Renal Function

Fractional Excretion of Sodium (FENa)

Distinguish prerenal from intrinsic renal AKI using the fractional excretion of sodium.

When to use: Use FENa in the workup of acute kidney injury (AKI) to differentiate prerenal (volume-depleted, heart failure, sepsis) from intrinsic renal causes (ATN, glomerulonephritis). Collect a spot urine sample for sodium and creatinine simultaneously with a serum creatinine and sodium. FENa is unreliable in patients who have received diuretics — in that scenario, calculate fractional excretion of urea (FEUrea) instead, as urea excretion is less affected by diuretics.

Calculator

Urine Sodium(mEq/L)

Plasma (Serum) Creatinine(mg/dL)

Plasma (Serum) Sodium(mEq/L)

Urine Creatinine(mg/dL)Spot urine creatinine from same sample as urine sodium

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Formula

FENa (%) = (Urine Na × Plasma Cr) / (Plasma Na × Urine Cr) × 100 <1%: Prerenal/hepatorenal | 1–2%: Indeterminate | >2%: Intrinsic renal (ATN)

Key Points for NEET PG

FENa <1% = Prerenal AKI: kidney working correctly but underperfused (volume depletion, cardiogenic shock, hepatorenal syndrome).
FENa >2% = Intrinsic AKI: tubular damage (ATN) → cannot reabsorb sodium → FENa rises.
Exception: contrast nephropathy and myoglobinuria (rhabdomyolysis) can cause ATN with FENa <1% — due to intense renal vasoconstriction.
FEUrea <35% suggests prerenal when FENa is unreliable (post-diuretics); formula: (Urine Urea × Plasma Cr) / (Plasma Urea × Urine Cr) × 100.
RIFLE and KDIGO AKI criteria use serum creatinine rise (×1.5 baseline or +0.3 mg/dL in 48h) and urine output (<0.5 mL/kg/h for 6h).

References

Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure · JAMA (1976)

KDIGO Clinical Practice Guideline for Acute Kidney Injury · Kidney Int Suppl (2012)

For educational purposes only. Not for clinical decision-making without professional oversight.