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The Use of Selected Urine Chemistries in the Diagnosis of Kidney Disorders

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The Use of Selected Urine Chemistries in the Diagnosis of Kidney Disorders CJASN ePress. Published on January 9, 2019 as doi: 10.2215/CJN.10330818 The Use of Selected Urine Chemistries in the Diagnosis of Kidney Disorders Biff F. Palmer1 and Deborah Joy Clegg2 Abstract Urinary chemistries vary widely in both health and disease and are affected by diet, volume status, medications, and disease states. When properly examined, these tests provide important insight into the mechanism and therapy of 1Division of various clinical disorders that are first detected by abnormalities in plasma chemistries. These tests cannot be Nephrology, interpreted in isolation, but instead require knowledge of key clinical information, such as medications, physical Department of examination, and plasma chemistries, to include kidney function. When used appropriately and with knowledge of Medicine, University of Texas Southwestern limitations, urine chemistries can provide important insight into the pathophysiology and treatment of a wide Medical Center, variety of disorders. Dallas, Texas; and Clin J Am Soc Nephrol 14: ccc–ccc, 2019. doi: https://doi.org/10.2215/CJN.10330818 2Department of Internal Medicine, University of California, Los Introduction values ,15 mEq/L. On the other hand, volume Angeles School of Urine chemistries can provide valuable insight into a expansion suppresses effector mechanisms and stimu- Medicine, Los wide range of clinical conditions. These tests are often lates release of atrial natriuretic peptide, leading to a Angeles, California underutilized because of the difficulty many physi- reduction in sodium reabsorption, causing urinary so- Correspondence: cians find in their interpretation. Whereas a basic dium concentration to be high. Thus, the urine sodium fi Dr. Biff F. Palmer, metabolic pro le obtained from a blood sample has concentrationisanindirectmeasureofvolumestatusand Department of Internal well defined normal values, there are no such values reflects the integrity of the kidney to regulate that status. Medicine, University for urine chemistries. Urinary excretion of electrolytes There are specific circumstances where measurement of Texas Southwestern vary widely as the kidney adjusts the rate of excretion of urine sodium no longer accurately reflects volume Medical Center, 5323 Harry Hines to match dietary intake and endogenous production. status. The urine sodium concentration is dependent on Boulevard, Dallas, The excretion of a dilute or concentrated urine or the amount of free water in the urine and is therefore TX 75390. Email: biff. administration of a drug can markedly alter the influenced by the rate of water reabsorption in the palmer@ concentration of urine electrolytes, potentially mis- kidneys. Under conditions of a water diuresis, the urine utsouthwestern.edu leading the clinician as to the absolute quantity in the sodium concentration may be reduced even though urine over a given amount of time. This review will be daily excretion is high, falsely suggesting the presence limited in scope and will concentrate on discussing the of a low volume state. Similarly, a concentrated urine clinical use and interpretation of urine chemistries to can increase urine sodium concentrationeventhoughthe include sodium, potassium, chloride, pH, creatinine, total amount of sodium is low, potentially masking the urea, and osmolality. Urine chemistries are best presence of volume contraction and falsely suggesting interpreted with knowledge of the clinical setting at euvolemia or volume expansion. The fractional excretion the time they are obtained. These tests can be di- of sodium (FENa)accountsfortheeffectofwater agnostic and offer mechanistic insight in the evalua- reabsorption in the kidney on urine sodium concentration: tion of AKI, volume status, disorders of plasma sodium and potassium, and acid-base disorders. FENa 5 ðUNa 3 PCreatinine=PNa 3 UCreatinineÞ 3 100% This formula expresses the percentage of filtered so- Urine Sodium dium excreted in the urine and provides a measure of Assessment of Effective Circulating Volume sodium handling that is independent of urinary con- Under normal conditions, sodium excretion by the centration. kidney equals dietary intake minus the small amount lost in sweat and feces, and typically ranges from 40 to 220 mEq/d. The ability to match dietary intake with Differentiation of Prerenal Azotemia versus Acute excretion enables extracellular fluid volume to be main- Tubular Necrosis tained within a narrow range. When effective circulatory Measurement of urine sodium concentration and volume is reduced, activation of effector mechanisms FENa are frequently utilized to determine whether such as sympathetic nerves and the renin-angiotensin- AKI is prerenal