AFP and age screening for Down syndrome.

The availability of maternal serum alpha-fetoprotein (AFP) values from neural tube defect screening programmes offers the opportunity to improve the effectiveness of screening for Down syndrome. By appropriately combining information on maternal serum AFP and maternal age fewer women would need an amniocentesis to detect a given number of Down syndrome pregnancies or, for a given number of women having amniocentesis, more cases of Down syndrome would be detected than if age alone were used. For example to detect 40% of pregnancies with Down syndrome using AFP and age, 7% of women would need an amniocentesis compared with 11% using age alone. If an amniocentesis and a chromosome analysis together cost $1,000 this is equivalent to a saving of $37,000 per 1000 women screened. The use of AFP as well as age in Down syndrome screening allows patients to be better informed of their risk of having an affected pregnancy. For example, a 35-year-old woman has a risk of a Down syndrome term pregnancy of about 1:380, the risk is 1:120 if the AFP level is 0.40 multiples of the normal median (MoM) and 1:1800 if it is 2.50 MoM. Software providing computer assisted test interpretation has been produced to facilitate the estimation of risk and to identify women with positive screening results.