Prevalence of Clinically Significant Differences in Sodium Measurements Due to Abnormal Protein Concentrations Using an Indirect Ion-Selective Electrode Method

Different serum sodium assay, different model for end stage liver disease - sodium scores in patients awaiting liver transplant: A cross-sectional study

INTRODUCTION AND AIMS

Sodium can be measured with direct or indirect methods; abnormal plasma total protein concentration can impact on sodium measured by indirect ion-selective electrodes (ISE). Serum sodium is an important item to determine the Model for End Stage Liver Disease Sodium (MELD-Na) score, commonly used for liver graft allocation. Patients with cirrhosis usually have hypoproteinemia. The aim of this study was to determine if there was a significant difference between the MELD-Na scores calculated based on the results of two different serum sodium ISE: indirect and direct.

METHODS

This was a retrospective study; we included 166 patients that underwent liver transplant assessment, and that had paired (i.e. same date and time) direct and indirect sodium determinations. We calculated the MELD-Na scores with both sodium determinations, and we compared them.

RESULTS

There was a significant difference between MELD-Na scores; the mean difference was 0.4±1.3. If MELD-Na score had been determined by the sodium measured by the direct ISE, 69 patients (42%) would have stayed in the same place on the waiting list, 67 patients (40%) would have moved up, and 30 patients (18%) would have moved down.

CONCLUSIONS

There was a statistically significant difference between the MELD-Na scores calculated based on the two different sodium concentrations, which would theoretically result in changes in the order of the waiting list. This finding should prompt studies to assess if MELD-Na calculated based on direct methods has a better performance to predict clinically relevant outcomes.

[Hyponatremia : Etiology, diagnosis and acute therapy]

Hyponatremia is one of the most common electrolyte disorders in emergency departments and hospitalized patients. Serum sodium concentration is controlled by osmoregulation and volume regulation. Both pathways are regulated via the release of antidiuretic hormone (ADH). Syndrome of inappropriate release of ADH (SIADH) may be caused by neoplasms or pneumonia but may also be triggered by drug use or drug abuse. Excessive fluid intake may also result in a decrease in serum sodium concentration. Rapid alteration in serum sodium concentration leads to cell swelling or cell shrinkage, which primarily causes neurological symptoms. The dynamics of development of hyponatremia and its duration are crucial. In addition to blood testing, a clinical examination and urine analysis are essential in the differential diagnosis of hyponatremia.

Pseudohyponatremia: Mechanism, Diagnosis, Clinical Associations and Management

Pseudohyponatremia remains a problem for clinical laboratories. In this study, we analyzed the mechanisms, diagnosis, clinical consequences, and conditions associated with pseudohyponatremia, and future developments for its elimination. The two methods involved assess the serum sodium concentration ([Na]_S) using sodium ion-specific electrodes: (a) a direct ion-specific electrode (ISE), and (b) an indirect ISE. A direct ISE does not require dilution of a sample prior to its measurement, whereas an indirect ISE needs pre-measurement sample dilution. [Na]_S measurements using an indirect ISE are influenced by abnormal concentrations of serum proteins or lipids. Pseudohyponatremia occurs when the [Na]_S is measured with an indirect ISE and the serum solid content concentrations are elevated, resulting in reciprocal depressions in serum water and [Na]_S values. Pseudonormonatremia or pseudohypernatremia are encountered in hypoproteinemic patients who have a decreased plasma solids content. Three mechanisms are responsible for pseudohyponatremia: (a) a reduction in the [Na]_S due to lower serum water and sodium concentrations, the electrolyte exclusion effect; (b) an increase in the measured sample's water concentration post-dilution to a greater extent when compared to normal serum, lowering the [Na] in this sample; (c) when serum hyperviscosity reduces serum delivery to the device that apportions serum and diluent. Patients with pseudohyponatremia and a normal [Na]_S do not develop water movement across cell membranes and clinical manifestations of hypotonic hyponatremia. Pseudohyponatremia does not require treatment to address the [Na]_S, making any inadvertent correction treatment potentially detrimental.

