Efficacy of the biparietal diameter/femur length ratio to detect Down syndrome in patients with an abnormal biochemical screen.

Abstract

Abnormal fetal biometry is considered a marker for fetal trisomy. We prospectively evaluated the biparietal diameter/femur length ratio to identify Down syndrome fetuses. This ratio was calculated when women (< 35 years old) underwent an amniocentesis for an abnormal biochemical screen for Down syndrome. Using reported ratio cut-offs (> 1.5 SD above the mean), the ratio had a sensitivity of 50% (3/6), specificity of 92% (244/264), positive predictive value of 13% (3/23), negative predictive value of 99% (244/247), and a relative risk of 10.8. Using our own population ratio, a cut-off > 1.5 SD had a sensitivity of 50% (3/6), specificity of 94% (249/252), positive predictive value of 17% (3/18), negative predictive value of 99% (249/252) and a relative risk of 13.9. A lower cut-off decreased the efficacy to detect Down syndrome. A ratio > 1.5 SD above the mean is a useful adjunct to identify Down syndrome in pregnancies at risk by an abnormal biochemical screen.