Case Report: Falsely Elevated Tacrolimus Concentrations in a Kidney Transplant Recipient.

Tacrolimus (TAC) is a cornerstone of immunosuppressive therapy after kidney transplantation but has a narrow therapeutic window, requiring strict therapeutic drug monitoring. Several analytical methods are available for serum concentration measurement and methodological interference may lead to clinically misleading results. We report the case of a 67-year-old female kidney transplant recipient who presented with persistently elevated tacrolimus concentrations despite drug discontinuation. TAC concentrations were initially measured using the affinity column-mediated immunoassay (ACMIA), showing markedly supratherapeutic values that prompted repeated suspension of the medication. Given the discrepancy between laboratory results and TAC withdrawal, measurement was repeated using the microparticle enzyme immunoassay (MEIA), which demonstrated undetectable results, confirming false-positive values associated with ACMIA interference. TAC was safely reintroduced at a reduced dose, with subsequent concentrations remaining within the therapeutic range. During follow-up, the patient developed antibody-mediated rejection, which was successfully treated and maintained good graft function. This case highlights the importance of recognizing analytical limitations of TAC assays and emphasizes that unexpected results should prompt confirmatory testing using alternative methodologies. Integration of clinical judgment with laboratory data is essential to avoid inappropriate immunosuppressive management and to ensure graft safety.