There are few widely accepted and operationally feasible models for predicting the mortality risk of patients in surgical intensive care unit (SICU). Although serum anion gap (AG) is known to be correlated with severe metabolic acidosis, no investigations have been reported about the association between AG level and the outcome during hospitalization in SICU. This study aimed to explore the predictive power of AG for 90-day all-cause mortality in SICU.

METHODS

Data of the eligible patients in SICU from 2008 to 2019 was obtained from the Medical Information Mart for Intensive Care IV version 2.0 (MIMIC-IV v2.0) database. Baseline clinical data of the selected patients was compared in different groups stratified by the outcome during their admission via univariate analysis. Restricted cubic spline (RCS) was drawn to confirm the relationship of AG and the short-term mortality. Kaplan-Meier survival curve was plotted in different AG level groups. Univariate and multivariate Cox analyses were performed, and Cox proportional-hazards models were built to investigate an independent role of AG to predict 90-day all-cause mortality risk in SICU. Receiver operating characteristics (ROC) curves analysis was performed to evaluate the predictive value of AG on the 90-day prognosis of patients.

RESULTS

A total of 6,395 patients were enrolled in this study and the 90-day all-cause mortality rate was 18.17%. Univariate analysis showed that elevated serum AG was associated with higher mortality (P < 0.001). RCS analysis indicated a positively linear relationship between serum AG and the risk of 90-day all-cause mortality in SICU (χ2 = 4.730, P = 0.193). Kaplan-Meier survival analysis demonstrated that low-AG group (with a cutoff value of 14.10 mmol/L) had a significantly higher cumulative survival rate than the counterpart of high-AG group (χ2 = 96.370, P < 0.001). Cox proportional-hazards models were constructed and confirmed the independent predictive role of AG in 90-day all-cause mortality risk in SICU after adjusting for 23 confounding factors gradually (HR 1.423, 1.246-1.625, P < 0.001). In the further subgroup analyses, a significant interaction was confirmed between AG and sepsis as well as surgery on the risk for the 90-day mortality. The ROC curve showed that the optimal cut-off value of AG for predicting 90-day mortality was 14.89 with sensitivity of 60.7% and specificity of 54.8%. The area under curve (AUC) was 0.602. When combined with SOFA score, the AUC of AG for predicting 90-day prognosis was 0.710, with a sensitivity and specificity of 70% and 62.5% respectively.

CONCLUSIONS

Elevated AG (≥ 14.10 mmol/L) is an independent risk factor for predicting severe conditions and poor prognosis of critical ill surgical patients.

Albumin Corrected Anion Gap and the Risk of in-Hospital Mortality in Patients with Acute Pancreatitis: A Retrospective Cohort Study

Purpose

To explore the prognostic value of albumin corrected anion gap (ACAG) within 24 hours of admission to the intensive care unit (ICU) for acute pancreatitis (AP).

Patients and Methods

This was a retrospective cohort study. Adult AP patients admitted to ICU from June 2016 to December 2019 were included in the study, who were divided into three groups according to initial serum ACAG within 24 hours upon ICU admission: ACAG ≤ 14.87 mmol/L, 14.87 < ACAG ≤ 19.03 mmol/L, and ACAG > 19.03 mmol/L. The primary study outcome indicator was in-hospital mortality. Age, sex, Glasgow Coma Scale score, and Acute Physiology and Chronic Health Evaluation II (APACHE II) score were matched through propensity score matching (PSM) method to balance the baseline between the survivors and non-survivors. Multivariate Cox regression was used to determine the relationship between ACAG and in-hospital mortality.

Results

A total of 344 patients (of them 81 non-survivors) were analyzed in this study. Patients with higher ACAG intended to present significantly higher in-hospital mortality, APACHE II score, creatine, lower albumin, and bicarbonate. Multivariate Cox regression analysis after matching demonstrated that white blood cell count, platelet count, and higher ACAG were independently associated with higher in-hospital mortality (ACAG ≤ 14.87 as a reference, 14.87 < ACAG ≤ 19.03 mmol/L with HR of 2.34 and 95% CI of 1.15-4.76, ACAG >19.03 with HR of 3.46 and 95% CI of 1.75-6.84).

Conclusion

Higher ACAG was independently associated with higher in-hospital mortality in patients with AP after matching the baseline between the survivors and non-survivors.

The Cause of Severe Metabolic Acidosis With Vomiting in a Neonate

We present a 22-day-old male born full term who presented with worsening non-projectile, non-bilious vomiting and failure to thrive (FTT) and was admitted to the pediatric intensive care unit (PICU) for severe metabolic acidosis with an elevated anion gap. Despite changing the formula, the patient continued to have spit-ups after feeds since birth. Before this admission, his vomiting worsened with every feed, which was now forceful along with two days of loose stools. Obstructive causes of emesis were ruled out with an upper gastrointestinal series, and a decision was made to evaluate for organic causes of FTT. Transient resolution of symptoms was noticed when the patient was placed NPO (nothing by os/mouth) briefly. His symptoms returned on resuming cow milk-based formula feeds. At this time, a presumptive diagnosis of cow milk protein allergy (CMPA) was made. Positive fecal occult blood supported the diagnosis, and his formula was changed to an extensively hydrolyzed formula (eHF). This is a case of severe CMPA with prolonged vomiting and FTT presenting with severe metabolic acidosis with an elevated anion gap. This case report highlights how CMPA can lead to severe dehydration with metabolic acidosis and increased anion gap.

Serum Anion Gap Level Predicts All-Cause Mortality in Septic Patients: A Retrospective Study Based on the MIMIC III Database

PURPOSE

Sepsis is a significant threat in the intensive care unit (ICU) worldwide because it has high morbidity and mortality rates. Early recognition and diagnosis of sepsis are essential for the prevention of adverse outcomes. The present study aimed to quantitatively assess the association between serum anion gap (AG) levels and 30- and 90-day all-cause mortality among sepsis patients.

METHODS

Clinical data of patients diagnosed with sepsis were extracted from the Medical Information Mart for Intensive Care III (MIMIC III) database. Kaplan-Meier curves and Cox proportional hazards models were used to evaluate the association between serum AG levels and all-cause mortality. A receiver operating characteristic (ROC) curve was drawn to quantify the efficacy of using the serum AG level to predict all-cause mortality.

RESULTS

A total of 3811 patients were included in the study. The Kaplan-Meier curves showed that patients with higher serum AG levels had a shorter survival time than those with lower levels. Serum AG levels were found to be highly effective in predicting all-cause mortality secondary to sepsis (30-day: AUROC = 0.703; 90-day: AUROC = 0.696). The Cox regression model further indicated that the serum AG level was an independent risk factor for 30- and 90-day mortality in sepsis (HR 3.44, 95% CI 2.97-3.99 for 30-day; HR 3.17, 95% CI 2.76-3.65 for 90-day, P < 0.001 for both).

