Toward the optimal clinical use of the fraction excretion of solutes in oliguric azotemia

The role of the clinical laboratory in diagnosing hyponatremia disorders

In clinical medicine, hyponatremia is highly prevalent and frequently misdiagnosed, leading to substantial mismanagement and iatrogenic morbidity. Its differential diagnosis includes numerous diseases with diverse etiologies, making accurate assessment challenging. Despite extensive literature and guidelines on hyponatremia, most patients do not receive adequate evaluation due to the limitations of diagnostic algorithms, which rely on low-value clinical signs and are unable to identify concurrent conditions. In this review, we examine the range of laboratory tests available for hyponatremia assessment. Understanding renal mechanisms of solute and water exchange (e.g., fractional excretion) is essential for selecting appropriate tests and interpreting their diagnostic value. Additionally, detailed electrolyte and acid-base assessments remain critical for establishing a definitive diagnosis. We comprehensively discuss the selection of laboratory tests for specific differential diagnoses of hyponatremia. Importantly, in cases of acute hyponatremia, rapid correction should take precedence over a complete diagnostic workup. Ultimately, a thorough understanding of laboratory evaluation is crucial for accurately diagnosing hyponatremia. This paper critically reviews the available literature and explores relevant diseases in the context of associated laboratory parameters.

Urinary albumin-to-creatinine ratio and serum albumin are predictors of acute kidney injury in non-ventilated COVID-19 patients: a single-center prospective cohort study

PURPOSE

Acute kidney injury (AKI) is a frequent complication among COVID-19 patients in the intensive care unit, but it is less frequently investigated in general internal medicine wards. We aimed to examine the incidence, the predictors of AKI, and AKI-associated mortality in a prospective cohort of non-ventilated COVID-19 patients. We aimed to describe the natural history of AKI by describing trajectories of urinary markers of hemodynamic, glomerular, and tubular injury.

METHODS

141 COVID-19 patients were enrolled to the study. AKI was defined according to KDIGO guidelines. Urine and renal function parameters were followed twice a week. Multivariate logistic regression was used to determine the predictors of AKI and mortality. Trajectories of urinary markers were described by unadjusted linear mixed models.

RESULTS

19.7% patients developed AKI. According to multiple logistic regression, higher urinary albumin-to-creatinine ratio (OR 1.48, 95% CI 1.04-2.12/1 mg/mmol) and lower serum albumin (OR 0.86, 95% CI 0.77-0.94/1 g/L) were independent predictors of AKI. Mortality was 42.8% in the AKI and 8.8% in the group free from AKI (p < 0.0001). According to multiple logistic regression, older age, lower albumin, and AKI (OR 3.9, 95% CI 1.24-12.21) remained independent predictors of mortality. Urinary protein-to-creatinine trajectories were diverging with decreasing values in those without incident AKI.

CONCLUSION

We found high incidence of AKI and mortality among moderately severe, non-ventilated COVID-19 patients. Its development is predicted by higher albuminuria suggesting that the originally damaged renal structure may be more susceptible for virus-associated effects. No clear relationship was found with a prerenal mechanism, and the higher proteinuria during follow-up may point toward tubular damage.

Diagnostic Performance of Fractional Excretion of Sodium for the Differential Diagnosis of Acute Kidney Injury: A Systematic Review and Meta-Analysis

BACKGROUND AND OBJECTIVES

AKI is classified as prerenal, intrinsic, and postrenal. Prerenal AKI and intrinsic AKI represent the most common causes for AKI in hospitalized patients. This study aimed to examine the accuracy of the fractional excretion of sodium for distinguishing intrinsic from prerenal AKI.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We searched MEDLINE, Embase, Cochrane Central Register of Controlled Trials, the Cochrane Library, and Scopus for all available studies that met the criteria until December 31, 2021. We included studies that evaluated fractional excretion of sodium in differentiating AKI etiologies in adults, whereas studies that did not have sufficient data to extract a 2×2 table were excluded. We assessed the methodologic quality using the Quality Assessment of Diagnostic Accuracy Studies-2 tool and extracted the diagnostic accuracy data for all included studies. We conducted a meta-analysis using the bivariate random effects model. We performed subgroup analysis to investigate sources of heterogeneity and the effect of the relevant confounders on fractional excretion of sodium accuracy.

