Nephrology
Fractional Excretion of Sodium (FENa)
Distinguishes pre-renal from intrinsic AKI.
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Background
The fractional excretion of sodium (FENa) is the percentage of sodium filtered by the kidney that is ultimately excreted in the urine, and it helps distinguish pre-renal from intrinsic causes of acute kidney injury (AKI). It is calculated from paired urine and plasma sodium and creatinine concentrations: (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine), multiplied by 100. In pre-renal states the tubules avidly reabsorb sodium to preserve volume, so FENa is low; in intrinsic tubular injury this reabsorption fails and FENa rises. Because creatinine appears in both numerator and denominator, its units cancel.
Interpreting the result
A FENa below 1% suggests a pre-renal cause such as hypovolaemia or poor renal perfusion, where the kidney is conserving sodium. A value above 2% suggests intrinsic injury, classically acute tubular necrosis, where sodium handling is impaired. Values between 1% and 2% are indeterminate and must be interpreted with the clinical context. The test is most useful in oliguric AKI and should ideally be measured before diuretics are given.
Worked example
In oliguric AKI, urine sodium is 15 mmol/L, plasma sodium 140 mmol/L, urine creatinine 4000 µmol/L and plasma creatinine 200 µmol/L. FENa = (15 × 200) ÷ (140 × 4000) × 100 ≈ 0.5%, a value below 1% that points towards a pre-renal cause.
Critical actions
Use consistent creatinine units (they cancel). Unreliable after diuretics or in CKD; FEUrea may be preferred if on diuretics.
Pearls / pitfalls
- Diuretics increase sodium excretion and falsely raise FENa — use the fractional excretion of urea (FEUrea) instead in patients already on diuretics.
- A low FENa is not unique to hypovolaemia: contrast nephropathy, early sepsis, hepatorenal syndrome and glomerulonephritis can also give values below 1%.
- It is unreliable in chronic kidney disease and in non-oliguric AKI, where baseline sodium handling is already altered.
- Use consistent creatinine units in numerator and denominator; because they cancel, mismatched units are a common avoidable error.
Evidence & validation
Popularised by Espinel (1976) and refined in subsequent AKI cohorts; it is a widely taught adjunct in nephrology, used alongside urinalysis, history and response to volume rather than as a stand-alone test.
Frequently asked questions
What does a FENa below 1% mean?
It suggests a pre-renal cause of AKI, where the kidney is reabsorbing sodium to defend circulating volume. However, several intrinsic conditions can also produce a low FENa, so interpret it in context.
Why is FENa unreliable on diuretics?
Diuretics force sodium loss in the urine, raising FENa regardless of the underlying cause. In this situation the fractional excretion of urea (FEUrea) is more reliable because urea handling is less affected.
When should the sample be taken?
Ideally in oliguric AKI and before diuretics are administered, as both factors affect the result. Paired urine and plasma samples should be collected at the same time.
Can a low FENa occur in intrinsic disease?
Yes — contrast-induced injury, early sepsis, glomerulonephritis and hepatorenal syndrome can all give a FENa below 1%. The test narrows but does not settle the diagnosis on its own.
Does it work in chronic kidney disease?
It is unreliable in CKD because baseline tubular sodium handling is already abnormal. The test is most informative in acute, oliguric presentations.
References
- Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579–581.
- KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012;2:1–138.
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