CONCLUSIONS

High serum AG may be considered as an alternative parameter for predicting the death risk in sepsis when other variables are not immediately available. Prospective large-scale studies are needed to support its predictive value in the clinic.

Transient loss of consciousness immediately after total pancreatectomy for pancreatic metastases from renal cell carcinoma: a case report

BACKGROUND

Total pancreatectomy (TP) is often selected for treatment of various pancreatic diseases. However, the resultant lack of autoregulation of glycometabolism necessitates careful postoperative management.

CASE PRESENTATION

A 77-year-old man who had undergone right nephrectomy for renal cell carcinoma 11 years previously presented with multiple histologically diagnosed pancreatic metastases. The patient had no notable comorbidities, including diabetes. Because no extrapancreatic organ metastasis was identified, he underwent TP as a curative treatment. He awoke from anesthesia and was extubated without any problems in the operating room. However, 15 min after entering the intensive care unit, he suddenly lost consciousness and became apneic, resulting in reintubation. Blood gas analysis revealed an increased glucose concentration (302 mg/dL) and mixed acid-base disorder (pH of 7.21) due to insulin insufficiency and fentanyl administration. After induction of continuous intravenous insulin infusion and termination of fentanyl, the glucose concentration and pH gradually improved. He regained clear consciousness and spontaneous ventilation and was extubated the next day with no difficulties or complications.

CONCLUSION

This case highlights the importance of active monitoring of the glycemic state and pH after TP because of the possibility of deterioration due to TP itself as well as the lingering effects of anesthesia.

The association between serum anion gap and all-cause mortality of unselected adult patients: A retrospective cohort study of >20,000 patients

BACKGROUND

Even though the serum anion gap (AG) is frequently measured in clinical practice, there is not much research that has examined long-term mortality in unselected adult patients. Our study's objective was to investigate how serum anion gap levels could be used to predict death in unselected participants.

METHODS

The relationship between baseline serum AG levels and short-, intermediate-, and long-term all-cause mortality in unselected adult patients is examined using the Cox proportional risk analysis, smoothed curve fitting, subgroup analysis, and Kaplan-Meier survival curves.

RESULTS

After screening the database using the appropriate method, a total of 26,270 patients were enrolled in our study for the final data analysis. Our study used smoothed curve fit plots and COX proportional risk regression models incorporating cubic spline functions to evaluate the association between AG levels and all-cause mortality in a non-selected population, and the results indicated a non-linear relationship. In the fully adjusted model, we found that AG levels were positively associated with 30-day, 90-day, 365-day, and 4-year all-cause mortality in unselected adult patients with HRs of 1.08 95% CIs (1.06, 1.09); 1.08 95% CIs (1.06, 1.09); 1.08 95% CIs (1.07, 1.08); 1.07 95% CIs (1.06, 1.07).

CONCLUSION

Serum anion gap levels were positively correlated with all-cause mortality in unselected adult patients, with increasing levels of serum anion gap increasing patient mortality.

Combining the anion gap with the sequential organ failure assessment score to evaluate the short-term prognosis of patients in the cardiac intensive care unit

BACKGROUND

We attempted to determine the predictive ability of the first-day Sequential Organ Failure Assessment (SOFA) score in the cardiac intensive care unit, as well as a new score combining the anion gap (AG) with the SOFA score (SOFA-AG).

METHODS

Information was obtained from the Medical Information Mart for Intensive Care III (MIMIC III 1.4) database. We plotted the relationship between the maximum first-day AG and 90-day mortality after admission to the care unit. Patients were divided into five groups based on the hazard ratio (HR) and assigned scores of 0, 1, 2, 3, or 4 points. We compared the area under the curve (AUC) for the receiver-operating characteristic curve of the SOFA and that of the SOFA-AG.

RESULTS

A total of 1316 patients were identified and divided into the following five groups: AG 8 to <16 mmol/L; AG 16 to <17 mmol/L; AG 17 to <19 mmol/L; AG 19 to <21 mmol/L; and AG ≥ 21 mmol/L. The SOFA-AG score had a greater AUC than the SOFA score at 7 days (0.770 vs. 0.711; P < 0.001), 14 days (0.751 vs. 0.692; P < 0.001), 28 days (0.741 vs. 0.684; P < 0.001), and 90 days (0.727 vs. 0.667; P < 0.001).

CONCLUSIONS

The SOFA score showed moderate predictive value only for 7-day mortality after admission to the cardiac intensive care unit, but the SOFA-AG score had improved predictive ability for up to 90 days after admission.

Bone metabolism during strict head-down tilt bed rest and exposure to elevated levels of ambient CO2

Astronauts on the International Space Station are exposed to levels of atmospheric carbon dioxide (CO₂) above typical terrestrial levels. We explored the possibility that increased levels of ambient CO₂ further stimulate bone resorption during bed rest. We report here data from 2 ground-based spaceflight analog studies in which 12 male and 7 female subjects were placed in a strict 6° head-down tilt (HDT) position for either 30 days at 0.5% ambient CO₂ or 60 days with nominal environmental exposure to CO₂. Bone mineral density (BMD) and bone mineral content (BMC) were determined using dual-energy X-ray absorptiometry (DXA). Blood and urine were collected before and after HDT for biochemical analysis. No change was detected in either BMD or BMC, as expected given the study duration. Bone resorption markers increased after bed rest as expected; however, elevated CO₂ had no additive effect. Elevated CO₂ did not affect concentrations of minerals in serum and urine. Serum parathyroid hormone and 1,25-dihydroxyvitamin D were both reduced after bed rest, likely secondary to calcium efflux from bone. In summary, exposure to 0.5% CO₂ for 30 days did not exacerbate the typical bone resorption response observed after HDT bed rest. Furthermore, results from these strict HDT studies were similar to data from previous bed rest studies, confirming that strict 30-60 days of HDT can be used to evaluate changes in bone metabolism. This is valuable in the continuing effort to develop and refine efficacious countermeasure protocols to mitigate bone loss during spaceflight in low-Earth orbit and beyond.

Relationship of Admission Serum Anion Gap and Prognosis of Critically Ill Patients: A Large Multicenter Cohort Study

Background

There were controversies over the relationship between Anion gap (AG) and mortality in critically ill patients. Therefore, a large multicenter cohort study was conducted to evaluate the association of AG and mortality in large-scale intensive care units (ICUs) patients.

Methods

This retrospective cohort study included adult ICU patients enrolled from eICU Collaborative Research Database. According to initial serum AG upon ICU admission, patients were divided into three groups: AG < 8 mmol/L, 8 ≤ AG ≤ 16 mmol/L, and AG > 16 mmol/L. Logistic regression models were built to investigate the association between serum AG and ICU and hospital mortalities. Serum AG was added into Acute Physiology and Chronic Health Evaluation (APACHE) IV score and the model discrimination was assessed by the area under the curve (AUC) of receiver operating characteristic curves. The relationship between serum AG and mortalities in patients with different acid-base status and serum lactate were also evaluated. An external validation was performed with the Critical care database comprising patients with infection at Zigong Fourth People's Hospital.

