*Purpose: Tacrolimus displays wide inter- and intra-patient variability, necessitating therapeutic drug monitoring. Assessing the duration of time in therapeutic range (TTR) is an additional tool to assess this drug therapy. The purpose of this study is to determine the impact of tacrolimus TTR on 12-month biopsy-proven rejection in an early corticosteroid withdrawal (ESW) population.

*Methods: Adult isolated renal transplant (RT) recipients maintained on tacrolimus between 1/1/2015 and 12/31/2018 were screened. Patients who underwent ABO incompatible, positive flow crossmatch RT, died <90 days of RT, were lost to follow-up <12 months post-transplant, or on non-protocol tacrolimus regimens were excluded. High tacrolimus TTR (TTR-H) and low tacrolimus TTR (TTR-L) were differentiated using whole cohort median. The primary outcome was to compare the incidence of 12-month biopsy-proven acute rejection (BPAR). Secondary outcomes included comparing rejection subtypes, presence of donor-specific antibody (DSA) and de novo DSA (dnDSA), and allograft function at 12 months. Tacrolimus TTR was calculated with the Rosendaal method using protocol goal tacrolimus levels (days 0 to 60: 8 – 12 ng/mL and beyond day 60: 5 – 10 ng/mL). Erroneous tacrolimus levels were excluded.

*Results: Patient demographics are demonstrated in table 1. Overall, BPAR was numerically higher but not statistically significant (TTR-L 19.5% vs TTR-H 10.6%, p=0.06). More antibody-mediated rejection occurred in the TTR-L group (8.0% vs 2.3%, p = 0.04). Incidence of DSA (figure 1) was significantly higher in TTR-L patients (36.7% vs. 16.7%, p=0.001) and dnDSA formation was also numerically higher (TTR-L 16.7% vs TTR-H 9.9%, p=0.18). There was no difference in eGFR at 12 months (p=0.29).

*Conclusions: In an ESW population, TTR-L was associated with a higher incidence of DSA and antibody-mediated rejection. Future studies to determine TTR thresholds in the setting of ESW are warranted.

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