Interpreting Plasma Estrogen Levels in Breast Cancer: Caution Needed

Plasma estrogen levels have been measured in many research studies and have been used to direct clinical management in patients with breast cancer. When used with samples from postmenopausal women, most assays are not adequately accurate for this purpose, and unfortunately there are few available assays that do have the required accuracy and precision. Authors and reviewers have failed to recognize that using the wrong assays has led to clearly inaccurate data in some recently published studies. This situation needs to be recognized promptly to avoid reinforcing incorrect interpretations that may affect clinical care. Errors resulting from this inaccuracy can lead to real harm and may affect large numbers of women because of the prevalence of estrogen receptor–positive (ER ) breast cancer, which results in many patients receiving endocrine therapy. This article reviews areas in which estrogen measurements are made in relation to breast cancer and highlights the dangers of erroneous measurement or interpretation.