Results

A total of 8520 patients entered the final cohort. There are 42 patients with serum AG < 8 mmol/L, 3238 patients with 8 ≤ AG ≤ 16 mmol/L, and 5240 patients with AG > 16 mmol/L. Serum AG > 16 mmol/L is related with increased ICU mortality (odds ratio [OR], 1.530; 95% confidence interval [CI], 1.305-1.794) and hospital mortality (OR, 1.618; 95% CI, 1.415-1.849), compared with 8 ≤ AG ≤ 16 mmol/L. Adding Serum AG to APACHE IV score could statistically improve the prediction of ICU (0.770 [0.761-0.779] to 0.774 [0.765-0.783], P = 0.001) and hospital mortalities (0.756 [0.747-0.765] to 0.761 [0.751-0.770], P = 0.012). The associations between serum AG and mortalities remain robust in patients with different acid-base statuses and serum lactate. The findings are validated in the external cohort.

Conclusions

Initial serum AG > 16 mmol/L after ICU admission is associated with increased mortality in critically ill patients.

Association of serum anion gap and risk of long-term mortality in patients following coronary artery bypass grafting: A propensity score matching study

BACKGROUND

The present study aimed to explore the relationship between serum anion gap (AG) and long-term mortality in patients undergoing coronary artery bypass grafting (CABG).

METHODS

Clinical variables were extracted among patients undergoing CABG from Medical Information Mart for Intensive Care III (MIMIC III) database. The primary outcome was 4-year mortality following CABG. An optimal cut-off value of AG was determined by the receiver operating characteristic (ROC) curve. The Kaplan-Meier (K-M) analysis and multivariate Cox hazard analysis were performed to investigate the prognostic value of AG in long-term mortality after CABG. To eliminate the bias between different groups, propensity score matching (PSM) was conducted to validate the findings.

RESULTS

The optimal cut-off value of AG was 17.00 mmol/L. Then a total of 3162 eligible patients enrolled in this study were divided into a high AG group (≥17.00, n = 1022) and a low AG group (<17.00, n = 2,140). A lower survival rate was identified in the high AG group based on the K-M curve (p < .001). Compared with patients in the low AG group, patients in the high AG group had an increased risk of long-term mortality [1-year mortality: hazard ratio, HR: 2.309, 95% CI (1.672-3.187), p < .001; 2-year mortality: HR: 1.813, 95% CI (1.401-2.346), p < .001; 3- year mortality: HR: 1.667, 95% CI (1.341-2.097), p < .001; 4-year mortality: HR: 1.710, 95% CI (1.401-2.087), p < .001] according to multivariate Cox hazard analysis. And further validation of above results was consistent in the matched cohort after PSM.

CONCLUSIONS

The AG is an independent predictive factor for long-term all-cause mortality in patients following CABG, where a high AG value is associated with an increased mortality.

Postoperative anion gap associates with short- and long-term mortality after cardiac surgery: A large-scale cohort study

Objective

To investigate whether postoperative anion gap (AG) is associated with short- and long-term mortality in patients following cardiac surgery.

Materials and methods

We conducted a retrospective cohort study of adults who underwent cardiac surgery from the Medical Information Mart for Intensive Care - III database. The generalized additive model (GAM), logistic regression, and Cox regression were performed to assess the correlations between AG levels and in-hospital, 90-day, and 4-year mortality. Linear regression was used to evaluate the associations between AG and length of stay (LOS).

Results

Totally, 6,410 subjects were enrolled in this study and classified into tertiles based on the initial AG levels. The GAM indicated a positive association between initial AG and in-hospital mortality after adjusting for potential confounders. Multivariate logistic analysis revealed that the risk of in-hospital mortality was higher among patients in tertile 2 (OR 2.05, 95% CI 1.11–3.76, P = 0.021) and tertile 3 (OR 4.51, 95% CI 2.57–7.91, P < 0.001) compared with those in tertile 1. For 90-day and 4-year mortality, multivariate Cox regression found similar associations between AG tertiles and mortality. The LOS in ICU and hospital also increased as AG tertiles increased. The E-value indicated robustness to unmeasured confounders.

Conclusion

This study found a positive association between postoperative AG levels and short- and long-term mortality among patients after cardiac surgery. This relationship warrants further research.

Association of Plasma Anion Gap with 28-Day Inhospital Mortality and 1-Year Mortality of Patients with Alcohol Use Disorder at ICU Admission: A Retrospective Cohort Study

Background

Alcohol use disorder (AUD) is common in critically ill patients. Plasma anion gap (AG) was known as a feasible parameter and was associated with outcomes of various diseases. This study is intended to explore whether AG is related to 28-day inhospital mortality and 1-year mortality of critically ill patients with AUD.

Method

We extracted data from the Medical Information Mart for Intensive Care IV (MIMIC-IV) database. The association of plasma AG with 28-day inhospital mortality and 1-year mortality of critically ill AUD patients was assessed using Cox proportional hazard regression models and stratification analyses, allowing AG as a time-varying covariate in the models. To evaluate the accuracy of AG in predicting different endpoints, receiver operator characteristic (ROC) curves were used.

Result

Among the 3993 critically ill patients with AUD, AG was positively associated with 28-day inhospital mortality and 1-year mortality after adjusting confounders (p < 0.001 for all). Compared with lower AG (<12 mmol/L), patients in different groups (12 ≤ AG < 14 mmol/L, 14 ≤ AG < 17 mmol/L, 17 ≤ AG < 20 mmol/L, and AG ≥ 20 mmol/L) had different HRs (95% CIs) for 28-day inhospital mortality (1.105, (0.906, 1.347); 1.171, (0.981, 1.398); 1.320, (1.108, 1.573); and 1.487, (1.254, 1.763), respectively) and 1-year mortality (1.037 (0.898, 1.196); 1.091 (0.955, 1.246); 1.201 (1.052, 1.371); and 1.3093 (1.149, 1.492), respectively).

Conclusion

Increased AG is associated with greater 28-day inhospital mortality and 1-year mortality. The effect of AG on all-cause mortality is linear in critically ill AUD patients.