RESULTS

We included 19 studies with 1287 patients. In a subset of 15 studies (872 patients) that used a threshold of 1%, the pooled sensitivity and specificity for differentiating intrinsic from prerenal AKI were 90% (95% confidence interval, 81% to 95%) and 82% (95% confidence interval, 70% to 90%), respectively. In a subgroup of six studies (511 patients) that included CKD or patients on diuretics, the pooled sensitivity and specificity were 83% (95% confidence interval, 64% to 93%) and 66% (95% confidence interval, 51% to 78%), respectively. In five studies with 238 patients on diuretics, the pooled sensitivity and specificity were 80% (95% confidence interval, 69% to 87%) and 54% (95% confidence interval, 31% to 75%), respectively. In eight studies with 264 oliguric patients with no history of CKD or diuretic therapy, the pooled sensitivity and specificity were 95% (95% confidence interval, 82% to 99%) and 91% (95% confidence interval, 83% to 95%), respectively.

CONCLUSIONS

Fractional excretion of sodium has a limited role for AKI differentiation in patients with a history of CKD or those on diuretic therapy. It is most valuable when oliguria is present.

Urine Electrolytes in the Intensive Care Unit: From Pathophysiology to Clinical Practice

Assessment of urine concentrations of sodium, chloride, and potassium is a widely available, rapid, and low-cost diagnostic option for the management of critically ill patients. Urine electrolytes have long been suggested in the diagnostic workup of hypovolemia, kidney injury, and acid-base and electrolyte disturbances. However, due to the wide range of normal reference values and challenges in interpretation, their use is controversial. To clarify their potential role in managing critical patients, we reviewed existing evidence on the use of urine electrolytes for diagnostic and therapeutic evaluation and assessment in critical illness. This review will describe the normal physiology of water and electrolyte excretion, summarize the use of urine electrolytes in hypovolemia, acute kidney injury, acid-base, and electrolyte disorders, and suggest some practical flowcharts for the potential use of urine electrolytes in daily critical care practice.

Acute kidney injury 2016: diagnosis and diagnostic workup

Acute kidney injury (AKI) is common and is associated with serious short- and long-term complications. Early diagnosis and identification of the underlying aetiology are essential to guide management. In this review, we outline the current definition of AKI and the potential pitfalls, and summarise the existing and future tools to investigate AKI in critically ill patients.

Utility of fractional excretion of urea in the differential diagnosis of acute kidney injury in children

INTRODUCTION

The fractional excretion of sodium (FENa) has been used as an index for the differential diagnosis of acute tubular necrosis (ATN) and prerenal acute kidney injury (AKI). The reliability of this index, however, decreases with the use of the diuretic agent furosemide. The fractional excretion of urea nitrogen (FEUN) has been shown to be useful in such settings in adults. The objective of this study was to examine whether FEUN is also useful in these settings in children.

METHODS

We assessed 102 episodes of AKI in 74 children, classifying these into three groups based on history, physical examination, urine examination and subsequent clinical course: (1) prerenal AKI without furosemide (N = 37), (2) prerenal AKI with furosemide (N = 32) and (3) ATN (N = 33).

RESULTS

Of the 37 prerenal AKI episodes without furosemide, 35 showed low FENa of <1 %, with an overall average of 0.35 ± 0.11 %, whereas prerenal AKI with furosemide (1.63 ± 0.37 %) and ATN (8.76 ± 2.11 %) were associated with a higher FENa. FEUN in the clinical setting of prerenal AKI was lower than that in ATN (27.9 ± 2.1 vs. 51.6 ± 3.8 %, respectively) and, in contrast to FENa, not significantly different between the categories of prerenal AKI with and without furosemide (29.2 ± 3.1 vs. 25.1 ± 2.9, respectively). The sensitivity of FEUN <35 % was 75 % in prerenal AKI with furosemide, whereas that of FENa was 53 %.

CONCLUSIONS

FEUN is useful in detecting prerenal AKI in children administered furosemide.

