Fractional Excretion of Sodium (FENa) Calculator
Use serum and urine samples taken as closely in time as possible. Confirm specimen type and units; serum and urine values are not interchangeable.
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4 fields are required.
FENa (fractional excretion of sodium)
Enter all four laboratory values to calculate.
FENa alone cannot establish the cause of acute kidney injury. Interpretation is limited with diuretics, chronic kidney impairment, nonoliguric AKI, sepsis, and urinary obstruction.
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Formula, units, worked example, and evidence
Equation and units
FENa (%) = (urine Na × serum creatinine) / (serum Na × urine creatinine) × 100. Use Na in mEq/L for both serum and urine, and creatinine in mg/dL for both. Do not round intermediate values.
Example (hypothetical samples)
Serum Na 140 mEq/L, serum creatinine 1.4 mg/dL, urine Na 20 mEq/L, and urine creatinine 80 mg/dL: 20 × 1.4 / (140 × 80) × 100 = 0.25%.
Applicability and limitations
Historical fractional excretion of sodium criteria were studied in oliguric acute renal failure; no etiology threshold should be transferred indiscriminately to other patient populations. This page provides arithmetic only, not a diagnosis, etiology classification, or treatment instruction.
Sources and review
Sources checked 2026-11-10.