Estimates for the sensitivity and false-positive rates for second trimester serum screening for Down syndrome and trisomy 18 with adjustment for cross-identification and double-positive results.

Second trimester screening for fetal Down syndrome and trisomy 18 is available through separate protocols that combine the maternal age-specific risk and the analysis of maternal serum markers. We have determined the extent to which additional Down syndrome affected pregnancies may be identified through trisomy 18 screening, and the extent to which additional cases of trisomy 18 may be screen-positive for Down syndrome. The combined false-positive rate, taking into consideration those pregnancies that are screen-positive by both protocols, has also been determined. Sensitivity and false-positive rates were determined by computer simulation of results that incorporated previously published statistical variables into the model. Using second trimester risk cut-offs of 1:270 for Down syndrome and 1:100 for trisomy 18, it was found that few additional cases of Down syndrome are identified through trisomy 18 screening. However, approximately 6-10% of trisomy 18 affected pregnancies will be screen-positive for Down syndrome but screen-negative for trisomy 18. For women aged 40 or more, the false-positive rate for trisomy 18 exceeds 1% and approximately half of these cases will also be screen-positive for Down syndrome. For a population with maternal ages equivalent to that in the United States in 1998, after adjusting for the cross-identification, the sensitivity for three-analyte trisomy 18 screening is 78%. If this testing is performed in conjunction with Down syndrome "triple" screening, the Down syndrome sensitivity is 75% and the combined false-positive rate is 8.5%. If the three-analyte trisomy 18 screening is performed with the Down syndrome "quad" screen, the trisomy 18 sensitivity remains at 78%, the Down syndrome sensitivity is 79%, and combined false-positive rate is 7.5%. Sensitivity and false-positive rates are also provided for other widely used Down syndrome and trisomy 18 risk cut-offs. Sensitivity and false-positive rates that take into consideration cross-identification and double-positives should be helpful for pre-test counseling and the evaluation of serum screening programs.