Do race-specific definitions of short long bones improve the detection of down syndrome on second-trimester genetic sonograms?

Objective. The purpose of this study was to determine whether the use of race-specific definitions of short femur and humerus lengths improves Down syndrome detection. Methods. This was a retrospective cohort study over 16 years. For each self-reported maternal race (white, African American, Hispanic, and Asian), we evaluated the efficiency of Down syndrome detection using published racespecific formulas compared with a standard formula for short femur and humerus lengths (observed versus expected lengths ≤0.91 and ≤0.89, respectively). The sensitivity, specificity, and 95% confidence intervals for each parameter were compared. Screening performance was compared by areas under the receiver operating characteristic curves. Results. Of 58, 710 women, 209 (0.3%) had a diagnosis of a fetus with Down syndrome. Although the race-based formula increased sensitivity in each population, the increase was statistically significant only in the white population, whereas a decrease in specificity was statistically significant in all 4 populations, as denoted by nonoverlapping confidence intervals. The area under the receiver operating characteristic curve for the model using the racespecific definition of short femur length was 0.67 versus 0.65 compared with the standard definition, and for humerus length it was 0.70 versus 0.71. Conclusions. The use of race-based formulas for the determination of short femur and humerus lengths did not significantly improve the detection rates for Down syndrome.