title: Down syndrome screening using first-trimester combined tests and contingent use of femur length at routine anomaly scan.
abstract: OBJECTIVE
  The objective of this study was to evaluate the performance of the contingent use of femur length (FL) at routine mid-trimester scan in screening for Down syndrome (DS) in women having previously undergone first-trimester screening with disclosure of risk estimates.
  
  
  METHODS
  Data from a prospective screening trial for DS in a population of 21 492 women with 80 observed DS were used. The performance of a contingent screening strategy based on adding short FL (FL < 5th percentile) as a soft marker in women at intermediate first-trimester risks was evaluated through simulated data.
  
  
  RESULTS
  In our population, the median (25th-75th percentile) maternal age was 30.7 years (28.0-33.9; range: 18.0-46.3). The median (25th-75th percentile) gestational age at ultrasound examination was 12 weeks 3 days (12 weeks and 12 weeks 6 days; range: 11 weeks to 13 weeks 6 days). Contingent screening allowed an improvement in screening performance. For example, using a first-trimester risk cut-off of 1/100 and an intermediate-risk population within (1/1000, 1/100) for the search of FL, a sensitivity (Se) of 88.4% at a 3% false-positive rate (FPR) was reached. With a cut-off of 1/200 and an intermediate-risk population within (1/1000, 1/200), screening would allow an Se of 92.3% at a 4% FPR.
  
  
  CONCLUSIONS
  Contingent screening could be used following first-trimester combined screening followed by second-trimester ultrasound soft markers. This could identify indications for early invasive testing in the highest risk cases and would allow efficient and simple ultrasound-based screening in the second trimester. This would provide an 88.4% Se for a 3% FPR, at no additional cost as compared to first-trimester combined screening and routine mid-trimester scan.
authors.affiliation: Department of Obstetrics and Gynecology, University Paris Descartes, Necker-Enfants Malades Hospital, Assistance Publique-Hôpitaux de Paris, Paris, France. laurentsalomon@gmail.com
authors.forename: Laurent J
authors.initials: LJ
authors.lastname: Salomon
authors.forename: Sylvie
authors.initials: S
authors.lastname: Chevret
authors.forename: Laurence
authors.initials: L
authors.lastname: Bussieres
authors.forename: Yves
authors.initials: Y
authors.lastname: Ville
authors.forename: Patrick
authors.initials: P
authors.lastname: Rozenberg
mesh_terms: D000293:Adolescent; D000328:Adult; D015331:Cohort Studies; D004314:Down Syndrome / Q000000981:diagnostic imaging*; D005189:False Positive Reactions; D005260:Female; D005269:Femur / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D006801:Humans; D008875:Middle Aged; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D011446:Prospective Studies; D018570:Risk Assessment; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal / Q000379:methods*; D055815:Young Adult