Fractional Excretion of Sodium (FeNa) Calculator
Calculate FeNa to differentiate between prerenal acute kidney injury (AKI) and intrinsic AKI (Acute Tubular Necrosis - ATN). Essential clinical tool in nephrology and ICU.
Clinical Interpretation of FeNa in Nephrology
Acute Kidney Injury (AKI) is a frequent and complex diagnosis in intensive care and emergency settings. To rapidly guide therapeutic management (fluid resuscitation vs. conservative management), determining the etiology is imperative. The Fractional Excretion of Sodium (FeNa) is the gold-standard urinary marker to differentiate prerenal (functional) AKI from intrinsic (organic) AKI.
How does FeNa work? (Pathophysiology)
FeNa measures the percentage of sodium filtered by the glomerulus that is ultimately excreted in the urine, relative to creatinine clearance. This ratio directly reflects the renal tubules' capacity to conserve sodium.
- In hypovolemia (Prerenal AKI): Faced with decreased renal blood flow (due to dehydration, hemorrhage, or heart failure), a kidney with intact tubules will activate the renin-angiotensin-aldosterone system. It massively reabsorbs sodium and water to restore intravascular volume. This results in highly concentrated, sodium-poor urine and a FeNa < 1%.
- In tubular damage (ATN): If renal ischemia is prolonged or in the presence of toxins (iodinated contrast media, aminoglycosides, myoglobin), renal tubular cells undergo necrosis. The kidney loses its physiological ability to reabsorb filtered sodium, leading to inappropriate sodium wasting in the urine and a FeNa > 2%.
Clinical Precautions (Diuretic Effect)
Using FeNa involves major clinical pitfalls. The most common cause of misinterpretation is the recent use of loop diuretics (e.g., Furosemide / Lasix).
- Diuretics chemically force the urinary excretion of sodium by blocking its reabsorption. Thus, even in severe prerenal AKI (where FeNa should be < 1%), the diuretic will artificially increase the FeNa (> 1% or 2%), falsely mimicking Acute Tubular Necrosis (ATN).
- Clinical Alternative: If the patient is on diuretics or has received them recently, it is strongly recommended to use the Fractional Excretion of Urea (FEUrea). A FEUrea < 35% suggests prerenal AKI, as tubular urea reabsorption is not significantly blocked by loop diuretics.
Differential Diagnosis of AKI based on FeNa
| FeNa Value (%) | Probable Etiology |
|---|---|
| < 1 % | Prerenal AKI (Hypoperfusion, Dehydration, Heart failure) |
| 1 % - 2 % | Gray zone: Indeterminate or mixed etiology |
| > 2 % | Intrinsic / Organic AKI (Acute Tubular Necrosis) |
References:
1. Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976.
[JAMA Network]
2. Zarich S, Fang LS, Diamond JR. Fractional excretion of sodium in diagnosing causes of oliguria. Effect of age and renal disease. Arch Intern Med. 1985.
[JAMA Internal Med]
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