Absent nasal bone: applying the right terminology and interpretation

Abstract

In a recent issue of this Journal, Sonek1 commented on the published studies of the association between an absent nasal bone and Down syndrome. He noted large differences in incidences of an absent nasal bone between chromosomally normal and abnormal fetuses. Furthermore, he stressed that these observations will have a major impact on the screening-test performance, in terms of improving Down syndrome detection rates and decreasing false-positive rates. I wish to make two comments. First, the evaluation of the nasal bone with ultrasound gives estimates of prevalence rather than incidence. Prevalence refers to the proportion of people in the target population with a certain condition/characteristic at a single point in time while incidence measures the occurrence of new cases during a specified period of time2. Given the high rate of attrition (i.e. spontaneous loss) of Down syndrome fetuses3 in early pregnancy and the strong association of an absent nasal bone with Down syndrome4, 5, it is expected that, apart from the ethnic variations, the prevalence of an absent nasal bone will be determined largely by the incidence of Down syndrome in the target population and the gestational age at the time of the ultrasound examination. Second, the addition of a new test (e.g. the evaluation of nasal bone with ultrasound) to already existing tests (e.g. first-trimester nuchal translucency scanning or biochemical tests) will improve both sensitivity and specificity, provided these tests are independent. From the published studies, however, it is not clear whether these tests are fully independent. In fact, if the assumption that these tests are completely independent is wrong, calculation of probability of Down syndrome from multiple tests would tend to overestimate the tests's value6. H. I. J. Wildschut*, * Erasmus University Medical Centre, Department of Obstetrics and Gynaecology, P.O. Box 2040, 3000 CA Rotterdam, The Netherlands