How this calculator works
Use paired serum and urine samples collected close together. Creatinine units must represent the same analyte, and sodium uses molar concentration. FENa describes the fraction of filtered sodium excreted relative to creatinine clearance. Diuretics, chronic kidney disease, sepsis and several intrinsic kidney disorders limit diagnostic interpretation; this page does not label an AKI cause. The result is an estimate for information only and is not medical advice.
Formula
FENa (%) = 100 × (urine sodium × serum creatinine) / (serum sodium × urine creatinine). The engine stores the displayed percentage as a fraction.
Worked example
Urine sodium 70, serum sodium 140 mmol/L, serum creatinine 1 and urine creatinine 100 mg/dL give 0.5%. With urine sodium 140, serum sodium 140, serum creatinine 2 and urine creatinine 100, the result is 2%.
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Frequently asked questions
Do I need a timed urine collection?
No. Flow cancels when sodium clearance is divided by creatinine clearance.
Does a low value establish a prerenal cause?
No. A low FENa can occur in some intrinsic disorders, and drugs or chronic disease can change it.
Can urine sodium be zero?
The arithmetic permits zero, but values below a laboratory detection limit are not exact zero measurements.