Clinical Results
Understanding the FENa Calculator
The Fractional Excretion of Sodium (FENa) is a crucial medical metric used in nephrology and internal medicine to differentiate between prerenal disease (decreased renal perfusion) and Acute Tubular Necrosis (ATN) in patients presenting with acute kidney injury (AKI).
FENa (%) = (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100
Note: Both serum and urine creatinine must be in the same units (usually mg/dL), and both sodium values must be in mEq/L or mmol/L.
Clinical Interpretation Matrix
Use this benchmark table to interpret the calculated FENa percentage in the context of oliguric acute kidney injury:
| FENa Value | Likely Diagnosis | Pathophysiological Interpretation |
|---|---|---|
| < 1% | Prerenal Azotemia | Kidneys are intact and actively conserving sodium and water in response to hypoperfusion (e.g., dehydration, heart failure). |
| 1% – 2% | Indeterminate / Postrenal | Equivocal range. May be seen in early ATN, postrenal obstruction, or a mixed clinical picture. |
| > 2% | Intrinsic / ATN | Tubular damage is present (Acute Tubular Necrosis). The kidneys are unable to conserve sodium despite hypoperfusion. |
Frequently Asked Questions
When is the FENa calculation unreliable?
FENa is primarily validated for oliguric patients. It is highly unreliable if the patient is taking diuretics (like furosemide or hydrochlorothiazide) because diuretics artificially elevate urinary sodium excretion, potentially pushing a prerenal state (>1%) into the ATN range. In these cases, the Fractional Excretion of Urea (FEUrea) is preferred.
Why calculate the U/P Creatinine Ratio?
The Urine to Plasma (Serum) Creatinine ratio evaluates the kidneys' ability to concentrate urine. A high ratio (>40) suggests functioning tubules actively concentrating urine (Prerenal), while a low ratio (<20) indicates tubular dysfunction commonly seen in Acute Tubular Necrosis.
Are there exceptions where FENa < 1% does not mean prerenal?
Yes. While classically prerenal, a FENa < 1% can also occur in intrinsic renal diseases associated with severe vascular changes or vascular volume depletion, such as contrast nephropathy, acute glomerulonephritis, and myoglobinuria (rhabdomyolysis).