Accuracy of Expected Risk of Down Syndrome Using the Second-Trimester Triple Test

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Abstract
Second-trimester maternal serum screening (MSS) for Down syndrome has been widely used in routine prenatal care in developed countries. The screening combines maternal age-specific risk of Down syndrome with risk estimation obtained by measuring maternal serum markers to assign women an expected risk of having a term Down syndrome pregnancy. Diagnostic tests were offered to women whose risk exceeded the risk cutoff determined by the screening program. The commonly used triple test, which involves the use of maternal age, serum α-fetoprotein, unconjugated estriol, and human chorionic gonadotropin, was expected to have a Down syndrome detection rate of 60–65% and false-positive rate of 5% (1). Although the expected screening performance has been achieved in many screening programs, the accuracy of individual risk calculated by a relatively complex computation based on a statistical model was not immediately obvious. Good agreement between the expected risk of Down syndrome and observed prevalence has been reported previously in several screening programs (2)(3)(4)(5). We evaluated the accuracy of expected risk of Down syndrome in a large provincial, multiple test center, MSS program in Ontario, Canada.