Interpreting the fractional excretion of sodium

Prevalence of exercise-associated hyponatremia in male ultraendurance athletes

OBJECTIVE

The prevalence of exercise-associated hyponatremia (EAH) has mainly been investigated in marathoners and Ironman triathletes. The aim of this study was to investigate the prevalence of EAH in male ultraendurance athletes in other disciplines, such as ultraswimming, ultracycling, and ultramarathon running.

DESIGN

Observational field study.

SETTING

"Marathon Swim" in Lake Zurich, the "Swiss Cycling Marathon," the "Swiss Bike Masters," the "100-km Lauf Biel," and the "Swiss Jura Marathon."

PARTICIPANTS

Fifteen ultraswimmers, 28 ultra-road cyclists, 37 ultra-mountain bikers, 95 ultramarathoners, and 25 mountain ultramarathoners.

MAIN OUTCOME MEASURES

Changes in body mass, plasma sodium, urinary specific gravity, and hematocrit were measured. The athletes recorded their intake of fluids.

RESULTS

Two swimmers (13%), 3 road cyclists (10.7%), no mountain bikers (0%), 5 ultramarathoners (5%), and 2 mountain ultramarathoners (8%) developed EAH. In the mountain bikers (r = -0.41) and the 100-km ultramarathoners (r = -0.52), fluid intake was significantly and negatively related to race time. In the mountain ultramarathoners, fluid consumption increased during the race.

CONCLUSIONS

The prevalence of EAH was no higher in ultraendurance athletes compared with existing reports on marathoners and Ironman triathletes. Of the 200 investigated ultraendurance athletes, 12 finishers (6%) developed EAH.

Urinary biomarkers in the clinical prognosis and early detection of acute kidney injury

BACKGROUND AND OBJECTIVES

Several novel urinary biomarkers have shown promise in the early detection and diagnostic evaluation of acute kidney injury (AKI). Clinicians have limited tools to determine which patients will progress to more severe forms of AKI at the time of serum creatinine increase. The diagnostic and prognostic utility of novel and traditional AKI biomarkers was evaluated during a prospective study of 123 adults undergoing cardiac surgery.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Urinary neutrophil gelatinase-associated lipocalin (NGAL), cystatin C (CyC), kidney injury molecule-1 (KIM-1), hepatocyte growth factor (HGF), π-glutathione-S-transferase (π-GST), α-GST, and fractional excretions of sodium and urea were all measured at preoperative baseline, postoperatively, and at the time of the initial clinical diagnosis of AKI. Receiver operator characteristic curves were generated and the areas under the curve (AUCs) were compared.

RESULTS

Forty-six (37.4%) subjects developed AKI Network stage 1 AKI; 9 (7.3%) of whom progressed to stage 3. Preoperative KIM-1 and α-GST were able to predict the future development of stage 1 and stage 3 AKI. Urine CyC at intensive care unit (ICU) arrival best detected early stage 1 AKI (AUC = 0.70, P < 0.001); the 6-hour ICU NGAL (AUC = 0.88; P < 0.001) best detected early stage 3 AKI. π-GST best predicted the progression to stage 3 AKI at the time of creatinine increase (AUC = 0.86; P = 0.002).

CONCLUSION

Urinary biomarkers may improve the ability to detect early AKI and determine the clinical prognosis of AKI at the time of diagnosis.

Comparison between fractional excretions of urea and sodium in children with acute kidney injury

