Fractional Excretion of Sodium (FENa) Calculator

How it is calculated

FENa compares how much sodium leaves in urine with how much creatinine does: FENa % = (urine Na × plasma Cr) ÷ (plasma Na × urine Cr) × 100. Both sodium values must be in the same unit (for example mmol/L), and both creatinine values in the same unit (for example mg/dL); the units cancel, so the result is a percent. The tool also shows two parts of the formula: urine Na ÷ plasma Na and urine Cr ÷ plasma Cr. Plasma sodium, urine creatinine and plasma creatinine must be above zero, and urine sodium cannot be negative; otherwise no result is shown. A urine sodium of 0 is accepted and gives 0.00%. It gives the percentage only and does not interpret it. FENa is less reliable after diuretics, and interpretation belongs with a clinician.

Common mistakes

• Swapping urine and plasma values. Swapping creatinine, for example, flips the ratio and gives a result thousands of times too large or small.
• Mixing units between the two sodium values or the two creatinine values, such as mg/dL in one box and µmol/L in the other.
• Using samples taken at very different times. Plasma and urine should be collected together for the numbers to mean anything.
• Leaving a creatinine or plasma sodium field as 0 because it is unknown. Zero in any of those three fields is rejected and no result appears.
• Using FENa after diuretics without caution. The percentage is reported correctly, but its usual meaning is less reliable then, so a clinician should interpret it.