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

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.