To the Editor.
—The fractional excretion of filtered sodium (FENa) has been shown to be a reliably discriminating test between prerenal azotemia (PRA) and acute tubular necrosis.1 However, and as was reviewed by Zarich et al2 in the January Archives, exceptions for the diagnostic value of FENa have been reported repeatedly. Zarich et al limited their discussion to those categories of acute renal failure (ARF) that are associated with low FENa values. Thus, in addition to PRA, low FENa values may be encountered in acute glomerulonephritis; early ureteral obstruction; contrast-induced ARF; pigment-induced ARF; acute interstitial nephritis; and ARF associated with burns, sepsis, cirrhosis, captopril therapy, or cardiac surgery.We wish to expand the discussion to involve those exceptions whereby a high FENa value may be encountered. In addition to acute tubular necrosis and late urinary tract obstruction,1 high FENa values are expected in PRA associated with metabolic