Tacrolimus is a narrow therapeutic index medication, which requires therapeutic drug monitoring to optimize dosing based on systemic exposure. MITRA microsampling offers a convenient, minimally invasive approach for the collection of capillary blood samples from a fingerprick versus conventional venous blood sampling for quantitation of tacrolimus blood concentrations. However, the suitability of MITRA microsampling for the determination of tacrolimus concentrations requires assessment in clinical settings.
Paired venous (2 mL) and capillary (10 μL) blood samples were collected pre tacrolimus dose and 1 and 3 h postdose during routine outpatient visits from stable adult liver or kidney transplant patients receiving prolonged-release tacrolimus. Tacrolimus concentrations were determined by liquid chromatography-tandem mass spectrometry, and the concentrations obtained by the two sampling methods were compared by linear regression and Bland-Altman agreement analyses.
Samples were available for 82 transplant recipients (kidney, n=41; liver, n=41). A high correlation was observed between tacrolimus concentrations in capillary and venous blood samples (Pearson’s correlation coefficient, 0.97; Lin’s concordance coefficient, 0.87; slope of the fitted line, >1.0). Tacrolimus concentrations in capillary samples were 22.5% higher on average than in the corresponding venous blood samples (95% limits of agreement, 0.5% to 44.6%). Similar results were observed in both transplant subgroups.
MITRA fingerprick sampling provides a convenient alternative to venipuncture for therapeutic drug monitoring in transplant recipients maintained on prolonged-release tacrolimus. When using the fingerprick MITRA method, the positive bias in tacrolimus concentrations observed with this technique, when compared with venipuncture, needs to be taken into consideration.

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