Challenge in hyponatremic patients - the potential of a laboratory-based decision support system for hyponatremia to improve patient's safety

OBJECTIVES

Hyponatremia is the most frequent electrolyte disorder in hospitalized patients with increased mortality and morbidity. In this study, we evaluated the follow-up diagnostic, the risk of inadequate fast correction and the outcome of patients with profound hyponatremia (pHN), defined as a blood sodium concentration below 120 mmol/L. The aim was to identify a promising approach for a laboratory-based clinical decision support system (CDSS).

METHODS

This retrospective study included 378,980 blood sodium measurements of 83,315 cases at a German tertiary care hospital. Hospitalized cases with pHN (n=211) were categorized into two groups by the time needed for a follow-up measurement to be performed (time to control, TTC) as either <12 h (group 1: "TTC≤12 h", n=118 cases) or >12 h (group 2: "TTC>12 h", n=93 cases). Length of hospital stay, sodium level at discharge, ward transfers, correction of hyponatremia, and risk of osmotic demyelination syndrome (ODS) due to inadequate fast correction were evaluated with regard to the TTC of sodium blood concentration.

RESULTS

pHN was detected in 1,050 measurements (0.3%) in 211 cases. Cases, in which follow-up diagnostics took longer (TTC>12 h), achieved a significantly lower sodium correction during their hospitalization (11.2 vs. 16.7 mmol/L, p<0.001), were discharged more frequently in hyponatremic states (<135 mmol/L; 58 (62.4%) vs. 43 (36.4%), p<0.001) and at lower sodium blood levels (131.2 vs. 135.0 mmol/L, p<0.001). Furthermore, for these patients there was a trend toward an increased length of hospital stay (13.1 vs. 8.5 days, p=0.089), as well as an increased risk of inadequate fast correction (p<0.001).

CONCLUSIONS

Our study shows that less frequent follow-up sodium measurements in pHN are associated with worse outcomes. Patients with a prolonged TTC are at risk of insufficient correction of hyponatremia, reduced sodium values at discharge, and possible overcorrection. Our results suggest that a CDSS that alerts treating physicians when a control time of >12 h is exceeded could improve patient care in the long term. We are initiating a prospective study to investigate the benefits of our self-invented CDSS (www.ampel.care) for patients with pHN.

Paraproteins and electrolyte assays: exclusion effect and effect of paraprotein elimination

Paraproteins are a potential source of error for electrolyte analyses. The exclusion effect itself causes a discrepancy between direct and indirect ion selective electrode assays (dISE and iISE, respectively). We tested the applicability of different pretreatment methods and the difference of dISE and iISE with paraprotein-rich samples. We analysed chloride (Cl-), potassium (K+), and sodium (Na+) on 46 samples with paraproteins up to 73 g/L. We compared pretreatment methods of preheating, precipitation, and filtration to the native sample. All induced a statistically significant difference (p-value <0.05). Clinically significant difference was induced by precipitation for all analytes, and filtration for Cl- and Na+, but for none by preheating. The difference in electrolyte measurements with either dISE or iISE on native samples was explained by total protein concentration (TP). There was a statistically significant difference in all electrolyte measurements. On average, there was a clinically significant difference in Na + but not in Cl- and K + measurements. Paraprotein concentration (PP) or heavy chain class did not induce a statistically significant effect. The regression analysis and comparison to the theoretical exclusion effect supported the conclusion that TP is the only explanatory factor in the difference between dISE and iISE. We conclude that preheating is a suitable pretreatment method for all the studied analytes. Precipitation is not valid for any of them, and filtration can be considered only for K+. Because the difference between dISE and iISE was explained by the exclusion effect caused by TP, dISE is the more suitable method to analyse paraprotein-rich samples.

A case of pulmonary hyalinizing granuloma characterized by pseudohyponatremia due to hyperproteinemia

A 57-year-old man presented with multiple pulmonary nodules. Thoracoscopic lung biopsy led to a pathological diagnosis of pulmonary hyalinizing granuloma (PHG) at the age of 39 years. The disease was progressive, refractory to therapy, and necessitated home oxygen therapy 10 years after the diagnosis. Hyponatremia progressed gradually along with lung disease. His serum sodium level was 129 mEq/L but serum osmolality was normal (287 mOsm/kg). Concomitant hyperproteinemia (12.1 g/dL) was attributable to hyperglobulinemia. Direct ion-selective electrode measurement revealed a normal sodium level (137 mmol/L). We herein report a case of PHG characterized by pseudohyponatremia due to hyperproteinemia, an uncommon finding in this rare entity. A left lung transplant was successfully performed, and no pseudohyponatremia was observed. Pseudohyponatremia should be suspected and diagnosed to prevent a misdiagnosis that could lead to complications from inappropriate treatment with sodium supplementation or restriction of drinking water. The direct ion-selective electrode measurement was useful for diagnosing pseudohyponatremia.

