FENa / FEUrea
FENa / FEUrea - a deterministic tool in Sophie Well's Clinical Math & Conversions group.
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What this is
Compute the fractional excretion of sodium (FENa, Espinel 1976) and the fractional excretion of urea (FEUrea, Carvounis 2002) from a paired urine and serum sample. FENa < 1% classically suggests a prerenal pattern and FENa > 2% an intrinsic tubular pattern; FEUrea < 35% suggests prerenal physiology and remains interpretable when the patient is on a loop or thiazide diuretic, which invalidates FENa. Sophie Well returns both fractions side-by-side with the conventional band labels.
When to use it
Use this in the workup of acute kidney injury when the volume question is genuinely ambiguous and the answer will change management - i.e., before committing to aggressive fluids, diuresis, or a renal-replacement decision. Both indices are screening numbers, not diagnoses: glomerular disease, contrast nephropathy, rhabdomyolysis, and early obstruction can all sit at FENa < 1% despite intrinsic injury, and chronic kidney disease blunts both numbers. Read FENa/FEUrea alongside the urine sediment, the BUN:Cr ratio, the bladder-scan / Foley output, and the bedside volume exam before acting on either threshold.
References
Espinel CH JAMA 1976 (FENa); Carvounis CP Kidney Int 2002 (FEUrea).
Worked example: FENa ~0.57% (prerenal); FEUrea ~20% (prerenal).
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