Falsely elevated serum oestradiol due to exemestane therapy

In this study, we present a case of falsely elevated oestradiol (E 2 ) concentration, determined by two immunoassays, in a breast cancer patient receiving exemestane therapy. The positive bias of immunochemical measurements was revealed using liquid chromatography tandem mass spectrometry which showed undetectable E 2 concentration. The discrepancy is expected to be a consequence of the structural resemblance of E 2 and exemestane sharing the same steroidal backbone. Inaccurate laboratory findings in therapy monitoring, as in this case, may lead to unnecessary changes of therapy.