Second trimester trisomy 21 maternal serum marker screening. Results of a countrywide study of 854,902 patients.

Objectives In France, maternal serum marker screening is governed by specific legislation. We conducted a study of the countrywide trisomy 21 screening based on second trimester maternal serum markers. Methods We reviewed the medical records of 854 902 patients prospectively screened for second trimester maternal serum markers in the 60 authorised laboratories over the two-year period 1997–1998. All patients screened in France were included. The risk of trisomy 21 was calculated from the combination of maternal age and maternal serum markers. The same cut-off (1/250) was used in all laboratories. Results In 1998, 65% of pregnant women underwent maternal serum screening. In the 837 765 patients under 38 years of age who were screened, 54 321 (6.48%; 5% CI 6.42–6.53%) had a calculated risk >1/250. Of the 884 Down syndrome cases observed, 626 were detected by maternal serum markers (70.8%; 5% CI 67.8–73.8%). These good results can be explained by a strict quality control of all steps. For the 13 891 patients over 38 years of age, the Down syndrome detection rate was 98.9% for a 34% false-positive rate. Conclusions Strict rules covering prenatal trisomy 21 screening are of benefit to patients, practitioners and laboratories alike, and ensure good quality control, a high trisomy 21 detection rate and a low amniocentesis rate. Copyright © 2002 John Wiley & Sons, Ltd.