Positive and negative predictive values for combined first-trimester screening and cell-free DNA screening for the three common trisomies

The "general population" is defined as the maternal ages associated with live births in the United States in 2015 while the "high-risk population" is defined as all women ages 35 and older in that same population. The PPV is the proportion of true positives divided by the sum of true positive and false positives (TP/(TP+FP)). The NPV is the proportion of true negatives divided by the sum of true negative and false negatives (TN/(TN+FN)). The first-trimester "combined" test includes nuchal translucency measurements along with serum measurements, such as PAPP-A and hCG. The estimated detection and false-positive rates are 85% and 5%, respectively. The first-trimester prevalence of trisomies 21, 18, and 13 in the general population is 1:340, 1:1100, and 1:3500, respectively. The first-trimester prevalence of trisomies 21, 18, and 13 in the high-risk population (women ages 35 and older) is 1:100, 1:340, and 1:1200, respectively.