A direct competitive inhibition time-resolved fluoroimmunoassay for the detection of unconjugated estriol in serum of pregnant women

Unconjugated estriol (uE(3)) is one of the most important serum markers for prenatal screening. The abnormally low content of uE(3) is used as an indicator of fetal DS (Down syndrome) during the second trimester in pregnant women. In the present study, we developed a time-resolved fluoroimmunoassay to detect uE(3) by employing microtiter plates with pre-captured primary antibodies. E-3-3-CME-BSA (estriol-3-carboxymethyl ether-bovine serum albumin) conjugates served as labels and Eu3+ (europium) as the probe for signal detection. The detection limit of this assay was 0.35 nmol L-1. The within-run and between-run imprecision values for serum control detection were less than 5.0% and 6.0% respectively. The mean recovery was 102.6%. The long-term stability (2-8 degrees C, 15 months) and thermostability (37 degrees C, 10 days) were excellent. The uE(3) concentrations measured by the present assay in 1168 Chinese maternal serum samples correlated well with those obtained by the chemiluminescence immunoassay assay (r = 0.948). The reference range in normal singleton pregnancies in Southern China was established which provided reference data to adjust the uE(3) medians for biochemical screening.