Received on June 4, 2004
Accepted on July 26, 2004

General Clinical Chemistry

Maternal Serum Invasive Trophoblast Antigen As a Screening Marker for Down Syndrome during the Second Trimester

¹ Foundation for Blood Research, Scarborough, ME
² Quest Diagnostics Nichols Institute, San Juan Capistrano, CA

^* To whom correspondence should be addressed. E-mail: palomaki{at}fbr.org.

Background: Approximately two million pregnancies in the United States are screened for Down syndrome annually by use of second-trimester maternal serum markers. At present, a combination of four markers can identify 75% of affected pregnancies when 5% of screened women are classified as candidates for amniocentesis. Although not currently included in screening panels, invasive trophoblast antigen (ITA) is a promising screening marker in serum or urine in both the second and first trimesters. This study aims at better defining the screening performance of serum ITA in the second trimester.

Methods: In an earlier study, serum samples from an unbiased sampling of 45 Down syndrome (cases) and 238 unaffected (control) pregnancies between 14 and 20 weeks of gestation were collected from various centers in the United States. Samples were aliquoted and stored at -20 °C for 8 years. We measured ITA in these samples and determined the screening performance both univariately and in combination with other screening markers.

Results: The median ITA in Down syndrome pregnancies was >3.00 multiples of the median, higher than that found for human chorionic gonadotropin (hCG) or free -hCG. At a 5% false-positive rate, ITA univariately detected 38% and 40% of Down syndrome pregnancies, respectively, when assigned by date of last menstrual period or ultrasound date. Modeling yielded rates of 45% and 48%. ITA correlated strongly with hCG and free -hCG. When substituted for either of these in a multiple marker panel, ITA performed comparably.

Conclusions: This study indicates that serum ITA is an effective marker for Down syndrome. It is highly correlated with both hCG and free -hCG and could replace either of them in a multiple marker panel.