FENa — Fractional Excretion of Sodium
Differentiate pre-renal from intrinsic acute kidney injury. A bedside test that guides early AKI workup.
AKI staging, etiology workup, every visit
EasyClinic tracks creatinine trends, computes KDIGO AKI stage, and shows FENa/FEUrea on the same screen — no more flipping calculators.
Also Known As
Formula
FENa (%) = (UNa × PCr) / (PNa × UCr) × 100
It expresses the fraction of filtered sodium that is excreted in urine. In pre-renal states, tubules avidly reabsorb sodium (FENa <1%). In ATN, tubular injury impairs sodium reabsorption (FENa >2%).
Interpretation
- FENa <1%: Pre-renal AKI — volume depletion, CHF, cirrhosis, hepatorenal, early sepsis, NSAIDs, ACE/ARB, contrast (early)
- FENa >2%: Intrinsic AKI — most commonly ATN; also AIN, glomerulonephritis
- FENa 1-2%: Indeterminate — consider clinical context, FEUrea
When FENa is unreliable — use FEUrea instead
FENa is misleading in these settings:
- Diuretic use (most common pitfall) — loop and thiazide diuretics force natriuresis
- Pre-existing CKD with baseline tubular dysfunction
- Glycosuria, bicarbonaturia (forces Na excretion)
- Contrast-induced AKI (can show FENa <1% despite intrinsic injury)
FEUrea (%) = (UUrea × PCr) / (PUrea × UCr) × 100. FEUrea <35% suggests pre-renal AKI even in patients on diuretics.
Frequently Asked Questions
▸My patient has FENa <1% but is clinically euvolemic — pre-renal or not?
FENa <1% can occur in early contrast nephropathy, hepatorenal syndrome, acute glomerulonephritis, and rhabdomyolysis with myoglobin cast nephropathy. Look at urine sediment, recent contrast exposure, liver function, CK level.
▸What about FENa in CKD on top of AKI?
Pre-existing tubular damage makes the cut-off less reliable. Combine with clinical context — change in weight, JVP, urine output, response to fluid challenge.
▸Can FENa replace fluid challenge?
No. FENa is one data point. In hemodynamically stable AKI, a careful fluid challenge (250-500 mL crystalloid) remains a gold-standard bedside test.
▸Is FENa useful in hepatorenal syndrome?
Classically very low (<1%) — but so is functional pre-renal AKI. The combination of cirrhosis with ascites, FENa <1%, and no response to albumin + terlipressin trial supports HRS.
References
- Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA 1976;236(6):579-81
- KDIGO Clinical Practice Guideline for Acute Kidney Injury 2012
- Indian Society of Nephrology AKI Working Group recommendations
AKI workup, automated
EasyClinic auto-computes FENa, FEUrea and AKI staging from your routine labs and flags the likely cause before the nephrology consult.
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