Is the Affinity Column–Mediated Immunoassay Method Suitable as an Alternative to the Microparticle Enzyme Immunoassay Method as a Blood Tacrolimus Assay?

BackgroundTacrolimus is a potent immunosuppressive drug used in organ transplantation. Because of its substantial toxic effects, narrow therapeutic index, and interindividual pharmacokinetic variability, therapeutic drug monitoring of whole-blood tacrolimus concentrations has been recommended. We investigated the comparability of the results of 2 immunoassay systems, affinity column–mediated immunoassay (ACMIA) and microparticle enzyme immunoassay (MEIA), comparing differences in the tacrolimus concentrations measured by the 2 methods in relation to the hematologic and biochemical values of hepatic and renal functions.MethodsA total of 154 samples from kidney or liver transplant recipients were subjected to Dimension RxL HM with a tacrolimus Flex reagent cartilage for the ACMIA method and IMx tacrolimus II for the MEIA method.ResultsTacrolimus concentrations measured by the ACMIA method (n = 154) closely correlated with those measured by the MEIA method (r = 0.84). The Bland-Altman plot using concentration differences between the 2 methods and the average of the 2 methods showed no specific trends. The tacrolimus levels determined by both the MEIA method and the ACMIA method were not influenced by hematocrit levels, but the difference between the 2 methods (ACMIA − MEIA) tended to be larger in low hematocrit samples (P < .001).ConclusionThe ACMIA method used for a tacrolimus assay is precise and has advantages, including the lack of a required pretreatment procedure. Furthermore, it is only slightly influenced by the hematologic or biochemical status of the samples.