FENa (fractional excretion of sodium)
Separate prerenal azotaemia from acute tubular necrosis in oliguric AKI
Sources last checked: 2026-10-13
FENa (fractional excretion of sodium)
Separate prerenal azotaemia from acute tubular necrosis in oliguric AKI
Validated in oliguric acute renal failure. Diuretics raise FENa (use FEUrea instead); contrast nephropathy, pigment nephropathy, sepsis and early obstruction can give a low FENa despite tubular injury, and CKD raises baseline FENa.
Interpretation
| < 1 % — prerenal pattern | In Espinel’s oliguric patients, prerenal azotaemia had FENa below 1 %. |
|---|---|
| 1–3 % — indeterminate | Between the two groups in the derivation study; interpret with the clinical picture and urine sediment. |
| > 3 % — tubular pattern | In Espinel’s patients, acute tubular necrosis had FENa above 3 %. |
What is the FENa (fractional excretion of sodium)?
The fractional excretion of sodium (FENa) is the percentage of filtered sodium that ends up in the urine. Espinel described it in 1976 in oliguric acute renal failure: patients with prerenal azotaemia had FENa below 1 %, those with acute tubular necrosis above 3 %. Miller and colleagues confirmed its value in a prospective series in 1978.
When to use it
- Oliguric acute kidney injury when prerenal and intrinsic causes need to be separated
- Spot urine and plasma samples taken at the same time, before diuretics
- Supporting the clinical assessment of volume status, not replacing it
When not to use it
- After loop or thiazide diuretics — FENa rises artificially; use FEUrea
- Chronic kidney disease, where baseline FENa is already high
How it is calculated
FENa (%) = (urine Na × plasma creatinine) / (plasma Na × urine creatinine) × 100
Clinical pearls and limitations
- Validated in oliguric acute renal failure. Diuretics raise FENa (use FEUrea instead); contrast nephropathy, pigment nephropathy, sepsis and early obstruction can give a low FENa despite tubular injury, and CKD raises baseline FENa.
- Contrast nephropathy, rhabdomyolysis, sepsis, hepatorenal syndrome and early obstruction can show FENa < 1 % despite intrinsic injury.
- Non-oliguric AKI was not the derivation population; interpret with more caution there.
- Units only need to match pairwise: both creatinines in µmol/L (or mg/dL), both sodiums in mmol/L.
Frequently asked questions
What is a normal FENa?
In healthy people on a normal salt intake FENa is usually below 1 %; it is interpreted mainly in oliguric AKI. There, < 1 % suggests a prerenal cause and > 3 % suggests tubular necrosis.
Can I use FENa after furosemide?
It becomes unreliable because diuretics increase sodium excretion. The fractional excretion of urea is the usual alternative.
How do I enter urine creatinine in mmol/L?
Multiply by 1000 to get µmol/L, or enter both plasma and urine creatinine in the same unit — only the ratio matters.
Evidence and guidelines
- Primary source: Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA 1976;236(6):579-581.
- Miller TR et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med 1978;89(1):47-50.
- 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.
Have a follow-up question?
Medelix answers clinical questions from current guidelines and studies — every statement with its source.
Ask Medelix about the FENa (fractional excretion of sodium)Related calculators
Embed this calculatorFree to use on your website, intranet or teaching page. Copy the code below.
<iframe src="https://medelix.ai/en/embed/fena" title="FENa (fractional excretion of sodium) – Medelix" width="100%" height="620" style="border:0;max-width:720px" loading="lazy"></iframe>
<p><a href="https://medelix.ai/en/calculators/fena-fractional-excretion-sodium">FENa (fractional excretion of sodium) calculator by Medelix</a></p>For healthcare professionals and medical students. These tools support, and do not replace, clinical judgement. Always check results against the original source and your local protocols.