fena

Fractional excretion of sodium (FeNa) is a calculation used in nephrology to estimate the percentage of filtered sodium that is excreted in the urine. It is particularly helpful in evaluating acute kidney injury and distinguishing prerenal azotemia due to decreased renal perfusion from intrinsic renal injury such as acute tubular necrosis.

Calculation: FeNa = (U_Na × P_Cr) / (P_Na × U_Cr) × 100, where U_Na is urine sodium concentration, P_Na is plasma sodium, U_Cr is urine creatinine, and P_Cr is plasma creatinine. All concentrations are typically measured from a spot urine sample and a concurrent plasma sample; units cancel to give a percent.

Interpretation: In prerenal azotemia, FeNa is typically less than 1% (often below 0.5%), reflecting intact tubular reabsorption of sodium. In intrinsic renal injury such as ATN, FeNa is typically greater than 2%, reflecting impaired tubular reabsorption. Values between 1% and 2% are inconclusive and must be interpreted in the clinical context.

Limitations: Use of diuretics can artificially raise FeNa, reducing reliability; in patients on diuretics, the test may incorrectly suggest intrinsic kidney injury. Other factors such as chronic kidney disease, recent renal transplant, rhabdomyolysis, or contrast nephropathy can also affect the result. In such cases, the fractional excretion of urea (FEUrea) may be a more reliable alternative when diuretics are used.

See also: prerenal azotemia, acute kidney injury, acute tubular necrosis, FEUrea.