FENa <1% suggests pre-renal azotemia — the kidneys are functioning normally and appropriately retaining sodium in response to decreased renal perfusion (dehydration, heart failure, hepatorenal syndrome). The kidney tubules are intact and avidly reabsorbing sodium and water.

FENa >2% suggests intrinsic renal disease — the kidney tubules are damaged and cannot properly reabsorb sodium. The most common cause is acute tubular necrosis (ATN). Other causes include interstitial nephritis and acute glomerulonephritis.

FENa is unreliable in patients on diuretics (use FEUrea instead), in the setting of glycosuria, contrast dye administration, chronic kidney disease with adapted tubular function, and early obstruction. Also, some causes of intrinsic renal disease can have FENa <1% (contrast nephropathy, myoglobinuria, early ATN).