Fractional Excretion of Sodium (FENa)
All calculations should be independently verified prior to clinical use. These calculators are intended to supplement, not replace, clinical judgment.
Calculate the fractional excretion of sodium from urine and plasma sodium and creatinine concentrations. FENa helps distinguish prerenal from intrinsic renal causes of acute kidney injury.
The fractional excretion of sodium is the percentage of filtered sodium that is ultimately excreted in the urine:
Because the units of sodium (mEq/L) and creatinine (mg/dL) cancel within their respective pairs, the result is dimensionless and expressed as a percentage. All four values must be drawn from the same spot urine and concurrent serum sample.
Interpretation guide (adult and pediatric reference ranges):
| FENa | Interpretation | Common causes |
|---|---|---|
| < 1% | Prerenal or tubular Na+ avidity | Dehydration, congestive heart failure, hepatorenal syndrome, acute glomerulonephritis, partial urinary tract obstruction |
| 1 – 3% | Indeterminate | May be seen in early ATN, contrast nephropathy, myoglobinuria, or transitional states |
| > 3% | Intrinsic renal injury | Acute tubular necrosis (ATN), severe bilateral urinary tract obstruction |
Important limitations: FENa is unreliable in patients who have recently received diuretics, as diuretics increase urinary sodium excretion independently of renal perfusion. In this setting, the fractional excretion of urea (FEUrea) — which is less affected by diuretics — may be more informative. FENa may also be low (< 1%) in contrast-induced nephropathy, myoglobinuric AKI, and early obstruction despite tubular injury.
In neonates and young infants, FENa is normally higher than in older children and adults due to immature tubular sodium reabsorption. Values up to 2–5% may be normal in premature neonates.
References
- Stein JH, ed. Internal Medicine. 4th ed. Mosby–Year Book; 1994.
- Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579–581.
- 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;62(6):2223–2229.