FENa Calculator (Fractional Excretion of Sodium)
Fractional excretion of sodium from urine and serum sodium and creatinine (µmol/L, mmol/L or mg/dL) — prerenal (< 1%), indeterminate or intrinsic AKI (> 2%) — with the optional FEUrea for patients on diuretics, the renal failure index, urine/plasma creatinine and BUN/creatinine ratios, and a worked solution.
How it works
FENa (%) = (urine Na × serum creatinine) ÷ (serum Na × urine creatinine) × 100 — < 1% prerenal, 1–2% indeterminate, > 2% intrinsic (ATN) · FEUrea (%) = (urine urea × serum creatinine) ÷ (serum urea × urine creatinine) × 100 — ≤ 35% prerenal, > 50% intrinsic · renal failure index = urine Na ÷ (urine creatinine ÷ serum creatinine)
When the kidney is underperfused but structurally intact, its tubules avidly reabsorb sodium to defend blood volume, so very little of the filtered sodium reaches the urine — a fractional excretion below 1%. In acute tubular necrosis the damaged tubules leak sodium, pushing the fraction above 2%. Expressing sodium excretion as a fraction of the filtered load (via the creatinine ratio) removes the effect of urine concentration that confounds a raw urine sodium. Each pair of measurements only has to share a unit, but the calculator lets you enter urine creatinine in µmol/L, mmol/L or mg/dL and serum creatinine in µmol/L or mg/dL and converts them for you. Loop and thiazide diuretics force sodium out regardless of volume status, so a FENa taken within a day of a diuretic is falsely high; urea is reabsorbed mainly in the proximal tubule and is far less affected, which is why the fractional excretion of urea (FEUrea) is the usual alternative — switch it on and enter urine and serum urea (or BUN). The results also give the urine sodium alone, the urine-to-plasma creatinine ratio, the renal failure index and, with urea, the BUN-to-creatinine ratio, set against their usual cut-offs in a table. The test has well-known blind spots: chronic kidney disease blunts sodium handling, the cut-offs were derived in oliguric patients, and several intrinsic conditions — contrast nephropathy, rhabdomyolysis, early glomerulonephritis, hepatorenal syndrome — present with a low FENa.
Worked example
Urine sodium 15, serum sodium 138 mmol/L, urine creatinine 8,000 µmol/L, serum creatinine 180 µmol/L, no diuretics: FENa = (15 × 180) ÷ (138 × 8,000) × 100 = 2,700 ÷ 1,104,000 × 100 = 0.24% — a prerenal pattern, backed by a urine:plasma creatinine ratio of 44 and a renal failure index of 0.34. If the same patient had received furosemide, FEUrea with urine urea 250 and serum urea 20 mmol/L is (250 × 180) ÷ (20 × 8,000) × 100 = 28.1% — still prerenal (≤ 35%).
Sources: Espinel CH (1976) — The FENa test: use in the differential diagnosis of acute renal failure, JAMA 236:579–581 · Carvounis CP, Nisar S, Guro-Razuman S (2002) — Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure, Kidney Int 62:2223–2229 · MDCalc — Fractional Excretion of Sodium (FENa)
Frequently asked questions
What if my lab reports creatinine in mg/dL?
Pick mg/dL from the unit menu next to each creatinine field. Strictly the units cancel as long as urine and serum creatinine share a unit, but many labs report urine creatinine in mmol/L or mg/dL and serum creatinine in µmol/L — mixing them without converting gives an answer off by a factor of 88.4 or 1,000.
How is FEUrea different?
Urea reabsorption happens mostly in the proximal tubule and is largely unaffected by loop and thiazide diuretics, so FEUrea at or below 35% suggests prerenal AKI even after furosemide, while values above 50% favour ATN. Without diuretics, FENa is usually at least as accurate.
What FENa is normal?
In a healthy person on a normal diet, FENa is usually around 1% or below. It is only interpreted as a diagnostic test in acute kidney injury, ideally oliguric, where < 1% points to a prerenal cause and > 2% to tubular injury.
Why can ATN show a low FENa?
Non-oliguric ATN, rhabdomyolysis, contrast nephropathy and sepsis can all keep FENa below 1%, and acute glomerulonephritis or hepatorenal syndrome characteristically do. A low FENa means the tubules are still conserving sodium, not that the kidney is undamaged.
What is the renal failure index?
It is urine sodium divided by the urine-to-plasma creatinine ratio — FENa without the serum sodium term. Values below 1 favour a prerenal cause and above 1 intrinsic injury; it adds little beyond FENa but some references still report it.
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Clinical decision tool for education and for use by trained clinicians alongside judgement — not a diagnosis and not a substitute for medical assessment. Scores validated in specific populations may not apply to every patient.