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The "Genetic Sonogram": Comparison of the Index Scoring System with the Age-adjusted US Risk Assessment¹

¹ From the Department of Radiology, Division of Ultrasound (T.C.W.), and the Department of Obstetrics and Gynecology, Division of Perinatal Medicine (S.B.U.), University of Washington Medical Center, Seattle; and the Center for Perinatal Studies, Swedish Medical Center, Seattle, Wash (V.L.S., D.A.N.). Received May 5, 1999; revision requested July 15; revision received August 25; accepted August 30. Address correspondence to T.C.W., Department of Radiology, University of Wisconsin Hospital, E3/311 CSC Box 3252, 600 Highland Ave, Madison, WI 53792.

PURPOSE: To compare two ultrasonographic (US) methods for prenatal detection of fetal Down syndrome.

MATERIALS AND METHODS: Genetic amniocentesis was successfully performed in 3,303 consecutive women with high-risk pregnancies (mean gestational age, 17.1 weeks). All patients underwent a complete "genetic US" examination prospectively. Risk was assessed by using (a) various modifications of the index scoring system (ISS) and (b) the age-adjusted US risk assessment (AAURA).

RESULTS: The prevalence of Down syndrome in this population was 1.6% (53 of 3,303). By using a threshold of at least 2 points to detect trisomy 21, the best ISS had a sensitivity of 45.3%, false-positive rate of 4.9%, likelihood ratio of 9.3, and positive predictive value in the high-risk population in this study of 13.3%. Lowering the threshold to 1 point increased the sensitivity to 60.4% but increased the false-positive rate to 15.8%. Adding points for age increased the sensitivity to 67.9% but increased the false-positive rate to 24.3%. Results of using AAURA to detect trisomy 21 were nearly identical, with a sensitivity of 43.4% and false-positive rate of 4.9% at a 1 in 36 risk threshold and a sensitivity of 69.8% and false-positive rate of 26.1% at a 1 in 200 threshold. Trisomies 18 and 13 were detected with sensitivities of 80.0% and 100.0%, respectively, with either system.

CONCLUSION: The modified ISS and AAURA are equivalent in screening for Down syndrome, with detection of approximately half of all trisomy 21 fetuses at a 5% false-positive rate.

Index terms: Down syndrome, 856.87 • Fetus, abnormalities, 856.87 • Fetus, US, 856.1298, 856.86

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