Fractional excretion of sodium (FENa) utilizes urine chemistry to distinguish between different causes of acute kidney injury (AKI). Is it a prerenal cause (decreased perfusion), acute tubular necrosis (ATN), or a post-renal cause (obstruction)? Since AKI is often multifactorial, we must remember that prerenal injury may progress to or coexist with intrinsic renal disease. If taken in clinical context, FENa is a useful additional data point in patients whose volume status is difficult to assess.

Procedure

Collect urine and plasma electrolytes simultaneously. Use the calculation below to calculate the FENa. (U_Na = urine sodium_, P_Cr = plasma creatinine, P_Na = plasma sodium, U_Cr = urine creatinine). There are numerous online clinical calculators which make it easy to input patient data to derive the result.
 

Interpretation

	Pre-renal	Intrinsic	Post-renal
FENa	<1%	>1%	>4%
Causes	Hypovolemia Heart failure Renal artery stenosis Sepsis	Acute tubular necrosis Interstitial nephritis	Obstruction - Prostate enlargement - Bladder stone - Ureteral obstruction

 

Clinical Considerations

• Do not use in patients receiving diuretics or patients with chronic kidney disease.
• FENa percentage should not be considered in isolation. Always consider the patient’s history, physical exam, clinical context, and current medications.
• Non-volume depleted states which may cause low or borderline FENa include: acute glomerulonephritis, contrast-induced nephropathy, cardiorenal syndrome, and hepatorenal syndrome.

References:

Espinel C. H. (1976). The FENa test. Use in the differential diagnosis of acute renal failure. JAMA, 236(6), 579–581. https://doi.org/10.1001/jama.236.6.579

Fractional Excretion of Sodium (FENa). MDCalc. https://www.mdcalc.com/calc/60/fractional-excretion-sodium-fena

Okusa, M. and Chopra T. (2022, November 15). Fractional excretion of sodium, urea, and other molecules in acute kidney injury. UpToDate. https://www.uptodate.com/contents/fractional-excretion-of-sodium-urea-and-other-molecules-in-acute-kidney-injury

Pahwa, A. K., & Sperati, C. J. (2016). Urinary fractional excretion indices in the evaluation of acute kidney injury. Journal of hospital medicine, 11(1), 77–80. https://doi.org/10.1002/jhm.2501