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FENa — Fractional Excretion of Sodium
Differentiates pre-renal AKI from intrinsic renal disease
Required Inputs
Required (1–400)
Required (100–180)
Required (1–800)
Required (0.1–30)
Optional — FEUrea Calculation
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FENa (%)
Interpretation Reference
| FENa | Interpretation | Mechanism |
|---|---|---|
| < 1% | Pre-renal AKI / Contrast nephropathy | Avid Na reabsorption; tubular function intact |
| 1–2% | Indeterminate | Overlap zone — consider clinical context |
| > 2% | Intrinsic renal (ATN) | Tubular damage → impaired Na reabsorption |
| FEUrea < 35% | Pre-renal (even on diuretics) | Urea reabsorption preserved; useful with diuretic use |
FENa = (UNa × SCr) / (SNa × UCr) × 100
FEUrea = (UUrea × SCr) / (BUN × UCr) × 100
FEUrea = (UUrea × SCr) / (BUN × UCr) × 100
Important Exceptions: FENa may be <1% in early ATN caused by contrast nephropathy, myoglobinuria/pigment nephropathy, or NSAIDs. FENa is unreliable in patients on diuretics — use FEUrea instead. FEUrea <35% suggests pre-renal even with diuretic use.
