Combination Tests Make for Best Down's Screening

NEW YORK (Reuters Health) - Along with factoring in the mother's age, two blood tests combined with a sonogram conducted in the first trimester is the most accurate way to screen for trisomy 21 (Down's syndrome), UK researchers advise.

The combination of tests includes looking for fetal nuchal translucency -- a characteristic sign seen on ultrasound -- and measuring maternal blood levels of beta-human chorionic gonadotrophin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A).

In a study of more than 30,000 pregnancies in which the mothers' average age was 34 years, the detection rate of trisomy 21 (and other major chromosomal defects) by this method, carried out at 11 to 13 weeks, was "about 90 percent, for a false-positive rate of 5 percent," the investigators report in the American Journal of Obstetrics and Gynecology for June.

The detection rate with the combination approach is similar to that achieved with second-trimester screening, but "with a more than 10-fold reduction in the number of unnecessary invasive tests," Dr. Kypros H. Nicolaides from King's College Hospital, London, and colleagues point out.

In the opinion of the authors, "screening for chromosomal defects in the first rather than in the second trimester provides earlier reassurance for those with a normal result and less traumatic termination for those choosing this option."

SOURCE: American Journal of Obstetrics and Gynecology, June 2005.