Urine biochemistry assessment in critically ill patients: controversies and future perspectives

In the past, urine biochemistry was a major tool in acute kidney injury (AKI) management. Classic papers published some decades ago established the values of the urine indices which were thought to distinguish "pre-renal" (functional) AKI attributed to low renal perfusion and "renal" (structural) AKI attributed to acute tubular necrosis (ATN). However, there were a lot of drawbacks and limitations in these studies and some recent articles have questioned the utility of measuring urine electrolytes especially because they do not seem to adequately inform about renal perfusion nor AKI duration (transient vs. persistent). At the same time, the "pre-renal" paradigm has been consistently criticized because hypoperfusion followed by ischemia and ATN does not seem to explain most of the AKI developing in critically ill patients and distinct AKI durations do not seem to be clearly related to different pathophysiological mechanisms or histopathological findings. In this new context, other possible roles for urine biochemistry have emerged. Some studies have suggested standardized changes in the urine electrolyte composition preceding increases in serum creatinine independently of AKI subsequent duration, which might actually be due to intra-renal microcirculatory changes and activation of sodium-retaining mechanisms even in the absence of impaired global renal blood flow. In the present review, the points of controversy regarding urine biochemistry assessment were evaluated as well as future perspectives for its role in AKI monitoring. An alternative approach for the interpretation of measured urine electrolytes is proposed which needs further larger studies to be validated and incorporated in daily ICU practice.

Physicochemical analysis of blood and urine in the course of acute kidney injury in critically ill patients: a prospective, observational study

Background

Sequential physicochemical alterations in blood and urine in the course of acute kidney injury (AKI) development have not been previously described. We aimed to describe these alterations in parallel to traditional renal and acid–base parameters.

Methods

One hundred and sixty eight consecutive critically ill patients with no previous kidney disease, who had an indwelling urinary catheter at ICU admission and who remained with the catheter for at least two days without dialysis were included. A sample of blood and spot urine were collected simultaneously, once daily, until catheter removal or dialysis requirement. Traditional acid–base and renal parameters were sequentially evaluated in parallel to blood and urinary physicochemical parameters. Patients were classified during this period as having or not AKI and, for patients with AKI, duration (transient or persistent) and severity (creatinine-based AKIN stage) were evaluated.

Results

One hundred and thirteen patients (67.3%) had AKI: 92 at ICU admission and 21 during the observation period. AKI development was characterized in blood by increased values of phosphate and unmeasured anions (SIG), decreased albumin, and in urine by decreased values of sodium (NaU), chloride (ClU) as well as high urinary strong ion difference (SIDu). These alterations began to occur before AKI diagnosis, and they reverted in transient AKI but remained in persistent AKI. NaU, ClU and albumin decreased, and phosphate, SIG and SIDu increased with AKI severity progression. NaU and ClU values increased again when AKIN stage 3 was reached.

Conclusions

Simultaneous physicochemical analysis of blood and urine revealed standardized alterations that characterize AKI development in critically ill patients. These alterations paralleled AKI duration and severity. Future studies should consider including sequential evaluation of urine biochemistry as part of the armamentarium for AKI diagnosis and management.

Fractional excretion of uric acid as a predictor for saline responsiveness in long-term kidney transplant patients

BACKGROUND/AIMS

Subclinical hypovolemia may contribute to allograft dysfunction in long-term kidney transplant (KT) patients. In order to predict responsiveness to saline hydration, indices for tubular transport were investigated.

METHODS

Fifty-four clinically euvolemic long-term KT patients with recently aggravated azotemia were given intravenous hydration as follows: 0.9% saline 5 ml/kg over 1 h, followed by 0.9% saline 1 ml/kg/h over 12 h and 1 liter of 0.45% saline over the next 24 h. Serum and urine data were collected and analyzed to assess responses.

RESULTS

In all patients, saline hydration relieved azotemia, as shown by blood urea nitrogen (46.9 ± 17.2 vs. 39.3 ± 15.4 mg/dl; p < 0.01) and serum creatinine levels (2.9 ± 1.1 vs. 2.5 ± 1.1 mg/dl; p < 0.01) on day 0 versus day 2. In 38 patients, serum creatinine did not increase in the following month (70% responders). Compared with the nonresponders, the responders had a higher urine-to-plasma creatinine ratio and lower fractional excretion of sodium, uric acid and urea at admission. Multivariate logistic regression analysis revealed that responsiveness to saline hydration was independently associated with lower fractional excretion of uric acid.

CONCLUSION

Subclinical hypovolemia should be considered in long-term KT patients with azotemia of unexplainable causes. Fractional excretion of uric acid may predict responsiveness to saline hydration.