Assessment of the kidney's ability to conserve sodium in the setting of decreased GFR.
Required Parameters
Requires simultaneous (paired) blood and spot urine samples for Sodium and Creatinine.
Section 2
Formula & Logic
Physiological Rationale
FENa measures the percentage of sodium filtered by the kidney that is actually excreted in the urine. In prerenal states, the kidneys attempt to maximize volume by reabsorbing sodium. In ATN, tubular damage prevents efficient reabsorption, leading to "salt wasting."
Formula
FENa = [(Urine Na × Serum Cr) / (Serum Na × Urine Cr)] × 100
Interpretation
< 1%
Prerenal Azotemia (Hypovolemia, CHF, Hepatorenal)
1% – 2%
Indeterminate / Mixed picture
> 2%
Intrinsic AKI (Acute Tubular Necrosis)
Section 3
Pearls/Pitfalls
The Diuretic Problem
Loop diuretics inhibit sodium reabsorption in the thick ascending limb, artificially elevating FENa even in prerenal states. Use FEUrea (Fractional Excretion of Urea) instead if the patient has received diuretics within 24 hours.
FENa is less reliable in patients with pre-existing CKD, as damaged tubules in a chronic state may have a baseline defect in sodium handling regardless of the acute insult.
Section 4
Next Steps
If FENa < 1% (Prerenal)
01
Assess volume status: Consider fluid bolus if clinically hypovolemic.
02
Optimize hemodynamics: Address pump failure in CHF or vasodilation in cirrhosis.
03
Review medications: Discontinue NSAIDs, ACE inhibitors, or ARBs which impair renal autoregulation.
If FENa > 2% (Intrinsic)
01
Maintain euvolemia: Avoid fluid overload if patient is oliguric.
02
Nephrotoxin audit: Stop aminoglycosides, NSAIDs, or other culprits.
03
Monitor for complications: Hyperkalemia, metabolic acidosis, and uremia.
04
Evaluate for dialysis indications if GFR continues to decline.
Complementary Tools
Fractional Excretion of Urea (FEUrea)
Renal Failure Index (RFI)
Anion Gap
Section 5
Evidence Appraisal
Foundational Reference
The FENa test. Use in the differential diagnosis of acute renal failure.
Espinel CH. • JAMA. 1976;236(5):579-81. This seminal work established the 1% threshold as a discriminator between prerenal and ATN states.
Validation in Diuretic Use
Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure.
Carvounis CP et al. • Kidney Int.. 2002;Demonstrated that FEUrea is superior to FENa in patients on diuretics, with a threshold of <35% indicating prerenal status.
Section 6
Literature
The Search for Precision
Before the FENa, clinicians relied on simple urine sodium concentration. However, urine sodium is highly dependent on urine volume. Dr. Espinel's introduction of "fractional excretion" normalized sodium clearance to the GFR (using creatinine as a proxy), providing a far more accurate reflection of tubular function.
