The current evidence suggests that the FEUrea is most reliable in diagnosing prerenal azotemia in patients who have used diuretics when the FENa is high but the FEUrea is low. The FEUrea will be less than 35% in prerenal azotemia and greater than 50% in ATN. Because urea is absorbed and excreted in the proximal tubule, the value will theoretically not be altered by the use diuretics. 3 FEUrea is used primarily for diagnostic evaluation in patients who have an AKI with recent use of diuretics. 3 The FENa is best utilized when urine sodium and creatinine are collected at the same time as the serum values, because serum creatinine levels tend to fluctuate with time and are not often accurate markers of GFR. 2 Subsequent studies have shown that when patients are oliguric, the FENa is more accurate. When is the FENa reliable? FENa measurements were first validated and studied in patients with a marked reduction in glomerular filtration rate (GFR) and oliguria. blood loss), or more commonly, administration of diuretics or intravenous fluids, can also alter the interpretation of the FENa. FENa values above 3% can occur in volume contraction in patients with chronic kidney disease (CKD) or in elderly patients as their sodium reabsorption is impaired. Moreover, use of diuretics can falsely elevate the FENa due to inhibition of sodium reabsorption. 1,2 Approximately 10% of patients with nonoliguric ATN have a FENa less than 1.0%. Importantly, a FENa value of less than 1% occurs in a number of conditions other than prerenal azotemia due to dehydration, including hypervolemic prerenal states such as cirrhosis or heart failure AKI due to radiocontrast or heme pigments acute glomerulonephritis transition from prerenal to postischemic ATN or sepsis, and in acute interstitial nephritis (AIN). Values greater than 3% may be consistent with acute tubular necrosis (ATN) due to inappropriate sodium excretion in the setting of tubular damage. A FENa of less than 1% in oliguric patients may indicate prerenal azotemia, as an increased reabsorption of sodium is the appropriate response of functioning nephrons to decreased renal perfusion. Although these markers prove to be beneficial in the work-up of AKI, both the FENa and FEUrea have several limitations.įENa measures the ratio of sodium excreted in the urine compared to how much is filtered through the kidney. Acute kidney injuryīoth the fractional excretion of sodium (FENa) and the fractional excretion of urea (FEUrea) have long been used as part of the standard work-up for determining if acute kidney injury (AKI) is due to prerenal causes. We will discuss the diagnostic utility of each of the urine electrolytes in a variety of clinical scenarios. There is a lack of clear diagnostic guidelines, so ordering all the urine electrolytes in a “blanket” strategy is a common practice. To order and interpret diagnostic tests appropriately, a hospitalist needs to have a thorough understanding of the diagnostic utility of laboratory tests. Hospitalists have been on the forefront of efforts to tailor testing and resource utilization to eliminate wasteful practices in health care.
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