Data on the clinical, analytical, and laboratory factors associated with negative anion gaps at an academic medical center

The anion gap is a calculated parameter derived from the difference between the major plasma cations and anions in serum/plasma or whole blood, with a widely used simple equation utilizing concentrations of sodium, chloride, and bicarbonate. While there is extensive literature on the clinical significance and causes of elevated anion gaps, there is comparatively less data on low anion gaps. Occasionally, anion gap calculations result in a negative number (-1 or less). From the published literature, causes of these 'negative anion gaps' include laboratory error, specimen contamination or interference, hypoalbuminemia, extreme hyperkalemia, bromism, and paraproteins from multiple myeloma or similar pathologic processes. The data in this article present results from retrospective review of clinical chemistry and blood gas analysis testing at an academic medical center. The data include electrolyte concentrations and anion gap values derived from a total of 2,948,574 specimens (2,841,863 serum/plasma specimens analyzed on Roche Diagnostics clinical chemistry analyzers, 93,987 whole blood specimens analyzed on Radiometer blood gas analyzers, and 12,724 whole blood specimens on point-of-care chemistry devices) from 371,925 unique patients, clinical area where testing was ordered (for serum/plasma samples), sex, and age. For serum/plasma specimens with a negative anion gap, the data additionally include information from detailed chart review of possible factors and disease conditions contributing to the negative anion gap, pattern of electrolyte abnormalities, presence or absence of hypoalbuminemia, and corrected anion gap (if hypoalbuminemia is present).

Acid-base disorders: A primer for clinicians

An understanding of acid-base physiology is necessary for clinicians to recognize and correct problems that may negatively affect provision of nutrition support and drug therapy. An overview of acid-base physiology, the different acid-base disorders encountered in practice, a stepwise approach to evaluate arterial blood gases, and other key diagnostic tools helpful in formulating a safe and effective medical and nutrition plan are covered in this acid-base primer. Case scenarios are also provided for the application of principles and the development of clinical skills.

Obesity, Anion Accumulation, and Anion Gap Metabolic Acidosis: A Cohort Study

Background

Obesity is associated with low serum bicarbonate, an indicator of metabolic acidosis and a CKD risk factor. To further characterize acid-base disturbance and subclinical metabolic acidosis in this population, we examined prospective associations of body mass index (BMI) with elevated anion gap and whether anion gap values in obesity associate with low bicarbonate.

Methods

Data from adult outpatients (n=94,448) in the Bronx, New York were collected from 2010 to 2018. Mixed effects models and Cox proportional hazards models were used to examine associations of BMI with elevated anion gap and anion gap metabolic acidosis and of baseline anion gap with incident low bicarbonate and anion gap metabolic acidosis. Anion gap was defined using traditional and albumin-corrected calculations.

Results

Greater BMI was associated with higher anion gap over time and with progressively greater risk of developing an elevated anion gap (hazard ratio [HR] for body mass index [BMI]≥40 kg/m² versus 18 to <25 kg/m², 1.32; 95% confidence interval [95% CI], 1.23 to 1.42 for traditional and HR for BMI≥40 kg/m² versus 18 to <25 kg/m², 1.74; 95% CI, 1.63 to 1.85 for corrected). Higher BMI was also associated with increased risk of developing anion gap metabolic acidosis (HR for BMI≥40 kg/m², 1.53; 95% CI, 1.39 to 1.69). Among patients with obesity, higher anion gap was associated with increased risk of incident low bicarbonate (HR for fourth versus first quartile, 1.29; 95% CI, 1.23 to 1.44 for traditional and HR for fourth versus first quartile, 1.36; 95% CI, 1.26 to 1.48 for corrected) and higher risk of anion gap metabolic acidosis (HR for fourth versus first quartile, 1.78; 95% CI, 1.59 to 1.99).

Conclusions

Obesity is characterized by unmeasured anion accumulation and acid retention or overproduction. Modest elevations in anion gap among patients with obesity are associated with previously unrecognized anion gap metabolic acidosis.

Association Between Anion Gap and Mortality in Critically Ill Patients with Cardiogenic Shock

Background

No epidemiological study has determined the association between the anion gap (AG) and all-cause mortality in critically ill patients with cardiogenic shock (CS). This study was conducted to clarify the relationship between the AG and mortality in CS.

Methods

We extracted clinical data from the public database, MIMIC-III V1.4, by using a generalized additive model to identify the nonlinear relationship between the AG and the 30-day mortality in 1248 intensive care unit patients. Cox proportional hazard models were used to assess the association between the AG and the 30-day, 90-day, and 365-day mortality in CS.

Results

The AG and 30-day all-cause mortality showed a nonlinear relationship, indicated by a J-shaped curve. In the multivariate analysis, after adjusting for potential confounders, a high AG was associated with an increased risk of 30-day, 90-day, and 365-day all-cause mortality in patients with CS compared with patients who had low AG (hazard ratio [95% confidence interval] 1.62 [1.14-2.30]; 1.35 [1.04-1.84]; and 1.38 [1.03-1.84], respectively). Similar results were shown in Model I (adjusted for age, sex and ethnicity) and in Model II (fully adjusting for age, ethnicity, sex, acute kidney injury stage, CHF, renal disease, stroke, malignancy, respiratory failure, pneumonia, sodium, potassium, chloride, BUN, PT, WBC, pH, creatinine, albumin, glucose, bicarbonate, vasopressor use, diastolic blood pressure, respiration rate, temperature, the Elixhauser Comorbidity Index, SOFA score and SAPSII score).

Conclusion

The relationship between the AG and 30-day all-cause mortality followed a J-shaped curve. Higher AG was associated with an increased risk of 30-day, 90-day, and 365-day all-cause mortality in critically ill patients with CS.

The Serum Anion Gap Revisited: A Verified Reference Interval for Contemporary Use

BACKGROUND

The anion gap (AG) is often used to evaluate acid-base disorders. The reference interval for normal AG is used to differentiate between raised (gap) or normal AG (non-gap) acidosis. Historically accepted AG values may not be valid with the evolution of modern analytical techniques and the reference interval requires revalidation.

AIMS

To determine the reference interval for AG based on current laboratory techniques.

METHODS

During a health-screening exercise, 284 participants with no major illnesses volunteered surplus blood for analysis. The samples were tested in an internationally-accredited clinical laboratory. AG was calculated by (Na⁺ ) - (Cl^- ) - (HCO3^- ) and AG_K by (Na⁺ ) + (K⁺ ) - (Cl^- ) - (HCO₃ ^- ). The reference interval was determined at 2.5th-97.5th percentiles. Analysis was further undertaken for a sub-cohort of 156 individuals with no sub-optimal health indicators.

RESULTS

Median age was 35 years, BMI 23.4 kg/m² and glomerular filtration rate 106 mL/min/1.73m² . Median AG was 13 mmol/L and the reference interval for normal AG is 10-18 mmol/L with a 99% level of confidence. Statistically significant differences in AG were detected for sex, race, obesity and serum albumin but the difference was 1 mmol/L between subgroups. The reference interval was the same for the sub-cohort of 156 individuals. Median AG_K was 17.7 mmol/L and reference interval was 14.6-22.5 mmol/L.

CONCLUSIONS

The AG reference interval of 10-18 mmol/L is valid for laboratories with similar reference intervals for electrolytes. Lower values expected with current laboratory techniques were not observed. The median AG of 13 mmol/L may be used to differentiate gap acidosis, non-gap acidosis or mixed acid-base disorders. This article is protected by copyright. All rights reserved.

