OP01.14: First trimester absent nasal bone to detect fetuses with Down syndrome. A Meta‐analysis

Abstract

Screening for Down syndrome is an essential part of routine prenatal diagnosis. There is an accumulating literature showing evidence of the significance of absent fetal nasal bone in the first trimester in screening for Down syndrome. To determine the accuracy of first trimester absent nasal bone in detecting Down syndrome in fetuses. Articles published between January 1980 and December 2007 identified through a search of entries in electronic databases and references cited in original studies. Studies were included if they recorded first trimester findings of nasal bone presence or absence, and clinical outcome of Down syndrome. A total of 19 studies describing 77,961 unaffected fetuses and 1040 fetuses with Down syndrome were included in the analysis. Articles were independently selected, reviewed, and abstracted by 2 reviewers. For each study, the positive likelihood ratio (LR), negative LR, sensitivity, and specificity were calculated. Overall estimates of pooled positive LR, negative LR, sensitivity, specificity, and pooled odds ratio with 95% confidence intervals were calculated for absent nasal bone. When the absence of nasal bone was observed in the first trimester, the odds of Down syndrome increased 48-fold (positive LR, 48; 95% CI 29–79). When the nasal bone was seen, the risk of a fetus having Down syndrome was halved (negative LR, 0.44; 95%CI 0.33–0.59). The pooled estimates of sensitivity for detecting fetuses with Down syndrome was 65% (95% CI 61%–69%) and specificity was 99% (95% CI 98%–99%). The diagnostic odds ratio for absent nasal bone was 112 (95% CI 66–187). The absence of nasal bone when observed in the first trimester is a useful tool in distinguishing Down syndrome fetuses. The overall sensitivity of absent nasal bone allows us to use it as a practical marker for Down syndrome.