Validation of a monoclonal unconjugated estriol antibody for use in prenatal maternal serum screening.

Objectives: Unconjugated estriol (uE3) is used as a marker for fetal aneuploidy in maternal serum screening tests. The goal of this study was to examine the validity of a new immunoassay for uE3 that uses a monoclonal antibody (m-uE3) rather than the more commonly used polyclonal antibody (p-uE3).

Setting: Assays were performed in the Special Chemistry laboratory at Women and Infants Hospital of Rhode Island.

Methods: Residual fresh (n = 100) and frozen (n = 533) second trimester serum samples from routine clinical care were tested using p-uE3 and/or m-uE3 assays. Assay results were compared between methods using Bland-Altman plots. A median equation was developed for m-uE3 results. Down syndrome risks were compared between the two assays in a case-control sample set (21 cases each matched with five controls for the completed week of gestation, duration of freezer storage and race).

Results: Log-transformed serum uE3 levels were highly correlated between the assays (r = 0.93, p < 0.001), with the m-uE3 assay levels yielding, on average, 23% higher (standard deviation of differences in log uE3 concentrations = 0.07) results. Assay and gestation-based median equations were calculated and used to convert m-uE3 concentrations to multiples of the median (MoM). The m-uE3 MoM values fit a log Gaussian distribution well with a log standard deviation of 0.11. Down syndrome risk results were not significantly different between assays.

Conclusions: The m-uE3 assay, with results expressed in MoMs, is suitable for screening and as a monoclonal-based assay offers the advantage of a predictable and indefinite supply of antibody to perform the assay.