Tacrolimus Can Be Reliably Measured With Volumetric Absorptive Capillary Microsampling Throughout the Dose Interval in Renal Transplant Recipients.

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. 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. 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. 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.