Immunoassay of estradiol: unanticipated suppression by unconjugated estriol.

BACKGROUND: Accurate measurement of estradiol is important in clinical settings. The quality of laboratory estimations of estradiol may be assessed through external quality-assurance surveys.
METHODS: Estradiol was measured by microparticle enzyme immunoassay (MEIA) and other immunoassays. Proficiency testing of medical laboratories was conducted using samples prepared from normal male human serum supplemented with exogenous estradiol and other steroid and nonsteroid hormones, and participant laboratories measured estradiol by a variety of commonly used immunoassay techniques.
RESULTS: The imprecision (CV) for measurement of estradiol [100-300 ng/L (367-1102 pmol/L)] was </=22% for most analytical techniques. Greater imprecision, as high as 40% for the same concentration range, was observed for the (AxSYM) MEIA method in the proficiency testing event of September 2001. Results from this method were bimodal in distribution. We found that unconjugated estriol at concentrations >1.5 microg/L (>5.2 nmol/L) interfered with the MEIA method, leading to decreased recovery of added estradiol by up to 50%. This suppression in estradiol measurement was prevented by dilution of the specimen before measurement. Addition of unconjugated estriol gave a positive bias in some other immunoassay methods for estradiol. Poor comparability among the immunoassay methods for measurement of estradiol at clinically relevant concentrations [ approximately 60 ng/L (220 pmol/L)] was revealed.
CONCLUSIONS: A negative interference of unconjugated estriol with the MEIA method is a source of error for estradiol measurement. Lack of specificity and lack of comparability among immunoassay methods for estradiol may have detrimental effects on medical practice.