Fractional excretion of sodium (FENa) has been said to be the most sensitive index for differentiating prerenal failure (PRF) from intrinsic renal failure (IRF). However, there are several instances of high FENa (>2%) in cases of PRF and low FENa (<1%) in IRF patients. In contrast, the fractional excretion of urea nitrogen (FEUN) is primarily dependent on passive forces, and many confounding variables that affect FENa have little effect on FEUN, if any. To compare FEUN with FENa, pediatric patients with acute kidney injury (AKI) were prospectively evaluated by history, physical examination, and obtaining appropriate laboratory data during a 1-year interval. Diagnosis of PRF or IRF was made in each patient, and renal failure indices were compared between two groups using chi-square and t test, as appropriate. Probability value (P value) <0.05 was considered significant. Receiver operating characteristic (ROC) plots for FEUN and FENa were drawn to compare the discriminative power of each index. Forty-three patients were enrolled in the study. There were 27 patients in the PRF and 16 in the IRF group. FENa was 2+/-0.4 in PRF and 4.5+/- 1% in IRF patients (P<0.05), and low FENa (<1%) was only seen in 44.4% of PRF patients, which was not statistically different from those with IRF (P>0.05). FEUN was 23.6+/- 4.9% in PRF and 41.6+/-4.8% in IRF patients (P<0.05), and low FEUN (<35%) was seen in 77.8% of the PRF group (P<0.05). Cutoff values of 30% and 1.6% were reached for FEUN and FENa, respectively. In conclusion, FEUN <35% had higher sensitivity and specificity than FENa <1% for differentiation of PRF from IRF.

Clinical utility of anion gap in deciphering acid-base disorders

The anion gap (AG) measurement is a very useful tool in the evaluation of patients with acid-base disorders. Once metabolic acidosis is identified, AG will provide the important first step in the differential diagnosis of disorders that either increase the AG and those that leave the AG unchanged. Delta gap is the comparison between change (delta) in the AG and the change (delta) in bicarbonate (HCO(3)(-)). Delta ratio, defined as delta AG:delta HCO(3)(-) is usually 1:1 in patients with an uncomplicated high AG acidosis. A value below 1:1 suggests a combined high and normal AG acidosis. A value above 2:1 suggests a combined metabolic alkalosis and a high AG acidosis. Urine AG (unmeasured anions-unmeasured cations) is an indirect estimate of the urine NH(4)(+) excretion. It is typically negative in patients with normal AG metabolic acidosis secondary to diarrhoea. Utilisation of AG calculations helps clinicians in identifying and treating acid-base disorders.

Evaluation and initial management of acute kidney injury

The evaluation and initial management of patients with acute kidney injury (AKI) should include: (1) an assessment of the contributing causes of the kidney injury, (2) an assessment of the clinical course including comorbidities, (3) a careful assessment of volume status, and (4) the institution of appropriate therapeutic measures designed to reverse or prevent worsening of functional or structural kidney abnormalities. The initial assessment of patients with AKI classically includes the differentiation between prerenal, renal, and postrenal causes. The differentiation between so-called "prerenal" and "renal" causes is more difficult, especially because renal hypoperfusion may coexist with any stage of AKI. Using a modified Delphi approach, the multidisciplinary international working group, generated a set of testable research questions. Key questions included the following: Is there a difference in prognosis between volume-responsive and volume-unresponsive AKI? Are there biomarkers whose patterns (dynamic changes) predict the severity and recovery of AKI (maximal stage of AKI, need for RRT, renal recovery, mortality) and guide therapy? What is the best biomarker to assess prospectively whether AKI is volume responsive? What is the best biomarker to assess the optimal volume status in AKI patients? In evaluating the current literature and ongoing studies, it was thought that the answers to the questions posed herein would improve the understanding of AKI, and ultimately patient outcomes.

Diagnostic performance of fractional excretion of urea and fractional excretion of sodium in the evaluations of patients with acute kidney injury with or without diuretic treatment

BACKGROUND

The accuracy of fractional excretion of sodium (FENa) for the diagnosis of transient acute kidney injury (AKI) caused by decreased kidney perfusion is reported to be low in patients administered diuretics.

STUDY DESIGN

This is a prospective study of diagnostic accuracy comparing the performance of fractional excretion of urea (FEur) with that of FENa to distinguish between transient and persistent AKI.

SETTING & PARTICIPANTS

99 patients hospitalized at a tertiary-care center who developed AKI (>or=30% increase in serum creatinine level from baseline within 1 week).

INDEX TEST

FEur and FENa were calculated for each patient.

REFERENCE TEST & MEASUREMENTS

Patients were classified as having transient or persistent AKI according to the clinical context and whether serum creatinine level returned to baseline within 7 days. Each group also was subdivided according to exposure to diuretics. FEur of 35% or less and FENa of 1% or less were used to define transient AKI. Sensitivity, specificity, and receiver operating characteristic curves were generated for each index test.

