FENa Calculator

The fractional excretion of sodium (FENa) is the percentage of sodium filtered by the kidneys that ends up in the urine. In acute kidney injury (AKI) it helps distinguish a prerenal cause — where the kidneys are underperfused but the tubules work and hold on to sodium — from intrinsic tubular damage such as acute tubular necrosis (ATN), where sodium is lost. Espinel’s original 1976 study in oliguric acute renal failure found FENa below 1% in prerenal azotemia and above 3% in ATN; in practice values below 1% suggest prerenal and above 2% intrinsic disease.

Diuretics increase sodium excretion and can push FENa up even when the cause is prerenal. In that situation the fractional excretion of urea (FEUrea) is more useful: Carvounis and colleagues found FEUrea of 35% or less identified prerenal AKI in most patients on diuretics. Enter plasma and urine sodium and creatinine, and optionally BUN and urine urea nitrogen, from samples taken at about the same time.

Creatinine units cancel, so any unit works as long as plasma and urine creatinine use the same one (and likewise for BUN and urine urea). FENa can be low in some intrinsic conditions (contrast nephropathy, rhabdomyolysis, glomerulonephritis, early obstruction) and is less reliable in chronic kidney disease, so interpret it with the history, urinalysis and trend.

Fractional excretion compares how much of a substance is excreted with how much is filtered. Creatinine is used as the marker of filtration, so the ratio of urine to plasma sodium is divided by the ratio of urine to plasma creatinine. Because each substance appears as a urine ÷ plasma ratio, units cancel — they only need to match within each pair.

Conventional cut-offs: FENa below 1% suggests prerenal AKI, 1–2% is indeterminate and above 2% suggests intrinsic renal injury such as ATN. For FEUrea, 35% or less suggests prerenal and above 50% suggests intrinsic disease; in Carvounis’s study mean FEUrea was about 25–28% in prerenal patients and 59% in ATN.