*Purpose: This study aimed to evaluate the association of tacrolimus coefficient of variation (CV) with biopsy-proven acute cellular rejection in the first year following lung transplant (LT).

*Methods: This was a retrospective chart review of 229 adult LT recipients who received basiliximab induction and tacrolimus-based triple drug maintenance immunosuppression. Tacrolimus CV at 0-3, 3-6, and 6-12 months was compared to the median CV of the cohort to assign patients to a high or low variability group. Total acute rejection score (TRS) at 12-months was compared between these groups by two-sample t-tests. The TRS was calculated as the summed scores of all biopsy specimens with points allocated as follows: A0=0, A1=1, A2=2, A3=3, A4=4, and isolated B grade=1. The development of chronic lung allograft dysfunction (CLAD) and antibody-mediated rejection (AMR) were analyzed as secondary outcomes.

*Results: An average (SD) of 50 (16.9) levels per patient were collected between 0-12 months post-transplant. The mean (SD) of the average tacrolimus levels for each patient were 9.9 ng/mL (1.4 ng/mL), 10.5 ng/mL (2.4 ng/mL), and 10.5 ng/mL (1.8 ng/mL) between 0-3, 3-6, and 6-12 months, respectively. The median CV of tacrolimus levels was 40%, 30%, and 30% between 0-3, 3-6, and 6-12 months, respectively. The mean (SD) TRS was 1.6 (1.7). Patients with high tacrolimus variability at 0-3, 3-6, and 6-12 months on average scored 0.18 (mean 1.6 vs. 1.5; 95% confidence interval [CI]: -0.3 to 0.66, p=0.46), 0.14 (mean 1.7 vs. 1.5; 95% CI: -0.32 to 0.6, p=0.55), and 0.12 (mean 1.6 vs. 1.5; 95% CI: -0.34 to 0.58, p=0.62) point higher in twelve-month TRS, respectively. The incidences of CLAD and AMR were numerically greater in the high variability group throughout certain periods.

*Conclusions: High tacrolimus CV was not associated with twelve-month total acute rejection score. Further studies are needed to assess long-term outcomes with tacrolimus level variability.

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