Serum Creatinine and Tacrolimus Assessment With VAMS Finger-Prick Microsampling: A Diagnostic Test Study

Rationale & Objective: Kidney transplant re-cipients require frequent venipunctures. Micro -sampling methods that use a finger-prick draw of capillary blood, like volumetric absorptive microsamplers (VAMS), have the potential to reduce the pain, inconvenience, and volume of blood loss associated with venipuncture. This study aimed to provide diagnostic accuracy using VAMS for measurement of tacrolimus and creatinine compared to gold standard venous blood in adult kidney transplant recipients.Study Design: Diagnostic test study. Prospective blood samples for measurement of tacrolimus and creatinine were collected using Mitra VAMS and venipuncture immediately before and 2 hours after tacrolimus dosing.Setting & Participants: A convenience sample of 40 adult kidney transplant participants in the outpatient setting. Tests Compared:Method comparison was assessed by Passing-Bablok regression and Bland-Altman analysis. The predictive performance of VAMS measurement compared to venipuncture was also assessed through estimation of the median prediction error and median absolute percentage prediction error.Results: A total of 74 tacrolimus samples and 70 creatinine samples were analyzed from 40 participants. Passing-Bablok regression showed a systematic difference between VAMS and venipuncture when measuring tacrolimus and creatinine with a slope of 1.08 (95% CI, 1.03-1.13) and a slope of 0.65 (95% CI, 0.6-0.7), respectively. These values were then corrected for the systematic difference. When used for Bland-Altman analysis, corrected values of tacrolimus and creatinine showed a bias of-0.1 mu g/L and 0.04 mg/dL, respectively. Tacrolimus (corrected) and creatinine (corrected) microsampling values when compared to corresponding venipuncture values met median prediction error and median absolute percentage prediction error predefined acceptability limits of <15%.Limitations: This study was conducted in a controlled environment using a trained nurse to collect VAMS samples.Conclusions: In this study, VAMS was used to reliably measured tacrolimus and creatinine. This represents a clear opportunity for more frequent and less invasive sampling for patients.