Sequential Down Syndrome Screening: The Importance of First and Second TrimesterTest Correlations when Calculating Risk

Journal of Genetic Counseling
(Published online: 30 December 2005)

Authors:
Peter Benn (1) and Alan E. Donnenfeld (2)

Author Affiliations:

(1) Division of Human Genetics, Department of Genetics and Developmental Biology, University of Connecticut, Farmington, Connecticut
(2) Genzyme Genetics, 833 Chestnut Street Suite 1250, Philadelphia, 19107, Pennsylvania

Abstract:

Down syndrome screening can be particularly effective when both first and second trimester tests are performed. However, the counseling of women who have received sequential first and second trimester screening can be problematic. We evaluated an approximation where the post-test risk from the first trimester screening is used as the new a priori risk for the second trimester screening. The approximation disregards between-trimester test correlations. The Down syndrome detection rate based on the approximation (90.2%) would be close to that obtained when all correlations were considered (90.8%) but the false positive rate would be 26% higher (3.9% versus 3.1%, respectively). For any particular woman, the use of the approximation could result in highly inaccurate risks. We conclude that the correlations that exist between first and second trimester screening tests preclude the use of second trimester risks derived from the direct product of separate first and second trimester screening. Counseling issues in the delivery of sequential screening are discussed. Keywords first trimester screening - second trimester screening - Down syndrome - sequential screening.
Author Contact:

Alan E. Donnenfeld
Email: alan.donnenfeld@genzyme.com

(C) Journal of Genetic Counseling

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