Executive summary

Abstract

Objectives To identify the most effective, safe and cost-effective method of antenatal screening for Down's syndrome using nuchal translucency (NT), maternal serum and urine markers in the first and second trimesters of pregnancy, and maternal age in various combinations.Design A prospective study of women who booked for their antenatal care at about 8-14 weeks of gestation, with follow-up to identify pregnancies with Down's syndrome ascertained through second trimester screening or at birth.Setting Twenty-five maternity units (24 in the UK and one in Austria) offering second trimester Down's syndrome serum screening that agreed to collect observational data in the first trimester.Participants The results were based on 47,053 singleton pregnancies, including 101 pregnancies with Down's syndrome.Measurements and tests NT measurements were included if obtained between 9 and 13 weeks of pregnancy; serum and urine samples were also taken and stored. Another pair of serum and urine samples was collected in the second trimester and included if obtained between 14 and 20 weeks. Urine and serum samples from each affected pregnancy and five matched controls were tested for:Serum:alphafetoprotein (AFP)total human chorionic gonadotrophin (hCG)unconjugated oestriol (uE(3))pregnancy associated plasma protein A (PAPP-A)free beta-hCG.dimeric inhibin-A.Urine:invasive trophoblast antigen (ITA)beta-core fragmenttotal hCGfree beta-hCG.The matching criteria were gestation (using an ultrasound crown-rump length or biparietal diameter measurement), duration of storage, and centre. Screening performance of the individual markers and combinations of markers together with maternal age was assessed using standard methods. In addition pairs of first and second trimester serum samples from 600 controls were tested to secure a larger set in which screening performance could be determined using distribution parameters based on dates (time since first day of the last menstrual period).Main outcome measures The following were determined for different combinations of markers:efficacy (by assessing screening performance, focusing on the false-positive rate (FPR) for an 85% detection rate (DR))safety (focusing on the number of fetal losses due to amniocentesis (or chorionic villus sampling) in 100,000 women screened)cost-effectiveness (focusing on the cost of screening 100,000 women and the cost per Down's syndrome pregnancy diagnosed).Results Efficacy (screening performance) The false-positive rates for an 85% detection rate for the main screening tests are shown in the above table, in decreasing order of screening performance:With the serum integrated test, 10 weeks is the preferred time in pregnancy for the PAPP-A measurement. For the integrated test and the combined test, the timing of the measurement of the first trimester markers is less critical.Safety The lower false-positive rate with the integrated test compared with other tests means that at an 85% detection rate there would be nine diagnostic procedure-related unaffected fetal losses per 100,000 women screened compared with 44 using the combined test or 45 with the quadruple test.Cost-effectiveness Screening using the integrated test is less costly than might be expected because the extra screening costs tend to be offset by savings in the cost of diagnosis arising from the low false-positive rate. It was estimated that to achieve an 85% detection rate the cost to the UK NHS would be pound15,300 per Down's syndrome pregnancy detected. The corresponding cost using the second trimester quadruple test would be pound16,800 and using the first trimester combined test it would be pound19,000.Conclusions Implications for healthcare The results showed that screening performance in the first trimester of pregnancy was virtually the same as that in the second trimester, and in either it was much less effective than integrating screening measurements from both trimesters into a single test. In applying these results to screening practice several conclusions can be drawn. The following tests offer the most effective and safe method of screening:overall: the integrated testif an NT measurement is not available: the serum integrated testfor women who do not attend for antenatal care until the second trimester of pregnancy: the quadruple testfor women who choose to have a screening test in the first trimester: the combined test.At a constant detection rate, the cost-effectiveness of these four tests is broadly similar, any extra screening costs tending to be offset by fewer diagnostic costs. The evidence presented in this report does not support retaining the double test, the triple test, or NT measurements on their own (with or without maternal age) because each would lead to many more women having invasive diagnostic tests, without increasing the proportion of Down's syndrome pregnancies detected.