EFFICACY OF THE BIPARIETAL DIAMETER FEMUR LENGTH RATIO TO DETECT DOWN-SYNDROME IN PATIENTS WITH AN ABNORMAL BIOCHEMICAL SCREEN

Abnormal fetal biometry is considered a marker for fetal trisomy. We p rospectively evaluated the biparietal diameter/femur length ratio to i dentify Down syndrome fetuses. This ratio was calculated when women (< 35 years old) underwent an amniocentesis for an abnormal biochemical s creen 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 99 % (244/264), positive predicitive value of 13% (3/23), negative predic tive value of 99% (244/247), and a relative risk of 10.8. Using our ow n 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 defect Down syndrome. A ratio >1.5 SD above the mean is a useful adjunct to identify Down synd rome in pregnancies at risk by an abnormal biochemical screen.