Renal & electrolytes
FeNa calculator
Four numbers from paired samples, one percentage: is the kidney clinging to sodium, or wasting it?
How to read the result
FeNa (%) falls into these published bands.
An oliguric patient: urine sodium 20 mmol/L, plasma sodium 140 mmol/L, plasma creatinine 177 µmol/L (2.0 mg/dL), urine creatinine 8 mmol/L.
FeNa 0.32% — prerenal pattern
Avid sodium retention: hypovolaemia or a low effective circulating volume — a perfusion problem before it is a tubule problem, provided no diuretics are on board.
Who this is for
For clinicians working up acute kidney injury with paired urine and plasma sodium and creatinine. Below 1% the kidney is retaining sodium avidly — a prerenal picture; above 2% it is wasting sodium — intrinsic tubular injury; 1–2% is an overlap zone the clinical picture must resolve. Plasma creatinine goes in as µmol/L, matching South African laboratory reports.
How it works
What it calculates
The fraction of filtered sodium that reaches the urine, from paired urine and plasma sodium and creatinine — the classic discriminator between prerenal AKI and acute tubular necrosis.
Interpreting the result
Below 1%: avid sodium retention — a prerenal picture. Above 2%: tubular sodium wasting — intrinsic injury. Between the two is an overlap zone that the clinical picture must resolve.
Limitations
Diuretics invalidate it (use fractional excretion of urea instead), and contrast nephropathy, early sepsis, and glomerulonephritis can all sit below 1% despite intrinsic disease. References: Espinel 1976; Carvounis 2002.
Built on published evidence
- Espinel, JAMA 1976; Carvounis et al., Kidney Int 2002
Codification last reviewed 2026-08-28, and checked against published reference values in automated tests on every release — see the full evidence ledger.
Questions clinicians ask
Can I use FeNa if the patient is on diuretics?
No — diuretics force sodium into the urine and invalidate the result. Use the fractional excretion of urea instead in that setting (Carvounis et al., Kidney Int 2002).
Is a FeNa below 1% always prerenal?
No — contrast nephropathy, early sepsis, and glomerulonephritis can all sit below 1% despite intrinsic disease, so a low FeNa supports but never replaces the clinical assessment.
Which samples does the calculation need?
Sodium and creatinine measured on urine and plasma collected at the same time. The formula is (urine Na × plasma creatinine) / (plasma Na × urine creatinine) × 100 — the classic Espinel (JAMA 1976) discriminator, with the units reconciled for you.
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