RESULTS

Sensitivity and specificity of FEur were 48% and 75% in patients not administered diuretics and 79% and 33% in patients administered diuretics. Sensitivity and specificity of FENa were 78% and 75% in patients not administered diuretics and 58% and 81% in those administered diuretics. Receiver operating characteristic curves did not identify a better diagnostic cutoff value for FEur or FENa.

LIMITATIONS

Small sample size, variable exposure to diuretics, and a high proportion of preexisting chronic kidney disease.

CONCLUSIONS

In patients without diuretic use, FENa is better able to distinguish transient from persistent AKI. In patients administered diuretics, this distinction cannot be made accurately by means of FENa. FEur cannot be used as an alternative tool because it lacks specificity.

Association between vitamin D receptor gene polymorphisms and fasting idiopathic hypercalciuria in recurrent stone-forming patients

OBJECTIVES

To investigate the association between fasting idiopathic hypercalciuria (IHc), defined as IHc in the fasting state associated with normal parathyroid function, and ApaI, BsmI, and FokI polymorphisms of the vitamin D receptor (VDR) gene in 159 hypercalciuric recurrent stone formers. IHc contributes to the formation of calcium kidney stones in more than one half of reported cases.

METHODS

We examined 62 patients with fasting IHc (24 women, mean age 42.8 +/- 11.1 years, body mass index 25.7 +/- 4.8 kg/m2), 97 patients with absorptive IHc (41 women, mean age 43.5 +/- 10.8 years, body mass index 26.1 +/- 4.4 kg/m2), and 124 healthy control subjects (52 women, mean age 41.9 +/- 10.4 years, body mass index 25.4 +/- 5.1 kg/m2) without a history of nephrolithiasis and without IHc. The bone mass density and VDR genotype and haplotype frequencies were determined in the studied populations.

RESULTS

A reduced bone mass density was observed in fasting IHc patients compared with absorptive IHc patients (P = 0.009) and control subjects (P = 0.006). The prevalence of ApaI and BsmI VDR genotypes and alleles in patients with fasting IHc was significantly different statistically (P <0.05) from that observed in patients with absorptive IHc and control subjects, and the ba haplotype was overrepresented in these patients. No statistically significant difference in the distribution of FokI VDR genotypes and alleles was found between the studied groups.

CONCLUSIONS

Our results suggest a genetic association between 3' VDR alleles, fasting IHc, and reduced bone mass density in patients with recurrent stone formation.

Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure

BACKGROUND

Fractional excretion of sodium (FENa) has been used in the diagnosis of acute renal failure (ARF) to distinguish between the two main causes of ARF, prerenal state and acute tubular necrosis (ATN). However, many patients with prerenal disorders receive diuretics, which decrease sodium reabsorption and thus increase FENa. In contrast, the fractional excretion of urea nitrogen (FEUN) is primarily dependent on passive forces and is therefore less influenced by diuretic therapy.

METHODS

To test the hypothesis that FEUN might be more useful in evaluating ARF, we prospectively compared FEUN with FENa during 102 episodes of ARF due to either prerenal azotemia or ATN.

RESULTS

Patients were divided into three groups: those with prerenal azotemia (N = 50), those with prerenal azotemia treated with diuretics (N = 27), and those with ATN (N = 25). FENa was low only in the patients with untreated plain prerenal azotemia while it was high in both the prerenal with diuretics and the ATN groups. FEUN was essentially identical in the two pre-renal groups (27.9 +/- 2.4% vs. 24.5 +/- 2.3%), and very different from the FEUN found in ATN (58.6 +/- 3.6%, P < 0.0001). While 92% of the patients with prerenal azotemia had a FENa <1%, only 48% of those patients with prerenal and diuretic therapy had such a low FENa. By contrast 89% of this latter group had a FEUN <35%.

CONCLUSIONS

Low FEUN (</=35%) was found to be a more sensitive and specific index than FENa in differentiating between ARF due to prerenal azotemia and that due to ATN, especially if diuretics have been administered.

