FRACTIONAL EXCRETION OF TRACE LITHIUM AND URIC-ACID IN ACUTE PENAL FAILURE

The early distinction between prerenal azotemia, characterized by an a vid proximal tubular sodium reabsorption, and ATN, in which proximal t ubule function is depressed, remains an important but difficult clinic al task. Indices of acute renal failure based on urinary sodium excret ion may be helpful but have several limitations, among which is the us e of diuretics. The effectiveness of the fractional excretion of uric acid (FE(UA)) and that of endogenous lithium (FE(LI)) in the diagnosis of acute renal failure has been evaluated in an unselected group of 4 6 patients, 28 with prerenal azotemia and 18 with ATN. In the entire g roup, FE(LI) concurred with the clinical diagnosis in 78% of the patie nts, whereas the fractional excretion of sodium (FEN,) and FE(UA) were in agreement in only 63 and 50%, respectively. FE,, was more sensitiv e to identify hemodynamic renal failure, because 93% of prerenal failu re patients had a low FE,I, contrasting with a low FE(UA) in only 68% and a low FE(NA) in 75%. The major reason for the discrepancy between FE,, and FE(LI) was the administration of diuretics. In both acute ren al failure groups, FE(NA) was higher in the subgroups receiving diuret ics. In contrast, diuretic therapy had no effect on FE(LI) in either g roup. These results suggest that FE(LI) is more accurate than either F E(Na) or FE(UA) for distinguishing prerenal azotemia from ATN. The sup eriority of FE(LI) appears especially relevant in patients treated wit h the usual diuretics.