THE FRACTIONAL EXCRETION OF UREA - A NEW DIAGNOSTIC-TEST FOR ACUTE RENAL-ALLOGRAFT REJECTION

Fractional excretion of sodium (FE(Na)) has been used in the diagnosis of acute renal allograft failure on the assumption that poor allograf t perfusion should result in a low FE(Na). However, many patients rece ive medications which affect the active transport of Na+ and thus FE(N a). In contrast, the fractional excretion of urea (FE(urea)) is mostly dependent on passive forces and is therefore less influenced by drug therapy. To test the hypothesis that FE(urea) might be more useful tha n FE(Na) in evaluating graft failure, we compared FE(urea) with FE(Na) 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 tra nsplants. There was no significant difference between groups in FE(Na) . However, FE(urea) 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%). FE(ur ea) 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). FE(ure a) was also measured during stable graft function, 7-14 days prior to allograft dysfunction. CsA-Nx was associated with a 30.5 +/- 8.3% decr ease in FE(urea). FE(urea) did not change in patients with AR. Based o n these findings, we present an algorithm to aid in the differential d iagnosis of acute renal allograft failure.