Renal clearance of branched-chain L-amino and 2-oxo acids in maple syrup urine disease

In maple syrup urine disease (MSUD), branched-chain L-amino (BCAA) and 2-oxo acids (BCOA) accumulate in body fluids owing to an inherited deficiency of branched-chain 2-oxo acid dehydrogenase complex activity. In MSUD, little information is available on the significance of urinary disposal of branched-chain compounds. We examined the renal clearance of leucine, valine, isoleucine and alloisoleucine, and their corresponding 2-oxo acids 4-methyl-2-oxopentanoate (KIC), 3-methyl-2-oxobutanoate (KIV), (S)-(S-KMV), and (R)-3-methyl-2-oxopentanoate (R-KMV), using pairs of plasma and urine samples (n = 63) from 10 patients with classical MSUD. The fractional renal excretion of free BCAA was in the normal range (< 0.5%) and independent of the plasma concentrations. The excretion of bound (N-acylated) BCAA was normal and not significantly dependent on the BCAA plasma concentrations. The fractional renal excretion of BCOA was in the order KIC << KIV < R-KMV < or = S-KMV (range (%): KIC 0.1-25; KIV 0.14-21.3; S-KMV 0.26-24.6; R-KMV 0.1-35.9), significantly correlated with the KIC plasma concentrations, and generally higher than that of the related BCAA. The results show that the renal excretion of free BCAA as well as of the acylated derivatives is negligible. The renal excretion of BCOA, however, to some extent counteracts increases in BCAA concentrations and thus contributes to the lowering of total BCAA pools in MSUD.

Renal tolerance of digoxin-specific Fab fragments in the rat

Renal markers were investigated over 96 h in male Sprague-Dawley rats after i.v. administration of 10.7 mg kg(-1) of digoxin-specific Fab fragments (DSFab). The dose was calculated by an allometric equation as equivalent to a high clinical dose. DSFab both alone and pre-incubated with digoxin were tested. None of the markers (creatinine clearance, fractional excretion of sodium, urinary activities of gamma-glutamyl transferase and alkaline phosphatase) were significantly altered in treated rats compared to control rats. Thus no renal toxicity due to DSFab alone or bound to digoxin was observed when administered at 10.7 mg kg(-1) in rats.

Are the increasing clinical demands for osmolality measurements and their associated electrolytes appropriate?

An audit of urine and plasma osmolalities and their associated urea and electrolytes over a 4 week period found that there were 124 plasma and 96 urine osmolality requests from 67 patients. In 21 patients (31.3 per cent), the osmolality results were useful in reaching a more precise diagnosis. In a further 11 cases, urine osmolality rather than plasma would have been appropriate. Seventy-one per cent originated from the Intensive Therapy Unit and were largely requested reflexly by the hospital computer order communication system. Plasma osmolal gaps could be calculated on 80 occasions (65 per cent). The formula 1.89 Na + 1.38 K + 1.03 urea + 1.08 glucose + 7.45 proved to be more accurate than the formula [Na+K] x 2 + urea+glucose (in mmol/L) with the latter showing a positive bias when compared to measured values. The osmolal gap was > 10 mOsm/Kg using the more complex formula on 23 occasions in 16 patients but only twice using the simpler calculation. These 16 patients usually had organ failure and were very ill. Urine sodium and potassium were measured on 72 occasions in 27 of these patients but urine chloride was never requested. Urine sodium < 20 mmol/L was found in 7 patients all of whom had relative or absolute hypovolaemia. Urine sodium was measured in 73 per cent of patients investigated for SIADH in general wards. Data was available to calculate the urine osmolal gap on 52 occasions. The value was > 100 mmol/L in 10 cases and this may be used as an index of the renal ammonium response to acidosis. Much potential derived information from simple indices is unused. As a result of this study, there was an approximate halving of the subsequent request volume.

Triage of critically ill patients: an overview of interventions

The recognition of tissue hypoxia or cumulative oxygen debt is of fundamental importance for triage and resuscitation of critically ill patients during the ¿golden hour¿ in the emergency department (ED). The measurement of central venous blood oxygen saturation, plasma lactate concentration, cardiac output, systemic oxygen transport and use, and non-vital organ oxygenation and function can enhance the detection of systemic and regional hypoperfusion and tissue hypoxia. Systemic and organ-specific oxygenation indices may guide the choice of therapy to optimize resuscitation of the macrocirculation and microcirculation in critically ill ED patients.

