*Purpose: Sub-therapeutic tacrolimus trough levels have been associated with increased risk of rejection in renal transplant recipients and supra-therapeutic levels with infectious risks and drug toxicities. The objective of this study is to identify risk factors for sub- and supra-therapeutic tacrolimus levels in renal transplant patients receiving an initial fixed dose of tacrolimus 2 mg twice daily based on institution protocol.

*Methods: This is a single-center, retrospective cohort study of adults who received a renal transplant from 2013-2017. Patients were classified into 2 groups: those with first appropriate tacrolimus trough level drawn post-transplant within the institutional target trough of 8-10 ng/mL and those with first trough <8 or >10 ng/mL. Logistic regression will be utilized to evaluate patient demographics and identify potential risk factors. Additional study endpoints will include biopsy proven acute rejection and renal function.

*Results: A total of 300 patients were included in the preliminary results.

*Conclusions: Only 13% of patients had an initial tacrolimus trough between 8-10 ng/mL. Of patients with an initial trough <8 ng/mL, a higher dose of 7 mg/day was required to achieve a level >8 ng/mL (p<0.0001). Patients with sub-therapeutic troughs on average had a higher BMI, showing fixed-doses may not achieve therapeutic levels in overweight patients early post-transplant. Patients with an initial trough <8 ng/mL required an increase in weight-based dose (0.08 vs 0.06 mg/kg/day, p<0.0001) to attain a level >8 ng/mL compared to patients with an initial trough of 8-10 ng/mL. Mean time from first dose to first trough >8 ng/mL among patients with an initial trough <8 ng/mL was 7±1 days. There was a trend towards statistical significance between lower age and sub-therapeutic tacrolimus trough levels. Additional patient analysis and logistic regression will aim to identify patient characteristics associated with sub-therapeutic levels.

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