Background: Therapeutic drug monitoring is standard practice for the immunosuppressant tacrolimus (Tac). Venous blood sampling at outpatient clinics is time-consuming and impractical with regard to obtaining trough concentrations on clinical visit days. Home-based blood sampling may be patient friendly and pave the way for limited sampling strategies for the prediction of total drug exposure. The aim was to establish a Tac assay for dried capillary microsamples, ensuring reliable measurements during the full dose interval in renal transplant recipients. Methods: An assay based on volumetric absorptive microsampling and liquid chromatography tandem mass spectrometry was validated. The agreement between capillary microsamples and liquid venous samples was investigated in stable renal recipients on twice-daily Tac dosing. Sampling throughout the 12-hour dose interval was examined at 2 separate days, at least 1 week apart, for each participant. Two sets of samples were obtained at each time point, one delivered directly to the laboratory and one sent through mail. Results: Twenty-seven renal transplant recipients were included, of whom 26 were investigated twice. Tac was efficiently extracted from the dried microsamples (mean recovery 94%–103%). The between-series mean accuracy was 88%–98% with coefficients of variation ≤5.0% (≤11% at the lower limit of quantification), measurement range 0.70–60 mcg/L. The mean difference between parallel microsamples was 5%–7%. Overall, the mean differences between dried microsamples and liquid samples were −3.1% when mailed (n = 679) and −4.2% when directly delivered (n = 682). Less than 8% were outside ±20%. The microsamples were stable for 1 month at ambient temperature. Conclusions: The microsample method demonstrated acceptable performance. Tac concentrations can be reliably quantified throughout the dose interval by using volumetric absorptive microsampling in renal transplant recipients, and the results are not influenced by postal shipment.

High estimated clearance is significantly associated with increased risk of BPAR the first 90 days posttransplantation and may predict an increased risk of rejection in the early phase after renal transplantation.

Summary Annual assessment of adherence would strengthen long‐term outcome assessments from registry data. The objective of this study was to evaluate tools suitable for annual routine capture of adherence data in renal transplant recipients. A single‐centre open prospective trial included 295 renal transplant recipients on tacrolimus. Two‐thirds of the patients were included 4 weeks post‐transplant, randomized 1:1 to intensive or single‐point adherence assessment in the early phase and 1‐year post‐transplant. One‐third were included 1‐year post‐transplant during a cross‐sectional investigation. Adherence was assessed using multiple methods: The “Basel Assessment of Adherence to Immunosuppressive Medication Scale” (BAASIS©) questionnaire was used to assess self‐reported adherence. The treating clinician scored patient′s adherence and tacrolimus trough‐concentration variability was calculated. In the analyses, the data from the different tools were dichotomized (adherent/nonadherent). The BAASIS© overall response rate was over 80%. Intensive BAASIS© assessment early after transplantation increased the chance of capturing a nonadherence event, but did not influence the 1‐year adherence prevalence. The adherence tools generally captured different populations. Combining the tools, the nonadherence prevalence at 1 year was 38%. The different tools identified to a large degree different patients as nonadherent. Combining these tools is feasible for annual capture of adherence status.