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.
Collect urine and plasma electrolytes simultaneously. Use the calculation below to calculate the FENa. (UNa
= urine sodium,
= plasma creatinine, PNa
= plasma sodium, UCr
= urine creatinine).
Renal artery stenosis
|Acute tubular necrosis
- Prostate enlargement
- Bladder stone
- Ureteral obstruction
- 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.
Espinel, C.H. (1976). The FeNa Test: Use in the Differential Diagnosis of Acute Renal Failure. JAMA, 236(6), 579-581.
Miller, T.R., Anderson, R.J., Linas, S.L., Henrich, W.L., Berns, A.S., Gabow, P.A., Schrier, R.W. (1978). Urinary Diagnostic Indices in Acute Renal Failure: A Prospective Study. Annals of Internal Medicine, 89(1), 47-50.
Pahwa, A.K., Sperati, C.J. (2016). Urinary Excretion Indices in AKI. Journal of Hospital Medicine, 11(1), 77-80. doi: 10.1002/jhm.2501