Abstract
BACKGROUND: High tacrolimus intrapatient variability (tac IPV) is associated with poor outcomes in kidney transplantation, including rejection, donor-specific antibodies, and graft loss. A common cause of high tac IPV is related to patient nonadherence, but this is yet to be conclusively demonstrated. METHODS: This was a longitudinal cohort study comprising adult kidney recipients, who received transplants between 2015 and 2017, with follow-ups through February 2020. The goal of this study was to identify the most common etiologies of tac levels outside the typical range, which lead to high tac IPV, and assess the etiology-specific associations between high tac IPV and graft outcomes. Multivariate Cox regression was used to assess time-to-event analyses. RESULTS: In total, 537 adult kidney recipients were included; 145 (27%) were identified as having a high tac IPV (>40%) 3-102 months post-transplant. Common etiologies of tac levels significantly outside the standard goal range (6-12 ng/mL) leading to high tac IPV included patient nonadherence (20%), infections (19%), tac-related toxicities (17%), and undocumented issues (27%). In multivariable Cox modeling, those with high tac IPV because of nonadherence had a 3.5 times higher risk of late acute rejection (P = 0.019) and 2.2 times higher risk of late graft loss (P = 0.044). No other etiologies in the typical tac level range were significantly associated with either acute rejection or graft loss. CONCLUSIONS: Although high tac IPV has many causes, only high tac IPV caused by nonadherence is consistently associated with poor allograft outcomes.