Assessment of tubular reabsorption of sodium, glucose, phosphate and amino acids based on spot urine samples

Reference values for tubular transport of sodium, phosphate, glucose and amino acids are generally based on inulin or creatinine short-term clearances, which are difficult to obtain in children. Hence, quantitative assessment of tubular transport capacities is rarely performed. For a simplified procedure, reference values for fractional sodium excretion, phosphate reabsorption related to glomerular filtration rate, percent glucose and percent amino acid reabsorption were established in 62 children from spot urine and simultaneously obtained blood samples. Sodium excretion, and glucose and amino acid reabsorption were significantly lower in infants than children, whereas phosphate reabsorption decreased during the first year of life. Results using the proposed protocol and those obtained from timed urine specimens correlated well; the phenomenon of renal adaptation during childhood could equally well be demonstrated. Renal tubular dysfunction can be diagnosed without timed urine specimens.

Ifosfamide-induced subclinical nephrotoxicity and its potentiation by cisplatinum

Renal function was assessed in 72 children and adolescents 3.5 to 123 months after completion of chemotherapy employing ifosfamide (n = 39) or ifosfamide plus cisplatinum (n = 33). No patient had preexisting renal parenchymal disease. Whereas reduction in glomerular filtration rate was present in six of 69 patients (8.7%), impairment of tubular transport for phosphate, glucose, and amino acids was more frequent: 32.8% of the patients showed reduction in phosphate reabsorption, and glucose and amino acid reabsorption was lowered in 16.4% and 55.0%, respectively. Elevated sodium excretion was found only occasionally, and there was no evidence of renal tubular acidosis. Proximal tubular damage is related to ifosfamide chemotherapy, but correlation between ifosfamide dose and phosphate reabsorption was not linear. The most severe depletion of phosphate reabsorption was seen in patients treated with both ifosfamide and cisplatinum. On reexamination of phosphate reabsorption after a median interval of 8 months, the majority of patients with initially reduced values showed further deterioration of this function.

Tubular function and histological findings in ifosfamide-induced renal Fanconi syndrome--a report of two cases

Two patients developed renal Fanconi syndrome (RFS) after intensive long-term chemotherapy for metastatic Ewing sarcoma and disseminated neuroblastoma. Whereas RFS was diagnosed in patient 1 before he developed osteomalacia, patient 2 experienced severe rickets and growth retardation. Renal function studies revealed slight glomerular impairment and severe tubular defects leading to increased excretion of glucose, amino acids, inorganic phosphate and low molecular weight proteins, indicating proximal tubular damage. Patient 2 additionally showed distal tubular dysfunction with acidosis and diminished concentrating capacity. Renal biopsy in patient 1 revealed marked proximal tubular defects without interstitial lymphocytic infiltration. In both patients renal damage could most likely be ascribed to previous ifosfamide (IFOS) therapy. Our patients showed no improvement in renal function after cessation of IFOS treatment, indicating a poor prognosis of once established RFS after IFOS therapy. Measurement of tubular reabsorption capacities provides exact information on the extent of tubular toxicity induced by IFOS and may be used to monitor IFOS treated patients.

Evaluation of the natriuretic and beta-adrenoceptor-blocking effects of tienoxolol in normal volunteers

