Sequential integrated antenatal screening for Down's syndrome, trisomy 18 and trisomy 13.

OBJECTIVE: To estimate the performance of antenatal sequential Integrated screening for Down's syndrome (DS), trisomy 18 (T18) and trisomy 13 (T13), in which women have first trimester testing for each disorder; those above specified risk cut-offs are screen-positive, and the remainder continue to Integrated testing, using first and second trimester screening markers together.

METHODS: Published screening marker parameters and Monte Carlo simulation were used to calculate detection rates (DR's) and risk cut-off levels for specified false-positive rates (FPR's), and DR's and FPR's for specified risk cut-offs. We compared this screening performance with that based on all women having Integrated tests.

RESULTS: Sequential Integrated DS screening detects 71% of DS pregnancies at the first trimester stage at a 0.5% FPR. For an overall 2% FPR, the DS DR is 92%, the same screening performance as the Integrated test performed on all women. Sequential Integrated T18 and T13 screening detects 70% of T18 and 53% of T13 pregnancies at the first trimester stage at a 0.05% FPR for each. The overall T18 and T13 DR's are 96% and 72% respectively at 0.2% FPR, the same screening performance as with Integrated tests performed on all women. Increasing the overall FPR's does not materially increase the DR's for any of the three disorders.

CONCLUSION: The performance of sequential Integrated screening is similar to the performance if all women have an Integrated test, but has the advantage of identifying most DS, T18, and T13 pregnancies a few weeks earlier.