Elevated testosterone on immunoassay in a patient with metastatic prostate cancer following androgen deprivation therapy and bilateral orchiectomy

We present the case of an 83-year-old man with metastatic prostate cancer who had testosterone levels reading above castration range despite appropriate medical and surgical castration. Mass spectrometry was performed to confirm presence of testosterone, but no testosterone was detected. The elevated testosterone as measured by standard immunoassay was postulated to be secondary to heterophile antibodies in the patient's serum. This report highlights the need for a high index of suspicion for interference in testosterone immunoassays when levels remain mildly elevated. Mass spectrometry may provide a more reliable method by which to detect testosterone concentration prior to escalation of care.