Good correlation between tacrolimus concentrations using improved CMIA on the Alinity i analyzer and LC-MS/MS method from a reference laboratory but unexpected negative bias with another LC-MS/MS method from a different reference laboratory

Abstract Objectives We compared tacrolimus concentrations obtained by the more recently US Food and Drug Administration–approved tacrolimus assay (CMIA) on the Alinity i analyzer (Abbott Laboratories) with a liquid chromatography/tandem mass spectrometry (LC-MS/MS)–based method from 2 reference laboratories. We also investigated the correlation between the CMIA tacrolimus and Elecsys tacrolimus assays. Methods Tacrolimus concentrations were measured in EDTA whole blood by the chemiluminescent microparticle immunoassay (CMIA) using the Alinity i analyzer, and then 2 aliquots were sent to 2 reference laboratories, both using ascomycin as the internal standard for the LC-MS/MS method. Results The total precision of the CMIA tacrolimus assay was excellent. When tacrolimus concentrations obtained by the LC-MS/MS method from reference laboratory A were plotted on the x-axis and corresponding CMIA values were plotted on the y-axis, the following regression equation was observed: y = 0.9721x + 1.005 (r = 0.95), indicating no significant bias with the CMIA. However, when tacrolimus values obtained from reference laboratory B were used for comparison, the regression equation was y = 0.7664x + 1.775 (r = 0.93), indicating significant negative bias with the CMIA. When we compared tacrolimus concentrations obtained by reference laboratories A and B, we observed positive bias with tacrolimus concentrations obtained by reference laboratory B. However, tacrolimus concentrations obtained by both CMIA and Elecsys immunoassays were comparable. Conclusions Because of good correlation of tacrolimus concentrations using the CMIA and LC-MS/MS from reference laboratory A, our long-term reference laboratory for drug analysis, we concluded that the CMIA on the Alinity i can be used for therapeutic drug monitoring of tacrolimus.