Maternal serum screening in Ontario using the triple marker test.

Objectives: To summarise the experience and evaluate the performance of the Ontario maternal serum screening (MSS) programme.

Setting: The Ontario MSS programme between October 1993 and September 2000.

Methods: This study used information collected in the Ontario MSS database, which contains data on each screened pregnancy. In the Ontario MSS programme, women are screened between 15 and 20 weeks of gestation. The risk cut-off for Down's syndrome was >or= 1 in 385 at term and women with a serum alpha-fetoprotein >or= 2.2 multiples of the unaffected population median were defined as screen-positive for open neural tube defects.

Results: Between 1 October 1993 and 30 September 2000, 428410 women residing in Ontario were screened for open neural tube defects, and 423895 women were screened for Down's syndrome and trisomy 18. Approximately 48% of all pregnant women in the province had MSS. The uptake rate of amniocentesis following a positive Down's syndrome screening was 67%. Of 717 cases of Down's syndrome ascertained in the screened population, 531 were detected by MSS, giving a term detection rate (DR) of 70.6%, with a false-positive rate (FPR) of 7.2%. For neural tube defects, the DR was 72.7%, with a FPR of 2.0%. The screen also detected 50% of cases of trisomy 18 at term, with a FPR of 0.2%. Coincidentally, 113 cases of chromosome aneuploidies other than Down's syndrome and trisomy 18 were detected.

Discussion: In the Ontario MSS programme, MSS performed as expected in the detection of Down's syndrome, open neural tube defects and trisomy 18. MSS is an effective and practical method for large-scale second trimester screening for Down's syndrome, open neural tube defects and trisomy 18, and the MSS database is an extremely useful tool in monitoring the performance of this screen.