Kidney function

Fractional excretion of urea (FEUrea)

Calculate fractional urea excretion with matched urine and blood urea-nitrogen measurements.

Intended population: Paired urine and blood samples; both nitrogen concentrations in mg/dL and both creatinine concentrations in mg/dL.

Do not mix total urea with urea nitrogen. Diagnostic performance varies, including in critically ill patients; diuretic use does not make FEUrea definitive. Interpret with the complete clinical assessment.

Urine urea nitrogen (not total urea) (mg/dL)

Blood urea nitrogen (BUN) (mg/dL)

Urine creatinine (mg/dL)

Serum creatinine (mg/dL)

Sources and scoring references

AACC guidance on laboratory investigation of acute kidney injury (2021) ↗Carvounis et al.: fractional excretion of urea (2002) ↗

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