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

Abstract

Unconjugated estriol (uE₃) is one of the most important serum markers for prenatal screening. The abnormally low content of uE₃ 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 uE₃ by employing microtiter plates with pre-captured primary antibodies. E₃-3-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 °C, 15 months) and thermostability (37 °C, 10 days) were excellent. The uE₃ 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 uE₃ medians for biochemical screening.