The fractional excretion of urea: a new diagnostic test for acute renal allograft rejection.

Fractional excretion of sodium (FENa) has been used in the diagnosis of acute renal allograft failure on the assumption that poor allograft perfusion should result in a low FENa. However, many patients receive medications which affect the active transport of Na+ and thus FENa. In contrast, the fractional excretion of urea (FEurea) is mostly dependent on passive forces and is therefore less influenced by drug therapy. To test the hypothesis that FEurea might be more useful than FENa in evaluating graft failure, we compared FEurea with FENa during 79 episodes of acute renal allograft dysfunction due to acute rejection (AR), cyclosporine nephrotoxicity (CsA-Nx), viral infection, or bacterial infection in 32 children and young adults with renal transplants. There was no significant difference between groups in FENa. However, FEurea was significantly lower (P < 0.05) in patients with CsA-Nx (32.6 +/- 1.9%) and viral infection (32.9 +/- 3.2%) than those with AR (45.1 +/- 1.7%) or bacterial infection (38.9 +/- 2.5%). FEurea was < 35% in 20 of 28 (71.4%) episodes of CsA-Nx and 8 of 11 (72.2%) of viral infection, but only 5 of 36 (13.9%) of AR (P < 0.05). FEurea was also measured during stable graft function, 7-14 days prior to allograft dysfunction. CsA-Nx was associated with a 30.5 +/- 8.3% decrease in FEurea. FEurea did not change in patients with AR. Based on these findings, we present an algorithm to aid in the differential diagnosis of acute renal allograft failure.