Maternal serum invasive trophoblast antigen and first-trimester down syndrome screening

Abstract

Background: In the United States, Down syndrome screening is still performed mainly in the second trimester, using 3 or 4 markers. Moving screening into the first trimester has the advantage of earlier diagnosis. Currently, first-trimester screening typically includes maternal serum pregnancy-associated plasma protein-A (PAPP-A), the free β-subunit of human chorionic gonadotropin (free β), and ultrasound measurement of nuchal translucency thickness (NT). The current report describes a case-control study of serum invasive trophoblast antigen (ITA) and its possible inclusion in first-trimester screening for Down syndrome. Methods: As part of an earlier observational study, serum samples from 54 Down syndrome and 276 matched unaffected pregnancies were collected between 9 and 15 weeks of gestation. Samples had been aliquoted and stored at -20 °C for 8 years. ITA was measured and converted to weight-adjusted multiples of the median (MoM). The distributions of other first-trimester markers are from a single published study. Results: Median ITA MoM in Down syndrome pregnancies increase as gestational age increases (2.02 MoM at 11 and 2.44 MoM at 13 completed weeks). At 75% detection, maternal age in combination with ITA and PAPP-A measurements have an 8.0% false-positive rate, slightly lower than the 8.8% found for the free β and PAPP-A combination; adding NT measurements reduces false positives for the 2 combinations to 2.0% and 1.8%, respectively. Conclusion: Serum ITA appears to be a useful first-trimester Down syndrome marker that could replace free β measurements while maintaining performance. © 2005 American Association for Clinical Chemistry.