Background: Primary hyperoxaluria type 1 (PH1) is a rare metabolic defect that results in oxalate over-production by the liver and leads to renal disease due to oxalate nephropathy. The treatment of choice is combined liver and kidney transplantation or kidney transplantation alone in some patients. As excess oxalate tissue stores remain even after liver transplant, the transplanted kidney is at risk of recurrent oxalate nephropathy.

Methods: We evaluated protocol kidney transplant biopsies in PH1 patients for evidence of recurrent calcium oxalate (CaOx) deposits. H&E stained sections were examined under polarized light to illuminate CaOx crystals. CaOx deposits were counted and graded by the number of CaOx crystals per number of glomeruli in the sample (crystal score) to account for differences in sample size. Recurrence was defined as any degree of CaOx crystals in the kidney. Spearman correlation coefficients were used to correlate urinary chemistries with the CaOx crystal score.

Results: 26 PH1 patients received 27 kidney transplants (19 simultaneous liver/kidney and 8 kidney alone) and had protocol kidney transplant biopsies at 0,4,12, and 24 months (73 total biopsies). Of the 24 allografts with at least 2 years follow up, 21 were functioning with a mean eGFR of 60.2 ml/min/1.73m2. Two grafts were lost to recurrent oxalate nephropathy and one to rejection. Overall, the rate of recurrent CaOx crystals at any time post-transplant was 63%. The highest rate and concentration of CaOx crystals was seen at the 4 month time point, with CaOx seen on 55% of biopsies, with a mean crystal score of 0.15 (range 0-1.4). Later post-transplant biopsies showed both a lower rate of involvement and lower crystal score. The crystal score correlated with plasma oxalate at the time of biopsy (p=0.0002) and with 24 hour urinary oxalate (p=0.001) The CaOx concentration at 4 months correlated with increased graft interstitial fibrosis and tubular atrophy and worse graft function at 1 and 2 years post-transplant. No correlation was identified between CaOx recurrence and clinical parameters of transplant type, time on dialysis, or other pre-transplant variables.

Conclusion: Early post-transplant allograft protocol biopsy with examination for CaOx crystal deposition may help identify patients at risk of later graft dysfunction who may be candidates for intervention.

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