The early distinction between prerenal
azotemia, characterized by an avid proximal tubular
sodium reabsorption, and ATN, in which proximal tubule function is depressed, remains an important but difficult clinical task. Indices of
acute renal failure based on urinary
sodium excretion may be helpful but have several limitations, among which is the use of
diuretics. The effectiveness of the fractional excretion of
uric acid (FEUA) and that of endogenous
lithium (FELi) in the diagnosis of
acute renal failure has been evaluated in an unselected group of 46 patients, 28 with prerenal
azotemia and 18 with ATN. In the entire group, FELi concurred with the clinical diagnosis in 78% of the patients, whereas the fractional excretion of
sodium (FENa) and FEUA were in agreement in only 63 and 50%, respectively. FELi was more sensitive to identify hemodynamic
renal failure, because 93% of prerenal failure patients had a low FELi, contrasting with a low FEUA in only 68% and a low FENa in 75%. The major reason for the discrepancy between FENa and FELi was the administration of
diuretics. In both
acute renal failure groups, FENa was higher in the subgroups receiving
diuretics. In contrast,
diuretic therapy had no effect on FELi in either group. These results suggest that FELi is more accurate than either FENa or FEUA for distinguishing prerenal
azotemia from ATN. The superiority of FELi appears especially relevant in patients treated with the usual
diuretics.