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

Background: Tacrolimus 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. Methods: A 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. Results: Tacrolimus 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). Conclusion: The 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.