Fractional Excretion of Sodium Calculator

How it is calculated

The tool compares how much sodium reaches the urine with how much creatinine does: FENa % = (urine Na × plasma Cr) ÷ (plasma Na × urine Cr) × 100. Sodium is entered in mmol/L (mEq/L) and creatinine in mg/dL, using the same unit for the urine and plasma value of each substance; only the ratios matter, so each pair just has to match. Besides FENa the tool shows two ratios, urine Na ÷ plasma Na and urine Cr ÷ plasma Cr. Accepted ranges: urine sodium 0 to 500, plasma sodium 80 to 200, and both creatinine values above zero. Anything outside these ranges, or a FENa that would come out above 100 %, shows no result. The tool only calculates the figure. In the usual teaching, below 1 % points toward a prerenal cause and above 2 % toward tubular damage, with the range between unclear, and diuretics or other factors can distort it.

Common mistakes

• Mixing units inside a pair, such as urine creatinine in mg/dL and plasma creatinine in µmol/L. The ratio is then wrong by a factor of about 88.
• Swapping urine and plasma values. The formula is not symmetric, so the result can be far off, and a swap can push FENa above 100 %, which gives no result.
• Using FENa after diuretics without caution. The drug raises sodium excretion and can hide a prerenal picture. This is a known limitation, not an error in the arithmetic.
• Reading the percentage as the amount of sodium lost. It is the share of filtered sodium that is excreted, not grams or millimoles per day.
• Entering sodium in the creatinine box or the reverse. The order is urine Na, plasma Na, urine Cr, plasma Cr, and a plasma sodium below 80 or above 200 shows no result, which usually signals a mix-up.