P01.04: the Efficiency of Ultrasonographic Risk Assessment for Down Syndrome (URAD) and Calculated Final Risk for Down Syndrome (DS) Screening

To assess the efficiency of both URAD, and final risk assessment based on URAD, maternal age, and/or maternal serum screening for the detection of DS. Between 9/1/03 and 9/1/05, 1339 (22.3%) of 6000 patients at risk for DS were enrolled in our URAD screening program between 17–22 weeks gestation. Patients with a desire for a diagnostic test, a fetal anomaly, or declined testing were excluded from the URAD program. 81 patients (6%) failed to complete URAD due to inappropriate gestational age, unfavorable fetal position, maternal weight or inability to complete anatomic survey. URAD included 6 fetal markers. If any marker was noted, the URAD was considered positive. The markers used were: nuchal thickness (likelihood ratio [LR] = 11), echogenic bowel (LR = 6.7), short femur (LR = 1.5) or short humerus (LR = 5), echogenic intracardiac focus (LR = 1.8) and pyelectasis (LR = 1.5). If a single marker was noted, the initial risk, which was based on maternal age and/or maternal serum screening, was multiplied by the LR of that marker. When more than one marker was found, then the initial risk was multiplied by the highest LR or 6.2. The absence of a marker had a LR of 0.5. Eight patients in the study group had DS. The distributions of these patients are noted below on the basis of either the presence or absence of a URAD marker or if their final calculated risk was less than or greater than 1/270. The relative risk (RR) of having Down syndrome with a positive URAD is 10.6 with 95% confidence intervals (CI) of 2.6–43.9. The RR with a final risk of > 1/270 is 13.5 with 95% CI 1.7–109.6. Although the prevalence of DS in our study is low, both URAD and calculated final risk are efficient screening techniques. Final risk based on URAD, maternal age and/or maternal serum screening is the more efficient predictor of this condition over URAD alone.