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.