and correctable by restoring intravas- aldosterone system causes avid sodium retention in cular volume, or whether it is secondary to acute the kidneys, lowering urine sodium concentration to tubular necrosis, where administration of fluids might www.cjasn.org Vol 14 April, 2019 Copyright © 2019 by the American Society of Nephrology 1 2 Clinical Journal of the American Society of Nephrology be harmful by causing volume overload (1). A random The utility of the FEUrea to assess effective circulatory , urine sodium concentration of 15 mEq/L and a FENa of volume is lost when proximal reabsorption of salt and ,1% suggest the presence of a volume-responsive compo- water is impaired (22). This situation occurs after either nent to a reduced GFR. In patients with established acute administration of acetazolamide, an osmotic diuresis due tubular necrosis, loss of tubular function will prevent to administration of mannitol, glycosuria as in uncon- maximal sodium retention even when extracellular fluid trolled diabetes, or increased urea excretion resulting from volume depletion is present. These tests remain useful high protein intake or catabolism. Proximal tubule salt when evaluating a change in kidney function in patients with reabsorption may also be impaired in patients with cerebral previously stable CKD; however, the response to decreased salt wasting (23). kidney perfusion is delayed and the reduction in urine sodium The FEUrea has also been used to discriminate between will be less maximal as compared with normal kidneys (2) volume responsive azotemia and tubular necrosis in the (Supplemental Material, Case 1). absence of diuretic use. How this test compares to the FENa Therearesituationswheretheurinesodiumconcentra- for this purpose is difficult to determine because studies tion and FENa inadequately distinguish between azotemia included patients who were highly selected and excluded due to kidney parenchymal injury from volume responsive those with interstitial nephritis, GN, urinary obstruction, and azotemia (3). These conditions are characterized by vaso- exposure to radiocontrast material (19,20). Another potential constriction in the kidney causing a reduction in GFR and limitation of the FEUrea is its use in elderly patients and those filtered load of sodium in the setting where tubular with sepsis. Studies in experimental models show down- function remains relatively intact (4–12) (Figure 1). The regulation of urea transporters in the nephron with biochemical profile in the urine can change in a time endotoxemia and aging. This effect would tend to increase dependent manner from a prerenal picture to that of acute the FEUrea in septic and elderly patients even when volume tubular necrosis. Variability in timing of measurements depletion was the only cause of azotemia (24,25). likely explains disparities in the literature as to sensitivity Conditions in which volume contraction coexists with a and specificity of these tests in patients with AKI (13–15). The nonreabsorbable anion also cause urine sodium concentra- fi sensitivity and speci city of the FENa is greatest when tion and FENa to be increased. As discussed below, urinary applied to patients with oliguria and a reduced GFR (dis- chloride concentration is typically low in this setting. cussed further in Supplemental Material, Cases 1 and 2). In patients with severe congestive heart failure, advanced cirrhosis of the liver, and extensive burns, a reduction in the Urinary Chloride filtered load of sodium brought about by intense neurohu- Urinary excretion of chloride mirrors sodium excretion in moral activation can cause sodium retention in the kidneys to response to dietary intake. In a manner similar to sodium, the be of such a degree that urine sodium concentration and FENa urinary concentration of chloride and fractional excretion of remain low even when tubular necrosis is present, as chloride (FECL) can be used as indirect markers of effective manifested by granular casts and tubular cells in the urine circulatory volume. Despite these similarities, there are (16–19). Despite volume expansion, a low urinary sodium situations when the directional change in urinary concentration concentration is typically present early in acute GN when may differ, such that reliance solely on one or the other can lead tubular function is intact and the filtered load of sodium is to an erroneous assessment of effective volume status. These reduced because of the decrease in glomerular surface area differences are particularly evident when volume disturbances available for filtration (11). are accompanied by acid-base disorders (26). For this reason, Active diuretic use is another situation where the urine both urine sodium and chloride should be obtained when fl sodium concentration and FENa may not accurately re ect using urine electrolytes to assess volume or acid-base
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