FENa

Why use it

Key tool for AKI differential diagnosis. Guides fluid management decisions: volume-responsive (prerenal) vs. intrinsic injury. Simple calculation from readily available labs.

About

The Fractional Excretion of Sodium (FENa) helps differentiate prerenal azotemia from intrinsic acute kidney injury (AKI). It measures the percentage of filtered sodium that is excreted in urine. In prerenal states, the kidney avidly reabsorbs sodium (FENa < 1%), while in intrinsic renal damage (ATN), sodium reabsorption is impaired (FENa > 2%).

When to use

• Acute kidney injury of unclear etiology
• Distinguishing prerenal azotemia from acute tubular necrosis
• Oliguria workup to guide fluid resuscitation vs. diuresis

Formula

Pearls & pitfalls

• FENa is unreliable if patient has received diuretics — use FEUrea instead (< 35% = prerenal).
• FENa < 1% can occur in early obstruction, contrast nephropathy, myoglobinuria, and acute GN.
• Chronic CKD patients may have FENa > 1% at baseline due to impaired concentrating ability.
• Spot urine samples are adequate — 24-hour collection not needed.
• In patients on diuretics, FEUrea is the preferred alternative.
• Hepatorenal syndrome has FENa < 1% despite intrinsic-appearing injury.

Next steps

FENa < 1%: Prerenal — trial of IV fluids. FENa 1-2%: Indeterminate — consider clinical context. FENa > 2%: Intrinsic AKI (likely ATN) — avoid excessive fluids, nephrology consult.

Evidence

Described by Espinel (Am J Med, 1976). Extensively validated as a bedside tool for AKI differentiation. Limitations recognized in the setting of diuretics, CKD, and certain intrinsic causes that mimic prerenal physiology.

References

1. Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581. — link