Oxalate deposits in biopsies from native and transplanted kidneys, and impact on graft function

Background. The purpose of this study was to examine the incidence of oxalate deposits in native and renal allograft biopsies, and its impact on graft function. Methods. The renal biopsy files at The Johns Hopkins University between 2000 and 2006 were searched to identify biopsies with oxalate deposits, determine the density of oxalate deposits in renal graft biopsies, compare graft histology and function between allograft recipients with oxalate in the graft biopsies, and a control group of recipients without oxalate in the graft. Results. Oxalate crystal deposits were observed in 61 of 5160 biopsies of native kidneys, and in 76 of 1621 renal allograft biopsies, with a frequency of 1 and 4%, respectively. Sixty-three (9%) of 680 transplant recipients showed oxalate in graft biopsies obtained within the first year from transplantation, with 1.3 +/- 1.2 average number of oxalate deposits per mm(2) of biopsy tissue. The high oxalate density and decreased renal function were correlated in the first 2 years post-transplant (P = 0.037-0.05). Compared with a control group of 70 kidney graft recipients, the renal function was significantly lower in the oxalate group at 1 year, but not at 2 years post-transplant. High tubulo-interstitial scarring (P < 0.0001) was noted in repeated biopsies in the oxalate group, and was significantly greater than that in the control group (P = 0.027). No significant difference in graft loss was observed between oxalate and control groups, and although mortality was higher in the oxalate group, the difference was not significant. Conclusions. In summary, this study defines the frequency of oxalate deposition in native and allograft kidney biopsies, and suggests its possible negative impact on graft function beyond the early post-transplant period.