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FENa and Urine Sodium: AKI Differential Diagnosis Guide
FENa <1% suggests prerenal AKI (tubular sodium reabsorption intact); FENa ≥1% suggests intrinsic renal injury. Meta-analysis confirms sensitivity 90%, specificity 82% in ideal conditions. Both drop significantly in patients on diuretics — use FEUrea instead. Always interpret FENa in the full clinical context.
TL;DR: FENa <1% suggests prerenal AKI; FENa ≥1% suggests intrinsic renal injury. Meta-analysis confirms sensitivity 90%, specificity 82% in ideal conditions. In patients on diuretics, use FEUrea instead.
What Is FENa?
FENa measures what percentage of filtered sodium is excreted in urine. When kidneys are under-perfused, tubular function remains intact and FENa is <1%. When tubules are injured (ATN), FENa rises above 1%.
FENa (%) = (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100
Interpretation
| FENa | Interpretation |
|---|---|
| <1% | Prerenal AKI |
| 1–2% | Indeterminate |
| ≥2–3% | Intrinsic injury (ATN) |
When FENa Fails
- Diuretics: Force sodium excretion regardless of volume status → use FEUrea
- CKD: Impaired baseline tubular sodium conservation
- Contrast nephropathy: FENa typically <1% despite tubular injury
- Rhabdomyolysis: Initially low FENa despite ATN
FEUrea <35% = prerenal; ≥35% = intrinsic injury
Evidence
Abdelhafez M et al. (2022): FENa pooled sensitivity 90%, specificity 82%. In oliguric patients without CKD/diuretics: Se 95%, Sp 91%. On diuretics: specificity drops to 54%. Clin J Am Soc Nephrol. 2022;17(6):785–797. PMID: 35545442.
References
- Abdelhafez M et al. Clin J Am Soc Nephrol. 2022;17(6):785–797. PMID: 35545442.
- Babar F et al. J Community Hosp Intern Med Perspect. 2017;7(5):296–299. PMID: 29147470.
- Espinel CH. JAMA. 1976;236(6):579–581. PMID: 939128.
- Kellum JA et al. Kidney Int Suppl. 2012;2(1):1–138.