FENa Calculator (Fractional Excretion of Sodium)
This FENa calculator works out the fractional excretion of sodium from four lab values: sodium and creatinine, each in urine and in plasma. Example: urine sodium 20 mmol/L, plasma sodium 140 mmol/L, urine creatinine 100 mg/dL and plasma creatinine 2 mg/dL give a FENa of 0.29% – below 1%, the range described for prerenal causes of acute kidney injury.
How it is calculated
The fractional excretion of sodium (FENa) is the share of the sodium filtered by the kidneys that ends up in the urine. It is used in acute kidney injury with low urine output to help distinguish a prerenal cause – too little blood reaching the kidneys – from damage to the kidney tubules. Healthy tubules that are short of blood flow hold sodium back, so the value is low; damaged tubules cannot.
Formula
FENa (%) = (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine) × 100. Put differently: the urine-to-plasma ratio of sodium divided by the urine-to-plasma ratio of creatinine. Because creatinine appears in both the numerator and the denominator, its unit cancels out – as long as urine and plasma use the same one. SI reports often give plasma creatinine in µmol/L and urine creatinine in mmol/L; the calculator multiplies the urine value by 1,000 in that case.
Worked example
Urine sodium 20 mmol/L, plasma sodium 140 mmol/L, urine creatinine 100 mg/dL, plasma creatinine 2 mg/dL: (20 × 2) ÷ (140 × 100) × 100 = 0.29%. With urine sodium 60 mmol/L, urine creatinine 40 mg/dL and plasma creatinine 4 mg/dL the result is (60 × 4) ÷ (140 × 40) × 100 = 4.29%. In SI units the first example reads 8.84 mmol/L in urine and 177 µmol/L in plasma and gives the same 0.29%.
How the value is read
| FENa | Described for | Source |
|---|---|---|
| below 1% | prerenal azotemia | Espinel 1976; MSD Manual |
| above 1% | tubular injury | MSD Manual |
| above 3% | acute tubular necrosis | Espinel 1976 |
The calculator shows values between 1% and 3% as a zone of their own, because the two sources draw the line differently. The value shown is rounded to two decimals, and that rounded value is the one classified.
Limits of the test
The cut-offs come from patients with acute kidney injury and little urine; they are not reliable when urine output is normal. A low value also occurs without volume depletion – in heart or liver failure, acute glomerulonephritis, contrast or pigment nephropathy and acute urinary obstruction. A value above 1% occurs despite volume depletion when diuretics have been given or chronic kidney disease is present. Healthy people usually have a FENa below 1% as well; the number says something only in the setting of an acute rise in creatinine.
FEUrea when diuretics were given
Diuretics increase sodium excretion and push the FENa up. The fractional excretion of urea is calculated in the same way with urea instead of sodium and is influenced less. In one study a value of 35% or less pointed to a prerenal cause even in patients on diuretics. Example: urine urea nitrogen 300 mg/dL and BUN 60 mg/dL with the creatinine values above give (300 × 2) ÷ (60 × 100) × 100 = 10%.
Frequently asked questions
How is FENa calculated?
FENa = (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine) × 100. With 20, 2, 140 and 100 the result is 0.29%. All four values should come from samples taken at the same time.
What does a FENa below 1% mean?
In acute kidney injury with low urine output, values below 1% were described for prerenal causes: the tubules still work and hold sodium back. The same low value is found in several other conditions and in healthy people, so it cannot be read on its own.
What does a FENa above 1% or 3% mean?
In the original study, patients with acute tubular necrosis had values above 3%; other references already treat values above 1% as a sign of tubular injury. Diuretics and chronic kidney disease raise the value as well.
Can I use FENa in a patient on diuretics?
With caution: diuretics increase sodium excretion, so the FENa can be above 1% even when the cause is prerenal. The fractional excretion of urea is less affected; enter urine and plasma urea to see it.
Do the units matter?
Sodium must be in mmol/L or mEq/L in both samples. Creatinine may be in any unit, provided urine and plasma use the same one – choose the SI option if your report shows mmol/L for urine and µmol/L for plasma.
Do I need a 24-hour urine collection?
No. The formula uses concentrations, so a spot urine sample with a blood sample from the same time is enough.
Sources and legal basis
- Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA 1976;236(6):579–581, doi:10.1001/jama.236.6.579 (prerenal azotemia: FENa below 1; acute tubular necrosis: above 3)
- Steiner RW. Interpreting the fractional excretion of sodium. Am J Med 1984;77(4):699–702, doi:10.1016/0002-9343(84)90368-1 (exceptions: diuretics, chronic kidney disease, contrast media, glomerulonephritis, heart and liver failure)
- 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, doi:10.1046/j.1523-1755.2002.00683.x (FEUrea at most 35% in prerenal causes, also under diuretics)
- Miller TR, Anderson RJ, Linas SL, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med 1978;89(1):47–50, doi:10.7326/0003-4819-89-1-47
- MSD Manual Professional Edition: Acute Kidney Injury (AKI) – table “Urinary Diagnostic Indices” (FENa = U/P sodium ÷ U/P creatinine; prerenal below 1 %, tubular damage above 1 %; not reliable without oliguria; as of Feb 2026)
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