ABSTRACT
Tacrolimus is an immunosuppressive agent used to prevent post-transplantation rejection. Tacrolimus has a narrow therapeutic window and therefore, its whole blood concentration is measured for therapeutic drug monitoring. In this report, we present two cases of falsely elevated tacrolimus concentrations identified in recipients of solid organ transplants due to analytical interferences in the antibody-conjugated magnetic immunoassay (ACMIA) method used. Tacrolimus concentrations measured using ACMIA were 4- to 8-fold higher than the values obtained using liquid-chromatography-tandem mass spectrometry (LC-MS/MS) or chemiluminescent micro-particle immunoassay. The cause of this interference remains unknown, but the identification of a possible false elevation of tacrolimus is of paramount importance in clinical practice. Pre-treatment of samples by ethanol extraction or using alternative methods of tacrolimus measurement such as LC-MS/MS are necessary to obtain reliable results in the event of an analytical interference.