Falsely Elevated Tacrolimus Levels Caused by Immunoassay Interference Secondary to β‐Galactosidase Antibodies in an Infected Liver Transplant Recipient

Careful interpretation of tacrolimus levels is essential to ensure optimal immunosuppressive therapy while avoiding toxicity. Interference with tacrolimus assays may be an underreported event that has the potential to result in negative patient outcomes through unnecessary modifications of therapy. We describe a 55–year‐old liver transplant recipient who had falsely elevated tacrolimus levels that led to the eventual disruption of his immunosuppressive therapy and subsequent rejection of his allograft. Although his increased tacrolimus levels did not correlate with clinical signs and symptoms of tacrolimus toxicity, interruption of therapy in this patient was supported by an acute infection and a slight elevation in serum creatinine concentration. Tacrolimus levels were analyzed by using an antibody conjugated magnetic immunoassay method, and levels as high as 79.7 ng/ml were observed, despite discontinuation of tacrolimus. We conducted an evaluation for assay interference by using an alternative assay method (microparticle enzyme immunoassay), by testing plasma samples that were not hemolyzed, and by analyzing levels of an unrelated drug that uses the same technology as the initial tacrolimus assay. β‐Galactosidase antibodies were ultimately confirmed as the cause of the immunoassay interference. In patients receiving tacrolimus, spuriously high tacrolimus levels should be carefully evaluated, and drastic adjustments to therapy should be made only within the context of clinical toxicity.