False-negative diagnosis of high anion gap in patients with end-stage kidney disease

The traditional anion gap (AG) equation is widely used, but its misdiagnosis in end-stage kidney disease (ESKD) patients has not been investigated fully. Diagnostic accuracy to detect high AG was cross-sectionally evaluated using 3 AG equations in 1733 ESKD patients with an eGFR less than 15 mL/min/1.73 m². The prevalence of high AG was 67.9%, 92.1% and 97.4% by the traditional, albumin-adjusted AG (aAG) and full AG equations, respectively. The sensitivity, specificity, accuracy and Kappa coefficient obtained with the traditional AG vs aAG equation were 0.70 vs 0.94, 0.98 vs 0.93, 0.7 vs 0.94, and 0.103 vs 0.44, respectively. Next, we created a subcohort comprising only patients with high full AG and investigated how the traditional AG equation leads to misdiagnoses. Multivariable-adjusted regression analysis in 1688 patients revealed that independent factors associated with a false-negative AG diagnosis were ARB use, eGFR, blood leukocyte count, serum chloride, bicarbonate, ionized calcium, potassium, albumin and phosphate. 93.2% of our subcohort prescribed any of RAAS inhibitors, Loop diuretics or Alkali which could increase either serum chloride or bicarbonate. Frequent use of these possible AG-reducing medications may conceal high AG state in patients with ESKD unless they have incidental inflammation which may increase AG value.

The Δ Anion Gap/Δ Bicarbonate Ratio in Early Lactic Acidosis: Time for Another Delta?

Background

The ratio of Δ anion gap and Δ bicarbonate (ΔAG/ΔHCO₃) is used to detect coexisting acid-base disorders in patients with high anion gap metabolic acidosis. Classic teaching holds that, in lactic acidosis, the ΔAG/ΔHCO₃ is 1:1 within the first few hours of onset and subsequently rises to 1.8:1. However, this classic 1:1 stoichiometry in early lactic acidosis was derived primarily from animal models and only limited human data. The objective of this study was to examine the ΔAG/ΔHCO₃ within the first hours of the development of lactic acidosis.

Methods

Data were obtained prospectively from a convenience sample of adult (age >18 years) trauma-designated patients at a single level-1 trauma center. Venous samples, including a chemistry panel and serum lactate, were drawn before initiation of intravenous fluid resuscitation.

Results

A total of 108 patients were included. Of these, 63 patients had normal serum lactate levels (≤2.1 mmol/L) with a mean AG of 7.1 mEq/L, the value used to calculate subsequent ΔAG values. ΔAG/ΔHCO₃ was calculated for 45 patients who had elevated serum lactate levels (>2.1 mmol/L). The mean ΔAG/ΔHCO₃ for all patients with elevated serum lactate levels was 1.86 (SD, 1.40).

Conclusions

The mean ΔAG/ΔHCO₃ was 1.86 within the first hours of the development of lactic acidosis due to hypovolemic shock, confirming a small prior human study. This contradicts the traditional belief that, in lactic acidosis, the ΔAG/ΔHCO₃ is 1:1 within the first several hours. The classic 1:1 stoichiometry was determined on the basis of animal models in which lactic acid is infused into the extracellular space, facilitating extracellular buffering of protons by bicarbonate. In contrast, our results demonstrate a higher initial ΔAG/ΔHCO₃ ratio in early endogenous lactic acidosis in humans. Our analysis indicates this is likely due to unmeasured anions contributing to an elevation in AG.

Serum Albumin Levels: Who Needs Them?

OBJECTIVES

The purpose of this critical narrative review is to discuss common indications for ordering serum albumin levels in adult critically ill patients, evaluate the literature supporting these indications, and provide recommendations for the appropriate ordering of serum albumin levels.

DATA SOURCES

PubMed (1966 to August 2020), Cochrane Library, and current clinical practice guidelines were used, and bibliographies of retrieved articles were searched for additional articles.

STUDY SELECTION AND DATA EXTRACTION

Current clinical practice guidelines were the preferred source of recommendations regarding serum albumin levels for guiding albumin administration and for nutritional monitoring. When current comprehensive reviews were available, they served as a baseline information with supplementation by subsequent studies.

DATA SYNTHESIS

Serum albumin is a general marker of severity of illness, and hypoalbuminemia is associated with poor patient outcome, but albumin is an acute phase protein, so levels vacillate in critically ill patients in conjunction with illness fluctuations. The most common reasons for ordering serum albumin levels in intensive care unit (ICU) settings are to guide albumin administration, to estimate free phenytoin or calcium levels, for nutritional monitoring, and for severity-of-illness assessment.

RELEVANCE TO PATIENT CARE AND CLINICAL PRACTICE

Because hypoalbuminemia is common in the ICU setting, inappropriate ordering of serum albumin levels may lead to unnecessary albumin administration or excessive macronutrient administration in nutritional regimens, leading to possible adverse effects and added costs.

CONCLUSIONS

With the exception of the need to order serum albumin levels as a component of selected severity-of-illness scoring systems, there is little evidence or justification for routinely ordering levels in critically ill patients.

Anion Gap Was Associated with Inhospital Mortality and Adverse Clinical Outcomes of Coronary Care Unit Patients

Background

Anion gap (AG) has been proved to be associated with prognosis of many cardiovascular diseases. This study is aimed at exploring the association of AG with inhospital all-cause mortality and adverse clinical outcomes in coronary care unit (CCU) patients.

Method

All data of this study was extracted from Medical Information Mart for Intensive Care III (MIMIC-III, version 1.4) database. All patients were divided into four groups according to AG quartiles. Primary outcome was inhospital all-cause mortality. Lowess smoothing curve was drawn to describe the overall trend of inhospital mortality. Binary logistic regression analysis was performed to determine the independent effect of AG on inhospital mortality.

Result

A total of 3593 patients were enrolled in this study. In unadjusted model, as AG quartiles increased, inhospital mortality increased significantly, OR increased stepwise from quartile 2 (OR, 95% CI: 1.01, 0.74-1.38, P = 0.958) to quartile 4 (OR, 95% CI: 2.72, 2.08-3.55, P < 0.001). After adjusting for possible confounding variables, this association was attenuated, but still remained statistically significant (quartile 1 vs. quartile 4: OR, 95% CI: 1.02, 0.72-1.45 vs. 1.49, 1.07-2.09, P = 0.019). Moreover, CCU mortality (P < 0.001) and rate of acute kidney injury (P < 0.001) were proved to be higher in the highest AG quartiles. Length of CCU (P < 0.001) and hospital stay (P < 0.001) prolonged significantly in higher AG quartiles. Maximum sequential organ failure assessment score (SOFA) (P < 0.001) and simplified acute physiology score II (SAPSII) (P < 0.001) increased significantly as AG quartiles increased. Moderate predictive ability of AG on inhospital (AUC: 0.6291), CCU mortality (AUC: 0.6355), and acute kidney injury (AUC: 0.6096) was confirmed. The interactions were proved to be significant in hypercholesterolemia, congestive heart failure, chronic lung disease, respiratory failure, oral anticoagulants, Beta-blocks, angiotensin-converting enzyme inhibitor (ACEI)/angiotensin receptor blocker (ARB), and vasopressin treatment subgroups.

