What is the role of urine electrolyte testing in the workup of acute kidney injury (AKI)?

Updated: Dec 24, 2020
  • Author: Biruh T Workeneh, MD, PhD, FASN; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

Urine electrolyte findings also can serve as valuable indicators of functioning renal tubules. The fractional excretion of sodium (FENa) is the commonly used indicator. However, the interpretation of results from patients in nonoliguric states, those with glomerulonephritis, and those receiving or ingesting diuretics can lead to an erroneous diagnosis.

FENa can be a valuable test for helping to detect extreme renal avidity for sodium in conditions such as hepatorenal syndrome. The formula for calculating the FENa is as follows:

FENa = (UNa/PNa) / (UCr/PCr) × 100

Calculating the FENa is useful in AKI only in the presence of oliguria. In patients with prerenal azotemia, the FENa is usually less than 1%. In ATN, the FENa is greater than 1%. Exceptions to this rule are ATN caused by any of the following:

  • Radiocontrast nephropathy
  • Severe burns
  • Acute glomerulonephritis
  • Rhabdomyolysis

In patients with liver disease, FENa can be less than 1% in the presence of ATN. On the other hand, because administration of diuretics may cause the FENa to be greater than 1%, these findings cannot be used as the sole indicators in AKI.

Urea

In patients who are receiving diuretics, a fractional excretion of urea (FEUrea) can be obtained, since urea transport is not affected by diuretics. (FEUrea of less than 35% is suggestive of a prerenal state.) The formula for calculating the FEUrea is as follows:

FEUrea = (Uurea/Purea) / (UCr/PCr) X 100


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