Flowchart showing screening performance for the common autosomal trisomies in the general obstetric population

At the top left corner are the DRs, FPRs, and the expected prevalence of the three common trisomies. Overall, the FPR is expected to be approximately 0.15%. The flowchart shows the results of screening 100,000 women from the general population with singleton pregnancies (United States 2015 birth records). For example, among the 100,000 women with screening results, there will be 294 cases of Down syndrome. Among these, 292 will be screen positive, and 2 will be screen negative. Among the 99,589 euploid pregnancies, there will be 50 false-positive calls. Thus, the PPV for Down syndrome screening will be 85% (odds of 6:1). The table below the flowchart summarizes the results of Down syndrome screening as well as screening for trisomies 18 and 13. In summary, the overall DR is approximately 98.5% for the common trisomies with a 0.15% FPR. The PPV is highest for Down syndrome, with lower PPVs for trisomy 18, trisomy 13, and overall (approximately 3:1). The false-negative cases include 2 Down syndrome, 3 trisomy 18, and 1 trisomy 13 pregnancy. This figure does not account for the reported higher rate of test failure associated with trisomies 18 and 13.