Conclusion

AG was an independent risk factor of inhospital all-cause mortality and was associated with adverse clinical outcomes in CCU patients.

Predictors of acute metabolic decompensation in children with maple syrup urine disease at the emergency department

Acute metabolic decompensation (AMD) of maple syrup urine disease (MSUD) must be promptly recognized and treated. In this study, we aimed to identify simple variables associated with AMD in children with MSUD for use in emergency settings. Data were collected retrospectively from 115 emergency visits of 29 children with MSUD over a 4-year period in a major referral hospital. Variables in visits with and without AMD were compared using t test, Mann-Whitney U test, and chi-square test. Logistic regression was used to identify independent variables associated with decompensations. Cut-off values of laboratory variables were determined with receiver operating characteristic curves and correlations with Spearman's rank correlation. Most important variables independently associated with AMD were poor feeding, malaise, anion gap, and especially uric acid, which correlated with leucine levels. Vomiting, dehydration, neurological signs, ketonuria, and ketoaciduria were also associated with AMD. Although sodium, chloride, and glucose were lower in AMD, they had little diagnostic value.Conclusion: In children with MSUD, uric acid and anion gap are key markers for AMD. Poor feeding and malaise are clues before the onset of neurological symptoms. These simple parameters can help determine the presence of AMD in emergency settings.What is Known:• In maple syrup urine disease, acute metabolic decompensations are characterized by gastrointestinal and neurological findings.• Diagnosis requires detection of significantly elevated leucine, which may take a long time or not be available.What is New:• Poor feeding, malaise, hyperuricemia, and high anion gap are parameters that can help diagnose acute decompensations in children with maple syrup urine disease at emergency departments.• Uric acid may be a biomarker for acute decompensations because of its high sensitivity, specificity, and its strong correlation with leucine.

Progression of Metabolic Acidosis in Chronic Kidney Disease

BACKGROUND

Metabolic acidosis, which is classified into either high anion gap type (high-AGMA) or non-anion gap type (non-AGMA), is a common complication in chronic kidney disease (CKD), but its development in CKD is obscure.

METHODS

Records of venous blood gas at a general hospital (2015-2017) were assessed by the physiological approach. Excluding records of primary respiratory disturbances, parameters of high-AGMA and non-AGMA (∆AG and ΔΔ, respectively) were compared with the estimated glomerular filtration rate (eGFR).

RESULTS

ΔAG correlated with eGFR negatively (r = -0.397, p < 0.001), but ΔΔ did not correlate with eGFR (p = 0.51). Among the records grouped by the CKD stage (either G1-3, G4, or G5), ∆AG in G5 (0.9 ± 2.7) was higher than those in G1-3 (-2.2 ± 2.6, p < 0.001) and in G4 (-2.0 ± 2.1, p < 0.001). ∆∆ in G4 (4.0 ± 4.1) was higher than that in G1-3 (1.5 ± 3.7, p = 0.056). Between the subgroups in G5 (either G5a: eGFR 10-15, G5b: eGFR 5-10, or G5c: eGFR <5 mL/min/1.73 m²), ∆AG in G5c (3.8 ± 2.1) was higher than that in G5b (0.8 ± 2.4, p < 0.001), which was higher than that in G5a (-0.9 ± 1.8, p < 0.001). ΔΔ in G5a (5.6 ± 4.1) was higher than those in G4 (p = 0.041) and in G5b (3.2 ± 3.9, p = 0.001), which was higher than that in G5c (0.8 ± 3.8, p = 0.006).

CONCLUSION

High-AGMA developed and progressed in CKD stage G5. Non-AGMA generally progressed before the early phase of CKD stage G5 and regressed thereafter.

Interpreting Anion Gap Values in Adult and Pediatric Patients: Examining the Reference Interval

BACKGROUND

The anion gap is primarily used in the diagnosis of acid-base disorders. We conducted a study to determine the anion gap reference interval in our patient population, investigated the workup of abnormal vs normal anion gaps, and examined the anion gap variation upon repeated testing.

METHODS

A retrospective review was performed on 17137 adult and pediatric patients who presented to Yale-New Haven Hospital outpatient clinics, emergency department, or intensive care units between 2012 and 2017.

RESULTS

We derived a new reference interval of 7 to 18 mmol/L with a median of 13 mmol/L in healthy adults with no significant differences owing to partitioning by sex or age. Based on the new reference interval, 5%, 23%, and 18% of healthy, emergency department, and intensive care unit adult patients, respectively, were misclassified as having high values with the previous interval of 6 to 16 mmol/L. However, there were no significant differences in the number of tests ordered in patients with anion gaps above and below the upper limit of the previous reference interval. The majority of increased anion gaps that were repeated normalized by 12 h. In a subgroup of healthy adult patients with annual testing, the median percent change in each patient's anion gap from 2015 to 2016 was approximately 13%.

CONCLUSIONS

The anion gap should be used with an appropriate reference interval to avoid misclassification. There may be a moderate degree of individuality that argues for comparing the anion gap with its baseline value in the same patient pending further studies that formally derive its biological variation.

The arterial blood gas algorithm: Proposal of a systematic approach to analysis of acid-base disorders

Abnormalities in the acid-base balance are common clinical problems and can have deleterious effects on cellular function and be a clue to various disorders. Therefore, it is important for the clinician to make a precise diagnosis of the acid-base disorder(s) present for a proper treatment. Three approaches have been proposed to evaluate acid-base disorders: a bicarbonate-centric approach; the Stewart approach, and the base excess approach. Although the latter two have many adherents, we will only discuss the bicarbonate-centric approach. This approach is simpler to utilize at the bedside, has a physiological evaluation of the acid-base disorder, presents an easily understandable approach to assess severity, and provides a more solid foundation for the development of effective therapies. Therefore, the following discussion will be limited to an examination of this approach. In this case-centric review, important new concepts will be introduced first; their benefits and limitations discussed; and then their utilization to analyze actual cases will be shown. A systematic approach algorithm that incorporates these new concepts has been generated and will be highlighted.

Anion gap reference intervals show instrument dependence and weak correlation with albumin levels

BACKGROUND

Anion gap (AG) aids the differential diagnosis of acid-base disorders. Its value has decreased, because of new analytical methods. Our goal was to compare AG reference intervals for different instruments and Southeast Asian populations.

METHODS

We studied AG at three hospitals. One used the cobas 8000; two others, the Architect c16000. We included consecutive adults ≥18 years whose samples were sent for electrolytes and creatinine. We assessed AG for all patients and patients with normal electrolytes.

