*Purpose: Therapeutic drug monitoring of tacrolimus (TAC) requires venepuncture blood samples taken by specialists, requiring patients to attend an outpatient clinic; this can be inconvenient and may affect timing of collection. A new, less-invasive assay based on capillary blood samples (10μL), obtained by a fingerprick that can be self-performed using the MITRA^® Microsampler, has been developed and validated. This study compares TAC concentrations determined in venepuncture and fingerprick samples.

*Methods: During routine outpatient follow up visits, 3 paired whole blood samples were collected pre-dose, 1- and 3 hours post-dose from stable adult liver or kidney transplant recipients receiving prolonged-release TAC (ADVAGRAF, Astellas Pharma Europe, BV) -based therapy (NCT03465969). High-performance liquid chromatography tandem-mass spectrometry (HPLC-MS/MS) was used for determination of whole blood TAC concentrations, and the two sampling methods were compared by linear regression and Bland-Altman agreement analyses.

*Results: 82 transplant patients (kidney, n=41; liver, n=41) were studied. Mean±SD age and TAC daily dose were 51±14.8 yrs and 4.6±2.44 mg, respectively. Linear regression analysis showed high correlation between TAC concentrations determined by venepuncture and fingerprick sampling (Pearson’s correlation coefficient [r²], 0.970; Lin’s concordance coefficient, 0.870; slope of the fitted line, >1.0). Bland-Altman agreement analysis showed TAC concentrations in capillary samples were ~22.5% higher on average than in the corresponding venepuncture samples (95% limits of agreement, 0.5% to 44.6%) (Figure).

*Conclusions: This study showed a strong positive correlation between quantitative determination of whole blood TAC concentration by fingerprick MITRA^® assay and conventional venepuncture sampling in transplant patients maintained on ADVAGRAF.

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