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Nephrology / Renal
FENa (Fractional Excretion of Sodium)

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
FENaInterpretationMechanism
< 1%Pre-renal AKI / Contrast nephropathyAvid Na reabsorption; tubular function intact
1–2%IndeterminateOverlap 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
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.
⚕️ For qualified healthcare professionals only. Not a substitute for clinical judgment.

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