The beta-adrenoceptor blocking and diuretic properties of tienoxolol (150 mg and 300 mg) were investigated and compared to those of a placebo in a double-blind, cross-over trial in six healthy volunteers. Heart rate (HR), systolic and diastolic blood pressures, peak expiratory flow rate (PEFR) at rest and during vigorous exercise, plasma renin activity (PRA) and aldosterone levels at rest, and diuresis and urinary electrolyte excretion values were measured before and at intervals up to 24 h after oral administration of the drugs. In addition, the clearances of electrolytes, uric acid, and creatinine were calculated, as well as the fractional sodium excretion (Fe Na%) before and 4 h and 24 h after drug intake. Finally, tienoxolol plasma levels were measured. Tienoxolol significantly and dose-dependently reduced exercise-induced tachycardia. This effect started 1 h after drug administration, peaked between 4 h and 6 h (-12% and -17% from control values at 150 mg and 300 mg, respectively), and lasted at least 12 h. Resting HR was decreased at 300 mg (P less than 0.05), PRA was decreased at both doses (P less than 0.05), but PEFR was not drug-affected. 24-h cumulative sodium excretion was increased (+24% at 150 mg [NS], +38% at 300 mg [P less than 0.01]) as compared to placebo, and Fe Na% did not change, regardless of the dose administered. 24-h cumulative diuresis was moderately increased by tienoxolol (NS), whereas creatinine clearance rose after the 300-mg dose, suggesting that tienoxolol might increase glomerular filtration rate. Plasma aldosterone levels remained unchanged. Finally, the elimination half-life of tienoxolol was 7.5 h. Thus, in healthy volunteers, tienoxolol behaves as an early acting and relatively long-lasting selective beta 1-adrenoceptor blocking drug endowed with significant natriuretic properties.

Effects of 12-HETE on renal tubular epithelial cells

The interaction of 12-HETE with MDCK cells was investigated to determine whether this lipoxygenase product might influence tubular epithelial function. MDCK cells took up 12-HETE from both the luminal and interstitial surfaces. Much of the 12-HETE was incorporated into phospholipids but, unlike arachidonic acid, there was no specificity for the sn-2 position. 12-HETE was incorporated without chemical modification, suggesting that it might perturb tight packing in the lipid bilayer. 12-HETE uptake was reduced by the presence of arachidonic acid, but not by oleic acid. 12-HETE was removed from the cell lipids more rapidly than arachidonic acid and it was released into the medium in the form of both more and less polar metabolites. When MDCK cells were incubated with 12-HETE, their capacity to produce prostaglandin E2 was reduced. Since 12-HETE enters epithelial phospholipids and is converted to metabolic products and reduces prostaglandin production, it apparently has the capacity to modulate renal function if it is released in the proximity of the tubular epithelium.

The reversal of the hepatorenal syndrome in four pediatric patients following successful orthotopic liver transplantation

Four pediatric patients are presented in whom profound renal failure (hepatorenal syndrome) developed in association with severe end-stage liver disease. All four patients had successful orthotopic liver transplantation. Special emphasis is given to the preoperative and postoperative renal function in the patients, and the criteria used to establish the diagnosis of the hepatorenal syndrome are discussed. In the initial work on liver transplantation and reversal of the hepatorenal syndrome, two of the three patients recovered renal function but died in the perioperative period. The four patients presented in this report have not only had reversal of the hepatorenal syndrome after successful orthotopic liver transplantation but have also survived long term. The four patients have been followed up for periods ranging from 18 months to 4.5 years. Three of the four patients have maintained near normal renal function, whereas the fourth patient (who had a left nephrectomy for obstruction and sepsis) has had a significant decline in renal function.

Renal failure in children with hepatic failure undergoing liver transplantation

Over a 3 1/2 year period, 133 children with hepatic failure underwent orthotopic liver transplantation (OLT) at our center. Renal failure (creatinine clearance less than 20 ml/min/1.73 m2) was present in 19 (14.3%) of these children. In seven of the 19 children, renal failure was present before OLT, and in the other 12 after OLT. The causes of renal failure included hepatorenal syndrome in seven, postischemic acute tubular necrosis in five, severe prerenal azotemia in five, and cyclosporine nephrotoxicity in two. Eight other patients died of renal failure while awaiting emergency transplantation. Of the total of 31 deaths among 133 children who underwent OLT, nine occurred in the 19 patients with renal failure. Thus patients with OLT and renal failure had a significantly higher mortality than other patients with transplants (P less than 0.025). Dialysis was not associated with improved survival. The majority of deaths in patients with renal failure were related to severe hemorrhage, thromboembolic events, and systemic fungal infections. Our experience suggests that renal failure is common in children with hepatic failure and is associated with reduced patient survival after OLT.