Laying the foundation for enhancing safety of desmopressin in older people: Validation of capillary blood sodium levels

PURPOSE

We aim to make desmopressin a safe treatment option for (older) patients at risk for hyponatremia, by introducing a new way of sodium monitoring. The goal is to reduce the risk of hyponatremia, enhance patient safety and ultimately introduce self-monitoring of sodium levels. The first step in the aforementioned is to validate capillary sodium.

MATERIALS AND METHODS

100 randomly selected patients admitted to the urology department received a single finger prick to collect capillary blood (250 µl) in a lithium-heparin tube. Each patient acted as its own control for the capillary and venous blood sample. Venous and capillary plasma sodium were analyzed by indirect ion-selective electrode measurement. The primary outcome was the agreement between capillary and venous sodium measurements, measured by the intra-class correlation coefficient (ICC).

RESULTS

One hundred paired blood samples were obtained of which four were excluded. There was no significant statistical difference observed between venous and capillary sodium (-0.23 mmol/L, p = 0.374). The ICC for single measures between capillary and venous sodium was 0.82 (95% confidence interval 0.75-0.88). Inter-method differences analyzed by a Bland-Altman plot and a Passing-Bablock regression did not reveal a statistically significant difference between both groups.

CONCLUSIONS

We demonstrated that venous and capillary sodium levels are interchangeable, taken into account the inter- and intravariability between analyses. We provided the first step towards a simple and safe solution for frequent sodium monitoring through a minimal invasive capillary blood collection. The results are of direct clinical relevance to safely use desmopressin in (older) patients at risk.

Comparison of Point-of-Care versus Central Laboratory Testing of Electrolytes, Hemoglobin, and Bilirubin in Neonates

OBJECTIVE

Electrolyte, hemoglobin, and bilirubin values are routinely reported with point-of-care (POC) testing for blood gases. Results are rapidly available and require a small blood volume. Yet, these results are underutilized due to noted discrepancies between central laboratory (CL) and POC testing. The study aimed to determine the correlation between POC and CL measurement of electrolytes, hemoglobin, and bilirubin in neonates.

STUDY DESIGN

Electrolyte, hemoglobin, and bilirubin results obtained from capillary blood over a 4-month period were analyzed. Each CL value was matched with a POC value from the same sample or another sample less than 1-hour apart. Agreement was determined by measuring the mean difference (MD) between paired samples with 95% limits of agreement (LOA) and Lin's concordance correlation (LCC).

RESULTS

There were 355-paired sodium/potassium, 139 paired hemoglobin, and 197 paired bilirubin values analyzed. POC sodium values were lower (133.5 ± 5.8 mmol/L) than CL (140.2 ± 5.8 mmol/L), p <0.00001 with poor agreement (LCC = 0.49; MD = 6.7; 95% LOA: -13.6 to 0.14). POC potassium values were lower (4.6 ± 0.98 mmol/L) than CL (4.98 ± 1.24mEq/L), p < 0.0001, but with better concordance and agreement. (LCC = 0.6; MD = 0.4; 95% LOA: -2.3 to 1.4). There were no differences in hemoglobin between POC (14.3 ± 3.2 g/dL) and CL (14.4 ± 3.1 g/dL), p = 0.2 with good LCC (0.93) and in bilirubin values between POC (6.0 ± 3.2 mg/dL) and CL (5.8 ± 3.0 mg/dL), MD = 0.18, and p = 0.07.

CONCLUSION

POC Sodium values are lower than CL. POC potassium levels are also lower, but the differences may not be clinically important while hemoglobin and bilirubin levels are similar between POC and CL. As POC potassium, hemoglobin, and bilirubin levels closely reflect CL values, these results can be relied upon to make clinical judgments in neonates.