RESULTS

AG means differed significantly (P < 0.001) between the three hospitals for all patients and the normal electrolyte subgroup. AG reference intervals from all patients were 9-19, 5-15, and 5-15 mmol/L, and for the normal electrolyte subgroup, 10-17, 6-14, and 5-12 mmol/L, respectively. Compared to the normal albumin group, hypoalbuminemia patients showed lower AG in two hospitals (P < 0.001, P = 0.03), whereas patients with hyperalbuminemia demonstrated higher AG in all three hospitals (P < 0.001).

CONCLUSIONS

Different instruments produce different AGs. There is a weak correlation between albumin levels and AG. Laboratorians should verify reference intervals used when detecting laboratory errors and assisting clinicians in the differential diagnosis of acid base disorders and other medical conditions.

Can base excess and anion gap predict lactate level in diagnosis of septic shock?

Background

Lactate measurement is the key component in septic shock identification and resuscitation. However, point-of-care lactate testing is not widely used due to the lack of access to nearby test equipment. Biomarkers such as serum lactate, anion gap (AG), and base excess (BE) are used in determining shock in patients with seemingly normal vital signs.

Purpose

We aimed to determine if these biomarkers can be used interchangeably in patients with septic shock in the emergency setting.

Patients and methods

A prospective observational cohort study was undertaken at a tertiary hospital in southern Thailand. Baseline point-of-care BE, AG, and serum lactate were recorded in all patients presenting with septic shock at the emergency department. Overall correlations including area under the receiver operating characteristic curve (AUROC) for both BE and AG to predict serum lactate level were calculated.

Results

One hundred and fifteen patients were enrolled. Pearson correlation of serum lactate to BE was −0.59 (r² = 0.35; 95% confidence interval [CI], −0.69 to −0.44; P < 0.001) and BE to AG was −0.67 (r² = 0.49; 95% CI, −0.76 to −0.55; P < 0.001), and serum lactate to AG was 0.64 (r² = 0.41; 95% CI, 0.52 to 0.74; P < 0.001). A cut-off point of 15.8 for AG identified a lactate level ≥2 mmol/L (sensitivity, 71.4%; specificity, 80.7%; and AUROC, 0.76), and the best cut-off value to predict a lactate level ≥4 mmol/L was 18.5 (sensitivity, 64.2%; specificity, 85.5%; and AUROC 0.78).

Conclusion

In patients with septic shock, lactate and AG showed a strong correlation with each other, whereas lactate and BE showed a moderate correlation with each other. Thus, these biomarkers can be used interchangeably to help determine septic shock earlier in patients.

A systematic review and diagnostic test accuracy meta-analysis of the validity of anion gap as a screening tool for hyperlactatemia

Objective

This systematic review and meta-analysis seeks to determine the validity of the anion gap to screen for hyperlactatemia in critically ill patients. We have previously shown that the anion gap does not predict 31-day and in-hospital mortality in critically ill patients. The present review aims to add confirmatory evidence to identify whether the anion gap is a suitable tool for risk stratification in low-resource countries.

Results

Nine studies reporting on 4504 samples from 2111 patients were included. The anion gap failed to detect hyperlactatemia defined as lactate above 2.5 mmol/l but showed good discriminatory ability for the detection of severe hyperlactatemia defined as lactate over 4 mmol/l. At the 2.5 mmol/l threshold, the anion gap had high specificity but low sensitivity for the detection of hyperlactatemia. A meta-analysis of correlation coefficients yielded high statistical heterogeneity. Therefore, in keeping with our previous findings, the use of the anion gap for risk stratification as an alternative to lactate cannot be recommended. However, the strength of the evidence we have synthesised is adversely affected by the small number of studies included, inconsistency of effect measures and positivity thresholds reported, and selection bias within individual studies. PROSPERO Registration Number: CRD42015016470 (registered on the 4th February 2015).

Electronic supplementary material

The online version of this article (10.1186/s13104-017-2853-9) contains supplementary material, which is available to authorized users.

Low sensitivity of anion gap to detect clinically significant lactic acidosis in the emergency department

INTRODUCTION

Lactic acidosis represents the pathologic accumulation of lactate and hydrogen ions. It is important to efficiently diagnose lactic acidosis as delayed treatment will lead to poor patient outcomes. As plasma lactate levels may not be rapidly available, some physicians may use elevated anion gaps to test for the need to measure lactate. All Edmonton metropolitan hospitals have Radiometer blood gas/electrolyte instruments in the ED or close by. As lactate is measured for each set of electrolytes, we were able to determine the effectiveness of a screening anion gap for lactic acidosis.

METHODS

Two years of emergency department lactates and electrolytes from Edmonton's 5 metropolitan hospitals were analyzed. We determined the sensitivity, specificity and positive predictive value of detecting an elevated lactate, defined as ≥2.5mmol/L or ≥4mmol/L.

RESULTS

Depending on the elevated anion gap cut-off and the definition of elevated lactate, between 40-80% of elevated lactates are missed. In general, the positive predictive value approaches 40% for AGs ≥12mmol/L and 60% for AGs ≥16mmol/L.

CONCLUSIONS

Anion gap is an inadequate marker of lactic acidosis. We recommend that lactate be done with each set of electrolytes and/or blood gases. In this way lactic acidosis will not be missed.

Disorders of Acid-Base Balance: New Perspectives

BACKGROUND

Disorders of acid-base involve the complex interplay of many organ systems including brain, lungs, kidney, and liver. Compensations for acid-base disturbances within the brain are more complete, while limitations of compensations are more apparent for most systemic disorders. However, some of the limitations on compensations are necessary to survival, in that preservation of oxygenation, energy balance, cognition, electrolyte, and fluid balance are connected mechanistically.

SUMMARY

This review aims to give new and comprehensive perspective on understanding acid-base balance and identifying associated disorders. All metabolic acid-base disorders can be approached in the context of the relative losses or gains of electrolytes or a change in the anion gap in body fluids. Acid-base and electrolyte balance are connected not only at the cellular level but also in daily clinical practice. Urine chemistry is essential to understanding electrolyte excretion and renal compensations.

KEY MESSAGES

Many constructs are helpful to understand acid-base, but these models are not mutually exclusive. Electroneutrality and the close interconnection between electrolyte and acid-base balance are important concepts to apply in acid-base diagnoses. All models have complexity and shortcuts that can help in practice. There is no reason to dismiss any of the present constructs, and there is benefit in a combined approach.

Pyroglutamic acidosis in association with therapeutic paracetamol use

Long-term use of paracetamol (at therapeutic doses) can cause the accumulation of endogenous organic pyroglutamate, resulting in metabolic acidosis with an elevated anion gap. This occurs in the presence of malnutrition, infection, antibiotic use, renal failure and pregnancy. Given the prevalence of these risk factors, this condition is thought to be relatively common in a hospitalised population but is probably significantly underdiagnosed. Prompt recognition is essential because the condition is entirely reversible if the causative agents are withdrawn.Here we describe five cases of pyroglutamic acidosis that we have encountered in a tertiary referral hospital. Together they illustrate the common clinical risk factors and the excellent prognosis, once a diagnosis is made. We describe how a rudimentary acid-base analysis (calculation of the anion gap) usually leads to the diagnosis but how a more nuanced approach may be required in the presence of mixed acid-base disorders.

