Kidney function

Fractional excretion of sodium (FENa)

Calculate fractional sodium excretion from paired blood and urine samples.

Intended population: Paired samples with urine/serum creatinine in the same units and both sodium values in the same units.

Interpret alongside volume status, urine microscopy and clinical course. Diuretics, CKD, sepsis and several intrinsic kidney disorders limit diagnostic accuracy; no percentage alone proves or excludes a cause of AKI.

Urine sodium (mmol/L)

Serum sodium (mmol/L)

Urine creatinine (mg/dL)

Serum creatinine (mg/dL)

Sources and scoring references

AACC guidance on laboratory investigation of acute kidney injury (2021) ↗Mayo Clinic Laboratories: random urine sodium and FENa calculation ↗

Implementation references reviewed October 28, 2026. Verify applicability and unusual or treatment-changing results against the linked references and local guidance.