KEY POINTS

· Electrolyte, hemoglobin, and bilirubin are available as POC.. · POC sodium and potassium values are lower than CL results.. · Hemoglobin and bilirubin values are similar between POC and CL..

Sodium and Potassium Dysregulation in the Patient With Cancer

Sodium and potassium disorders are pervasive in patients with cancer. The causes of these abnormalities are wide-ranging, are often primary or second-order consequences of the underlying cancer, and have prognostic implications. The approach to hyponatremia should focus on cancer-related etiologies, such as syndrome of inappropriate antidiuretic hormone, to the exclusion of other causes. Hypernatremia in non-iatrogenic forms is generally due to water loss rather than excessive sodium intake. Debilitated or dependent patients with cancer are particularly vulnerable to hypernatremia. Hypokalemia can occur in patients with cancer due to gastrointestinal disturbances, resulting from decreased intake or increased losses. Renal losses can occur as a result of excessive mineralocorticoid secretion or therapy-related nephrotoxicity. The approach to hyperkalemia should be informed by historical and laboratory clues, and pseudohyperkalemia is particularly common in patients with hematological cancers. Hyperkalemia can be seen in primary or metastatic disease that interrupts the adrenal axis. It can also develop as a consequence of immunotherapy, which can cause adrenalitis or hypophysitis. Tumor lysis syndrome (TLS) is defined by the development of hyperkalemia and is a medical emergency. Awareness of the electrolyte abnormalities that can befall patients with cancer is vital for its prompt recognition and management.

Resolving Pseudohyponatremia: Validation of Plasma Sodium on Radiometer ABL800 Blood Gas Analyzers for Immediate Reflex Testing

OBJECTIVE

To perform validation of plasma sodium on blood gas analyzers to reflexively correct erroneous measurements by ion-selective electrodes (ISEs).

METHODS

We compared remnant specimens of whole blood and plasma collected by lithium heparin vacutainer with normal protein concentrations and no lipemia. Whole-blood specimens were tested for sodium concentration on the ABL800 Flex blood gas analyzer, followed by centrifugation for plasma separation, and repeat sodium determination on an aliquot of the plasma only. Also, plasma specimens were analyzed by indirect ISE on the Cobas 8000 series and by direct ISE on the ABL800 Flex for instrument comparison.

RESULTS

Plasma aliquots yielded comparable results to the parent whole-blood specimen, with an average change of -1.33 mmol/L (R2 = 0.9727). Comparison of indirect ISE to direct ISE similarly yielded comparable results, with an average change of + 0.8 mmol/L (R2 = 0.9016).

CONCLUSION

Plasma is a valid specimen matrix for use on blood gas analyzers for sodium determination, eliminating the need for re-collection of whole-blood specimens from patients with pseudohyponatremia.

Differential diagnosis of hyponatremia and hypernatremia

Dysnatremias are among the most common mineral imbalances encountered in clinical practice. Both hyponatremia and hypernatremia are associated with increased morbiditidy and mortality and represent negative prognostic factors regardless of their cause. Serum osmolality, extracellular fluid volume and sodium urine concentration are important parameters for evaluation the cause and differential diagnosis. The rate of onset of ionic disorder and severity of clinical symptoms are essential. While acute disorders with symptoms are treated immediately, in chronic disorders, thorough diagnostic evaluation and a careful approach to their correction are necessary. Especially with rapid substitution of chronic hyponatremia, there is a risk of osmotic demyelination syndrome. Therefore, a slow correction of the serum sodium level with frequent mineralogram checks is required.

Clinical factors within a week of birth influencing sodium level difference between an arterial blood gas analyzer and an autoanalyzer in VLBWIs: A retrospective study

