*Purpose: To investigate risk factors for calcium phosphate deposition in the kidney allograft.

*Methods: This is a single-institution, retrospective analysis of adult patients who underwent kidney transplantation between 10/2016 and 11/2017 and who had a 12-month allograft surveillance biopsy available for review. Three-month and 12-month surveillance biopsies were reviewed for calcium phosphate deposition. Pre- and post-transplant clinical variables such as serum calcium, serum phosphorus, parathyroid hormone (PTH), alkaline phosphatase, 25-hydroxyvitamin D, bone densitometry results, and dialysis vintage were collected and analyzed. Post-transplant tertiary hyperparathyroidism was defined as elevated PTH and hypercalcemia and/or cinacalcet use. Those patients with calcium phosphate deposition in the allograft were compared to those without calcium phosphate deposition. Univariable analysis and logistic regression was performed to identify significant independent predictor variables for calcium phosphate deposition.

*Results: Of the 100 adult patients who underwent a kidney transplant during the study period, 77 patients were included for analysis. Common reasons for exclusion were dual-organ transplant or absence of a 12-month surveillance biopsy. Thirty-three (43%) patients had calcium phosphate deposition on the 3- and/or 12-month allograft biopsies. Calcium levels and presence of tertiary hyperparathyroidism were the only statistically significant post-transplant predictors of calcium phosphate deposition on univariable analysis. Furthermore, those with tertiary hyperparathyroidism at 12 months (n=24) had a high incidence of calcium phosphate deposition (21/24, 87.5%) in their allograft at 3 months or 12 months regardless of cinacalcet use. Notably, nine of the 10 tertiary hyperparathyroid patients with calcium phosphate deposition on their 3-month allograft biopsy were on treatment with cinacalcet prior to the biopsy. Four patients had undergone subtotal parathyroidectomy prior to transplant and none of those patients had calcium phosphate deposition on biopsy nor developed tertiary hyperparathyroidism. There was no difference in creatinine or glomerular filtration rate (GFR) at the 12 months between the two groups.

*Conclusions: Persistent tertiary hyperparathyroidism in the first year after kidney transplantation is associated with calcium phosphate deposition as early as 3-months post-transplant. GFR and average creatinine did not differ between those with and without calcium phosphate deposition at the 12-month point.

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