Non-Anion Gap Metabolic Acidosis: A Clinical Approach to Evaluation

Acid-base disturbances can result from kidney or nonkidney disorders. We present a case of high-volume ileostomy output causing large bicarbonate losses and resulting in a non-anion gap metabolic acidosis. Non-anion gap metabolic acidosis can present as a form of either acute or chronic metabolic acidosis. A complete clinical history and physical examination are critical initial steps to begin the evaluation process, followed by measuring serum electrolytes with a focus on potassium level, blood gas, urine pH, and either direct or indirect urine ammonium concentration. The present case was selected to highlight the differential diagnosis of a non-anion gap metabolic acidosis and illustrate a systematic approach to this problem.

Anion gap as a prognostic tool for risk stratification in critically ill patients - a systematic review and meta-analysis

BACKGROUND

Lactate concentration is a robust predictor of mortality but in many low resource settings facilities for its analysis are not available. Anion gap (AG), calculated from clinical chemistry results, is a marker of metabolic acidosis and may be more easily obtained in such settings. In this systematic review and meta-analysis we investigated whether the AG predicts mortality in adult patients admitted to critical care settings.

METHODS

We searched Medline, Embase, Web of Science, Scopus, The Cochrane Library and regional electronic databases from inception until May 2016. Studies conducted in any clinical setting that related AG to in-hospital mortality, in-intensive care unit mortality, 31-day mortality or comparable outcome measures were eligible for inclusion. Methodological quality of included studies was assessed using the Quality in Prognostic Studies tool. Descriptive meta-analysis was performed and the I(2) test was used to quantify heterogeneity. Subgroup analysis was undertaken to identify potential sources of heterogeneity between studies.

RESULTS

Nineteen studies reporting findings in 12,497 patients were included. Overall, quality of studies was poor and most studies were rated as being at moderate or high risk of attrition bias and confounding. There was substantial diversity between studies with regards to clinical setting, age and mortality rates of patient cohorts. High statistical heterogeneity was found in the meta-analyses of area under the ROC curve (I(2) = 99 %) and mean difference (I(2) = 97 %) for the observed AG. Three studies reported good discriminatory power of the AG to predict mortality and were responsible for a large proportion of statistical heterogeneity. The remaining 16 studies reported poor to moderate ability of the AG to predict mortality. Subgroup analysis suggested that intravenous fluids affect the ability of the AG to predict mortality.

CONCLUSION

Based on the limited quality of available evidence, a single AG measurement cannot be recommended for risk stratification in critically ill patients. The probable influence of intravenous fluids on AG levels renders the AG an impractical tool in clinical practice. Future research should focus on increasing the availability of lactate monitoring in low resource settings.

PROSPERO REGISTRATION NUMBER

CRD42015015249 . Registered on 4th February 2015.

The Anion Gap is a Predictive Clinical Marker for Death in Patients with Acute Pesticide Intoxication

Pesticide formulation includes solvents (methanol and xylene) and antifreeze (ethylene glycol) whose metabolites are anions such as formic acid, hippuric acid, and oxalate. However, the effect of the anion gap on clinical outcome in acute pesticide intoxication requires clarification. In this prospective study, we compared the anion gap and other parameters between surviving versus deceased patients with acute pesticide intoxication. The following parameters were assessed in 1,058 patients with acute pesticide intoxication: blood chemistry (blood urea nitrogen, creatinine, glucose, lactic acid, liver enzymes, albumin, globulin, and urate), urinalysis (ketone bodies), arterial blood gas analysis, electrolytes (Na(+), K(+), Cl(-) HCO3 (-), Ca(++)), pesticide field of use, class, and ingestion amount, clinical outcome (death rate, length of hospital stay, length of intensive care unit stay, and seriousness of toxic symptoms), and the calculated anion gap. Among the 481 patients with a high anion gap, 52.2% had a blood pH in the physiologic range, 35.8% had metabolic acidosis, and 12.1% had acidemia. Age, anion gap, pesticide field of use, pesticide class, seriousness of symptoms (all P < 0.001), and time lag after ingestion (P = 0.048) were significant risk factors for death in univariate analyses. Among these, age, anion gap, and pesticide class were significant risk factors for death in a multiple logistic regression analysis (P < 0.001). In conclusions, high anion gap is a significant risk factor for death, regardless of the accompanying acid-base balance status in patients with acute pesticide intoxication.

Prevalence of Metformin Use and the Associated Risk of Metabolic Acidosis in US Diabetic Adults With CKD: A National Cross-Sectional Study

The use of metformin in chronic kidney disease (CKD) population has been intensely debated with conflicting evidence. Large population studies are needed to inform risk assessment and therapeutic decision-making. We evaluated the associations among metformin, metabolic acidosis, and CKD in a 10-year nationally representative noninstitutionalized civilian population in the United States.In this cross-sectional study, a total of 2279 diabetic adults aged 20 years or older in the National Health and Nutrition Examination Survey (NHANES) from 2003 to 2012 were included and had measurements of serum bicarbonate, sodium, potassium, and chloride. The exposure was metformin use. The outcome was subclinical and severe metabolic acidosis defined by serum bicarbonate <23 mEq/L and anion gap > 16mEq/L and by serum bicarbonate < 20 mEq/L, respectively.The prevalence of metformin use decreased from 67.2% among CKD-1 and -2, 40.6% among CKD-3, to 1.3% among advanced CKD-4 and -5. Across CKD stages up to CKD-3b, we observed a tendency of lower levels of serum bicarbonate that was significant in metformin users with CKD-2 and CKD-3a and marginally significant with CKD-3b compared to nonmetformin users. The corresponding tendency of higher anion gap in metformin users with the estimated glomerular filtration rate >60 mL/min/1.73 m was also observed. In multiple linear regression analysis, metformin was significantly associated with decreased serum bicarbonate levels (β = -0.45, 95% CI: -0.73, -0.17) and increased serum anion gap levels (β = 0.40, 95% CI: 0.19, 0.61). The adjusted odds ratio of subclinical high anion gap and severe metabolic acidosis for metformin users was 1.68 (95% CI: 1.11, 2.55) and 1.31 (0.49, 3.47), respectively. The association between metformin and serum bicarbonate was significantly modified by CKD status. No interaction was found between metformin and CKD stages for serum anion gap and acidosis.Metformin is associated with subclinical metabolic acidosis but not with severe metabolic acidosis. The propensity of serum bicarbonate-lowering effect was intensified in advanced CKD; however, such tendency was not associated with the risk of clinically defined acidosis. Our findings highlight a potential of cautious expansion of metformin use among CKD-3b patients with diabetes meriting further investigations.