Neonatologists often experience sodium ion level difference between an arterial blood gas analyzer (direct method) and an autoanalyzer (indirect method) in critically ill neonates. We hypothesize that clinical factors besides albumin and protein in the blood that cause laboratory errors might be associated with sodium ion level difference between the 2 methods in very-low-birth-weight infants during early life after birth. Among very-low-birth-weight infants who were admitted to Jeonbuk National Hospital Neonatal Intensive Care Units from October 2013 to December 2016, 106 neonates were included in this study. Arterial blood sample was collected within an hour after birth. Blood gas analyzer and biochemistry autoanalyzer were performed simultaneously. Seventy-six (71.7%) were found to have sodium ion difference exceeding 4 mmol/L between 2 methods. The mean difference of sodium ion level was 5.9 ± 6.1 mmol/L, exceeding 4 mmol/L. Based on sodium ion level difference, patients were divided into >4 and ≤4 mmol/L groups. The sodium level difference >4 mmol/L group showed significantly (P < .05) higher sodium level by biochemistry autoanalyzer, lower albumin, lower protein, and higher maximum percent of physiological weight than the sodium level difference ≤4 mmol/L group. After adjusting for factors showing significant difference between the 2 groups, protein at birth (odds ratio: 0.835, 95% confidence interval: 0.760-0.918, P < .001) and percent of maximum weight loss (odds ratio: 1.137, 95% confidence interval: 1.021-1.265, P = .019) were factor showing significant associations with sodium level difference >4 mmol/L between 2 methods. Thus, difference in sodium level between blood gas analyzer and biochemistry autoanalyzer in early stages of life could reflect maximum physiology weight loss. Based on this study, if the study to predict the body's composition of extracellular and intracellular fluid is proceeded, it will help neonatologist make clinical decisions at early life of preterm infants.

Interference in Ion-Selective Electrodes Due to Proteins and Lipids

BACKGROUND

Ion-selective electrodes (ISE) have become the mainstay of electrolyte measurements in the clinical laboratory. In most automated analyzers used in large diagnostic laboratories, indirect ISE (iISE) -based electrolyte estimation is done; whereas direct ISE (dISE) -based equipment are mostly used in blood gas analyzers and in the point-of-care (PoC) setting.

CONTENT

Both the techniques, iISE as well as dISE, are scientifically robust; however, the results are often not interchangeable. Discrepancy happens between the two commonly due to interferences that affect the two measuring principles differently. Over the last decade, several studies have reported discrepancies between dISE and iISE arising due to abnormal protein and lipid contents in the sample.

SUMMARY

The present review endeavors to consolidate the knowledge accumulated in relation to interferences due to abnormal protein and lipid contents in sample with the principal focus resting on probable solutions thereof.

Preventing pseudohyponatremia: Intralipid®-based lipemia cutoffs for sodium are inappropriate

BACKGROUND

Pseudohyponatremia describes an artifactual decrease in plasma sodium result in samples with high proteins and/or lipids when measured by an indirect ion-selective electrode (ISE) method. We suspected that Intralipid®-based lipemia cutoffs are inappropriate for detecting interfering lipids in human samples and a major contributing factor to the existence of pseudohyponatremia.

METHODS

We evaluated 2 approaches to derive a lipemia cutoff for sodium, one in which patient plasma samples were pooled and spiked to simulate hyperlipidemia using Intralipid® (commonly used approach by in-vitro diagnostics manufacturers), and another in which endogenous hyperlipidemic samples (n = 31) were measured by methods not affected by hyperlipidemia (i.e., direct ISE and post-ultracentrifugation indirect ISE). Triglycerides, lipemic index (L-index) and indirect ISE sodium concentrations of samples were measured on Roche Cobas® 8000 and direct ISE on Radiometer® ABL835 Flex analyzers. Endogenous hyperlipidemic samples were also ultracentrifuged on Beckman Coulter® Airfuge to clear excess lipids and re-analyzed for sodium by indirect ISE.

RESULTS

We discovered that Intralipid® is not an accurate emulation of the lipemic interference seen in pseudohyponatremia because it showed no effect up to the maximum level of lipemia tested (L-index = 2000). By contrast, endogenous hyperlipidemic samples demonstrated significant deviations in sodium concentration (≥4 mmol/l) when L-index approached or exceeded 700, and a strong positive correlation between L-index and the difference between the indirect and direct methods (i.e., extent of pseudohyponatremia).

CONCLUSIONS

Clinical laboratories should lower their tolerance for lipemia from the currently recommended L-index cutoff of 2000 on Roche Cobas 8000®. We recommend reflexing to direct ISE when L-index exceeds 700. Manufacturers and laboratories with other indirect ISE methods should evaluate the effect of lipid interference on their method using hyperlipidemic human samples not Intralipid®.