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.

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.