Limitations of direct immunoassays for measuring circulating estradiol levels in postmenopausal women and men in epidemiologic studies.

Serum estradiol (E(2)) serves as an important diagnostic marker in a variety of clinical conditions. In epidemiologic studies, E(2) is commonly used to define the etiologic role of estrogen in hormone-related cancers and chronic conditions. Having an accurate and reliable E(2) assay is of critical importance in these studies, especially when measuring the very low E(2) levels (<30 pg/mL) common in postmenopausal women and men, and for discerning the relatively small (usually <20%) case-control differences in E(2) levels. Because E(2) is metabolized to >100 metabolites in the body, some of which cross-react with E(2) antibodies, direct RIAs without purification steps lack specificity for E(2) and can substantially overestimate E(2) levels. Although direct E(2) RIAs using commercial kits are simpler, less time consuming, and less expensive and require less sample volume than conventional RIAs with preceding purification steps, their lack of sensitivity and specificity makes them invalid for measuring circulating E(2) levels in epidemiologic studies of postmenopausal women or men. Instead, we recommend the use of a well-validated RIA with purification steps to improve sensitivity and specificity and to help achieve the necessary accuracy and reliability needed for epidemiologic studies.