*Purpose: Routine therapeutic drug monitoring (TDM) of tacrolimus (Tac) is currently performed in venous bloods samples drawn at the hospital. The novel volumetric absorptive microsampling (VAMS^TM) devices allow the patients themselves to collect a finger prick capillary blood sample suitable for tacrolimus monitoring. In the present study we have validated a commercial product (Mitra tip^®) compared to simultaneously drawn venous samples in stable kidney transplant recipients.

*Methods: Two 12-hour Tac pharmacokinetic investigations (13 samples each), separated by at least one week, were performed in 27 renal transplant recipients with target Tac trough 4-7 µg/L. Patients were included 3 ± 1 weeks after transplantation and received mycophenolate mofetil and prednisolone in addition to twice daily tacrolimus as maintenance immunosuppressive therapy. At each sampling point 2 venous and 2 capillary samples (10 µL) were obtained; one pair (venous/capillary) went directly to the lab while the second pair was sent via ordinary mail service. Tacrolimus was analyzed using the standard mass spectrometry assay at the hospital and the microsampling method was validated according to the guideline on bioanalytical method validation of the European Medicines Agency (EMA).

*Results: A total of 682 pairs of venous and microsampled specimens from patients were assayed for Tac concentrations. Mean Tac dose and trough concentration were 3.5 ± 1.5 mg and 6.6 ± 1.5 µg/L, respectively. The micro sample concentrations were on the average 4.2% (95% CI: -5.1% to -3.4%) lower than the venous samples. All results of the method validation were within the criteria of the EMA guideline. Range of Tac concentrations tested was from 0.7 to 57 µg/L, the accuracy ranged from 88% to 98% and imprecision was below 5% (except for the 0.7 µg/L where CV was 11%). The Mitra tip® micro samples were stable for at least 30 days in room temperature and were not influenced by postal service shipment.

*Conclusions: Measurement of whole blood tacrolimus concentrations in 10 µL capillary blood from renal transplant recipients can be reliably and safely performed using the VAMS™ technique. Following a minimum of training, this also provides an option for self collection by patients.

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