title: Ultrasound screening of fetuses at increased risk for Down syndrome: how many missed diagnoses?
abstract: OBJECTIVE
  To determine the number of Down syndrome (DS) fetuses identified, amniocenteses performed, and procedure-related losses incurred when second-trimester ultrasound is used to screen high-risk patients in order to determine who should undergo an amniocentesis.
  
  
  METHODS
  A decision analytic model was designed for women at increased risk for a DS fetus due to either advanced maternal age (AMA) or a positive expanded maternal serum alpha fetoprotein (MSAFP) screening test, also known as a triple screen (+triple) test. The model compared: (1) the current standard of offering all at-risk women amniocentesis to (2) a policy of performing amniocentesis only when sonographic markers of DS are seen on ultrasound. Baseline assumptions included a sensitivity of 60.6% for targeted ultrasound and a screen-positive rate of 16.2%.
  
  
  RESULTS
  If targeted ultrasound is used to determine whether high-risk patients should undergo amniocentesis, 56% (3933/7025) of all DS fetuses are missed. The diagnosis of DS is missed most frequently among women at highest risk for DS (AMA and +triple). In this subgroup of patients, there are 3152 DS fetuses in the second trimester, of which 2815 (89%) are identified by triple screen test. When an ultrasound screen is added, only 1709 (54%) of DS fetuses will be identified.
  
  
  CONCLUSIONS
  Substantially fewer DS fetuses will be detected when targeted ultrasound is used as a screen among already identified high-risk patients.
authors.affiliation: Department of Obstetrics, Gynecology and Reproductive Sciences, University of California, San Francisco, 94080, USA. abcmd@berkeley.edu
authors.forename: Aaron B
authors.initials: AB
authors.lastname: Caughey
authors.forename: Deirdre J
authors.initials: DJ
authors.lastname: Lyell
authors.forename: A Eugene
authors.initials: AE
authors.lastname: Washington
authors.forename: Roy A
authors.initials: RA
authors.lastname: Filly
authors.forename: Mary E
authors.initials: ME
authors.lastname: Norton
mesh_terms: D003661:Decision Support Techniques; D003663:Decision Trees; D004314:Down Syndrome / Q000000981:diagnostic imaging*; D005260:Female; D005817:Genetic Counseling; D006801:Humans; D011237:Predictive Value of Tests; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal