Fractional Excretion of Sodium (FENa) [Updated]

Fractional Excretion of Sodium (FENa) quantifies the proportion of filtered sodium that is excreted in the urine and is widely used to help distinguish between different causes of acute kidney injury (AKI), particularly in oliguric patients. FENa provides a more reliable measure of renal sodium handling than urine sodium concentration alone, as it accounts for urine volume and the influence of antidiuretic hormone. Despite its utility, FENa interpretation requires careful consideration of the clinical context, as low values may also occur in non-volume-depleted states such as glomerulonephritis, hepatorenal syndrome, and cardiorenal syndrome. FENa is best applied in oliguric AKI patients without confounding factors such as diuretic use, chronic kidney disease, urinary tract obstruction, or acute glomerular diseases. No absolute FENa value can definitively diagnose pre-renal AKI, and serial measurements may provide additional diagnostic insight. The physiologic rationale for using a FENa <1% as a marker of pre-renal azotemia stems from the fact that at a normal glomerular filtration rate, this represents sodium excretion exceeding typical dietary intake. Ultimately, FENa should be interpreted as one piece of a broader clinical puzzle that includes history, physical examination, and other laboratory data.