What is FENa?
The fractional excretion of sodium (FENa) estimates the percentage of filtered sodium that the kidneys excrete in the urine rather than reabsorb. In acute kidney injury, this helps distinguish prerenal causes - where the tubules are intact and avidly reabsorbing sodium in response to reduced perfusion - from intrinsic causes like acute tubular necrosis, where damaged tubules can't reabsorb sodium normally.
How to calculate FENa
FENa (%) = (Urine Na x Serum Creatinine) / (Serum Na x Urine Creatinine) x 100
All sodium values in mEq/L, and creatinine values in matching units (mg/dL with mg/dL, or convert consistently) so the units cancel out correctly.
Interpretation
| FENa | Suggests |
|---|---|
| <1% | Prerenal azotemia |
| 1–2% | Indeterminate |
| >2% | Intrinsic renal injury (e.g. ATN) |
Limitations
FENa is not valid in patients who have recently received diuretics, since diuretics independently increase sodium excretion regardless of the underlying cause. In that situation, FEUrea (fractional excretion of urea) is used instead, as urea handling is less affected by diuretics. FENa is also less reliable in chronic kidney disease and certain other settings (e.g. contrast-associated nephropathy, early obstruction).
References
Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002.
See also: AKI Approach — classifying acute kidney injury by cause.