Ratio of nuchal thickness to humerus length for Down syndrome detection

OBJECTIVE: Ultrasonographic biometry markers are now being used clinically to adjust Down syndrome risk. The limitations are that the definitions of " abnormal" measurements used are arbitrary, thus reducing screening performa nce, and also that patient-specific Down syndrome risks cannot be calculate d. We report a new ultrasonographic algorithm that is sensitive for Down sy ndrome detection and that estimates individual risk. STUDY DESIGN: Overall in fetal populations with Down syndrome the humerus l ength is decreased, whereas the nuchal thickness is increased relative to t hat of a normal population. The nucha[ thickness/ humerus length ratio ther efore shows an even greater increase and magnifies the separation between D own syndrome and healthy groups. Prospective data were collected in midtrim ester amniocentesis cases. A regression equation for the median nuchal thic kness/humerus length ratio based on biparietal partial diameter was generat ed. The Down syndrome likelihood ratio, or the odds on the basis of the nuc ha[ thickness/ humerus length ratio (multiples of the median), was multipli ed by the age-related risk to give the posterior Down syndrome risk. Charts for rapid estimation of individual Down syndrome risk on the basis of mate rnal age and the nuchal thickness/humerus length ratio were constructed. RESULTS: There were 94 cases of Down syndrome and 4700 cases in which the k aryotype was normal. The mean (+/- SD) gestational age of the study populat ion was 16.1 +/- 1.6 weeks. Thirty-three fetuses with Down syndrome and 68 karyotypically normal fetuses had gross anomalies. The equation for the exp ected median nuchal thickness/humerus length ratio was as follows. 10(e)(1. 7163 - 0.0292) x BPD + 0.0003 x BPD2, where BPD is the biparietal[ diameter . In the overall study population the nuchal thickness/humerus length ratio and maternal age had a 79.8% detection rate at a 22.1% false-positive rate , compared with maternal age plus humerus length (sensitivity, 55.1%) or ma ternal age plus nuchal thickness (sensitivity, 66.7%) at the same false-pos itive rate. For women greater than or equal to 35 years old the values were 80% and 22.0%, respectively CONCLUSIONS: We report an ultrasonographic biometry algorithm that, in comb ination with maternal age, detects 79.6% of Down syndrome cases in a high-r isk group. Individual Down syndrome risk can be quickly calculated at the b edside and made available to women who desire this information before makin g a decision on amniocentesis. On the basis of published standards, ultraso nographic biometry as described would be a cost-effective alternative to am niocentesis in this high-risk group.