title: First-trimester contingent screening for Down syndrome can reduce the number of nuchal translucency measurements required.
abstract: BACKGROUND
  To assess the performance of a two-stage screening protocol for Down syndrome based on initial serum marker analysis for all women and nuchal translucency (NT) measurement only in women with intermediate risks.
  
  
  METHODS
  Biochemical marker and NT data in 10 189 women who had had combined ultrasound and biochemical (CUB) screening, were re-analysed using the contingent model. A risk was calculated from the results of the pregnancy-associated plasma protein A (PAPP-A) and free beta human chorionic gonadotrophin (FbetahCG) measurements and maternal age. For risks between 1 in 42 and 1 in 1000, the likelihood ratio from the NT measurement was incorporated and assessed against a final cut-off risk of 1 in 250.
  
  
  RESULTS
  A total of 3.1% unaffected and 61.4% Down syndrome pregnancies had risks>or=1:42. In women with risks<1 in 42 and >1 in 1000 (29%), a further 2.7% unaffected pregnancies and 27.3% Down syndrome pregnancies had risks above 1 in 250 when NT was incorporated. Overall detection rate was 88.6%, and false positive rate 5.8% (compared with 90.9% and 6.4% for CUB screening). NT measurements were required in 29% of women.
  
  
  CONCLUSIONS
  Within first-trimester, contingent screening provides good sensitivity and specificity with the potential for considerable saving in ultrasound resources.
authors.affiliation: Biochemical Genetics Department, Institute of Medical Genetics, Yorkhill, Glasgow G3 8SJ, UK.
authors.forename: Thenmalar
authors.initials: T
authors.lastname: Vadiveloo
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: Crossley
authors.forename: David A
authors.initials: DA
authors.lastname: Aitken
mesh_terms: D000328:Adult; D015415:Biomarkers / Q000097:blood; D018997:Chorionic Gonadotropin, beta Subunit, Human / Q000097:blood; D015331:Cohort Studies; D004314:Down Syndrome / Q000097:blood* / Q000000981:diagnostic imaging*; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D008423:Maternal Age; D048208:Nuchal Translucency Measurement; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D011266:Pregnancy-Associated Plasma Protein-A / Q000032:analysis; D012680:Sensitivity and Specificity
chemical_list: D015415:Biomarkers; D018997:Chorionic Gonadotropin, beta Subunit, Human; D011266:Pregnancy-Associated Plasma Protein-A