title: Outcome validation of the Beckman Coulter access analyzer in a second-trimester Down syndrome serum screening application.
abstract: BACKGROUND
  Mid-trimester maternal serum alpha-fetoprotein (AFP) and unconjugated estriol (uE3) are 30% lower and human chorionic gonadotropin (hCG) is twofold higher in Down syndrome pregnancies compared with unaffected pregnancies. In maternal serum screening, patient-specific risks are calculated using published gaussian frequency distribution parameters for these three markers obtained with previously available immunoassays. New immunoassays must generate similar distribution parameters if the accuracy of assigned risks and overall performance of prenatal screening are to be maintained.
  
  
  METHODS
  Agreement between the Beckman Coulter Access and the Bayer Immuno 1 assays for AFP and hCG and the Amersham Amerlex-M RIA for uE3 was assessed in 558 fresh sera. Precision was measured over 6 weeks. Median concentrations were calculated by regression of 568 Caucasian singleton pregnancy samples against gestational age in days. Frozen mid-trimester sera from 44 confirmed Down syndrome singleton pregnancies (cases) were selected without conscious bias for reanalysis, and each case was matched with five control specimens from unaffected pregnancies. Serum markers were expressed as the multiple of the median (MoM) concentration derived from the control samples, corrected for maternal weight and converted to their log-equivalent values. Normality was assessed using probability plots and the Shapiro-Wilk W-test. Gaussian distribution parameters were compared with established values, and Down syndrome risk calculations were assessed with a commonly used risk algorithm.
  
  
  RESULTS
  The Access AFP and hCG assays had consistent proportional agreement with the established assays, whereas agreement between the uE3 methods was less consistent. Analytical imprecision was 3-6% at mid-trimester concentrations. Normal distributions were obtained for the log MoM values of all three markers in both the Down syndrome and unaffected populations, and their gaussian distribution parameters compared well with established values. The performance of the Access assays in an established trivariate risk algorithm for Down syndrome was equal to the performance exhibited by traditional methods.
  
  
  CONCLUSIONS
  The Beckman Coulter Access analyzer provides valid mid-trimester serum AFP, uE3, and hCG results and risk assessments when applied in a prenatal Down syndrome screening service.
authors.affiliation: The Research Institute at Lakeridge Health Inc., 850 Champlain Ave., Oshawa, Ontario, L1J 8R2 Canada.
authors.forename: Andrew R
authors.initials: AR
authors.lastname: MacRae
authors.forename: H Allen
authors.initials: HA
authors.lastname: Gardner
authors.forename: Lynn C
authors.initials: LC
authors.lastname: Allen
authors.forename: Sonya
authors.initials: S
authors.lastname: Tokmakejian
authors.forename: Nathalie
authors.initials: N
authors.lastname: Lepage
mesh_terms: D015415:Biomarkers / Q000097:blood; D006063:Chorionic Gonadotropin / Q000097:blood; D004314:Down Syndrome / Q000097:blood / Q000175:diagnosis*; D004964:Estriol / Q000097:blood; D005260:Female; D006801:Humans; D007118:Immunoassay / Q000379:methods; D008403:Mass Screening / Q000379:methods; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D012372:ROC Curve; D016008:Statistical Distributions; D000509:alpha-Fetoproteins / Q000032:analysis
chemical_list: D015415:Biomarkers; D006063:Chorionic Gonadotropin; D000509:alpha-Fetoproteins; D004964:Estriol