OBJECTIVE We compared the results of 2 immunoassay systems (affinity column-mediated immunoassay [ACMIA] and microparticle enzyme immunoassay [MEIA]), regarding hematologic and biochemical values at 2 weeks after liver transplantation. METHODS We obtained 256 blood samples from 35 patients, at 2 weeks after liver transplantation, excluding those from patients who were treated with interacting medications or renal replacement therapy. We also excluded the early mortality cases within 2 weeks of liver transplantation. A Dimension RxL HM with the tacrolimus Flex reagent cartilage was used for the ACMIA and the IMx tacrolimus II for the MEIA method. RESULTS The tacrolimus concentrations measured by the ACMIA method correlated closely with those measured by the MEIA method (r = 0.953). However, the weighted concordance correlation coefficient for the repeated-measurement design was 0.74 (95% confidence interval, 0.66-0.85). The discrepancies in the tacrolimus level between the 2 methods was large among samples with low tacrolimus concentrations especially <5 ng/mL. When the difference ratio of the 2 methods ([ACMIA - MEIA]/ACMIA) was analyzed with a linear mixed-effects model to identify significant laboratory findings, there were no significant differences based on hematocrit, renal function, or hepatic function. However, the serum potassium level correlated with the difference ratio of the 2 methods (estimated slope, 10.173; P = .02). CONCLUSIONS Both the ACMIA and the MEIA methods are precise; however, the ACMIA method has the advantage of fewer pretreatment procedures. In the early liver transplant period, however, there was a difference between the serum tacrolimus concentrations measured by the 2 methods, especially at a low drug concentrations.