Renal
Fractional excretion of sodium (FENa)
FENa is the proportion of filtered sodium the kidney excretes, and it separates pre-renal acute kidney injury from intrinsic tubular injury. Under 1% suggests the kidney is holding on to sodium; over 2% suggests it cannot.
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The formula
- FENa = (urine Na × plasma creatinine) ÷ (plasma Na × urine creatinine) × 100
Worked example
Urine Na 10, plasma Na 140, urine creatinine 100, plasma creatinine 2.0 → (10 × 2) ÷ (140 × 100) × 100 = 0.14%
When to use it
- Distinguishing pre-renal azotemia from acute tubular necrosis in oliguric acute kidney injury
And when not to
- A diuretic invalidates it. Use fractional excretion of urea instead in a diuretic-treated patient.
- It assumes oliguria and a steady state, neither of which holds in early or recovering injury.
- It cannot be interpreted in chronic kidney disease.
References
- Urinary diagnostic indices in acute renal failure — Miller TR et al., Annals of Internal Medicine, 1978
Last reviewed 07/09/2026.
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These calculators are free and always will be. We also build Nirogix, a hospital management system for Indian clinics and hospitals. The same calculations are planned inside the consultation record, pre-filled from the chart.