*Purpose: High tacrolimus intrapatient variability (IPV) is associated with inferior outcomes in solid organ transplantation. Many evaluations of IPV empirically exclude the early post-transplant period ranging from 1 to 3 months due to an anticipated increase in IPV. Such differences have not been well quantified in the existing literature. In this study we aimed to describe the natural history of tacrolimus IPV during the first post-transplant year.

*Methods: This was a retrospective cohort study of adult patients who received a kidney transplant between August 1, 2016 and June 18, 2018. Coefficient of variation (SD/mean*100) was used as the measure of IPV. Patients with enough levels to calculate CV for the first 6 months and the 12 months (at least 3/month) were included. One-way ANOVA with repeated measures with a Greenhouse-Geisser correction was conducted. Pairwise comparisons were completed using Bonferroni correction.

*Results: 115 patients had CV values for the first 6 months. Among these patients the mean CV differed significantly between time points (p<0.001, Figure 1). Post hoc testing revealed significant differences between the first month and all subsequent months (p<0.001 after Bonferroni for all). The mean difference was also significant for month 2 compared to month 5 (p=0.02). The range of CV values remains large overtime (Figure 2). Forty patients had CV values for all 12 months. Overall a significant difference was noted (p<0.001) driven by month 1. No other pairwise comparisons were significant.

*Conclusions: Excluding the first month of tacrolimus levels from an overall analysis of CV in the first year appears warranted but there is little evidence a longer period is necessary.

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