title: Fetal nasal bone assessment in first trimester down syndrome screening.
abstract: OBJECTIVE
  To evaluate the contribution of nasal bone assessment in the first trimester Down syndrome screening.
  
  
  METHODS
  The fetuses which underwent first trimester screening with nuchal translucency (NT) measurement were evaluated for the absence or presence of nasal bone according to the instructions described by the Fetal Medicine Foundation, London.
  
  
  RESULTS
  Among the 1,807 fetuses included in the study, 9 had trisomy 21. The detection rate of Down syndrome with NT measurement was 77.8% (7/9) with a false-positive rate of 4.5%. Incorporation of biochemical tests (PAPP-A, and free beta-hCG measurement) into the screening increased the detection rate to 88.9% (8/9) and decreased the false-positive rate to 3.6%. The prevalence of absent nasal bone was 7/1,798 (0.39%) in chromosomally normal fetuses, and 3/9 (33.3%) in Down syndrome fetuses. Sensitivity, specificity, positive predictive and negative predictive values of absence of nasal bone for trisomy 21 are 33.3% (CI: 0.12-0.64), 99.6% (CI: 0.99-0.99), 30% (95% CI: 0.11-0.53) and 99.7% (95% CI: 0.99-0.99), respectively. The positive likelihood ratio of absent nasal bone was 85.6 (95% CI: 26.2-279.5), and the negative likelihood was 0.67 (95% CI: 0.42-1.06). When nasal bone assessment was incorporated into the NT risk assessment or combined test, the detection rate of trisomy 21 was not changed, however, the false-positive rate decreased to 3.4 and 3%, respectively.
  
  
  CONCLUSIONS
  The absence of fetal nasal bone has a high positive likelihood ratio for Down syndrome in the first trimester screening, and the presence of nasal bone may potentially lower the need for invasive testing.
authors.affiliation: Department of Obstetrics and Gynecology, Istanbul Medical Faculty, Istanbul University, Istanbul, Turkey. recephas@superonline.com
authors.forename: Recep
authors.initials: R
authors.lastname: Has
authors.forename: Ibrahim
authors.initials: I
authors.lastname: Kalelioglu
authors.forename: Atil
authors.initials: A
authors.lastname: Yuksel
authors.forename: Lemi
authors.initials: L
authors.lastname: Ibrahimoglu
authors.forename: Hayri
authors.initials: H
authors.lastname: Ermis
authors.forename: Alkan
authors.initials: A
authors.lastname: Yildirim
mesh_terms: D000328:Adult; D018568:Crown-Rump Length; D004314:Down Syndrome / Q000175:diagnosis / Q000000981:diagnostic imaging*; D005260:Female; D005865:Gestational Age; D006801:Humans; D016013:Likelihood Functions; D009295:Nasal Bone / Q000000981:diagnostic imaging* / Q000196:embryology; D048208:Nuchal Translucency Measurement; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D011296:Prenatal Diagnosis / Q000379:methods; D012680:Sensitivity and Specificity