Immunoassay of Estradiol: Unanticipated Suppression by Unconjugated Estriol

Background: Accurate measurement of estradiol is im-portant in clinical settings. The quality of laboratory estimations of estradiol may be assessed through exter-nal quality-assurance surveys. Methods: Estradiol was measured by microparticle en-zyme immunoassay (MEIA) and other immunoassays. Proficiency testing of medical laboratories was con-ducted using samples prepared from normal male hu-man serum supplemented with exogenous estradiol and other steroid and nonsteroid hormones, and participant laboratories measured estradiol by a variety of com-monly used immunoassay techniques. Results: The imprecision (CV) for measurement of es-tradiol [100–300 ng/L (367–1102 pmol/L)] was <22 % for most analytical techniques. Greater imprecision, as hig