title: Medians for second-trimester maternal serum markers: geographical differences and variation caused by median multiples-of-median equations.
abstract: OBJECTIVE
  To establish gestational age-specific mid-trimester normal medians for the prenatal serum markers alpha fetoprotein (AFP), human chorionic gonadotropin (HCG) and unconjugated oestriol (uE3) for a Belgian population by using the Beckman Coulter Access chemiluminiscent immunoassays; to compare these data with data obtained from other geographical regions; to propose regression coefficients for regressed medians and analyse variation induced by different regression equations; to evaluate the effect of formulas used for gestation correction on estimating risk in Down's syndrome.
  
  
  METHODS
  Data derived from 862 fresh serum samples from women being screened for Down's syndrome pregnancy, composed of selected pregnancies deemed to be normal, were examined in a retrospective study. Regressed medians were calculated by using a first-degree logarithmic-linear fit of the raw data. Multiples-of-median (MoM) values estimated by using a simple logarithmic-linear equation were compared with those calculated with higher-degree polynomials chosen with a goodness-of-fit analysis. Model-specific variation was estimated and the effect on risk for Down's syndrome was evaluated.
  
  
  RESULTS
  Regressed medians (Y) for Access serum markers AFP (IU/ml), HCG (IU/ml) and uE3 (nmol/l) for a Belgian population can be estimated with the equation Y = 10((A+BX)) with X = decimal weeks. The best fit was obtained with a third-degree and a second-degree polynomial for AFP and uE3, respectively. Differences between the medians and among the slopes of the geographical populations were found to be significant (analysis of covariance, p<0.001).
  
  
  CONCLUSIONS
  Belgian marker medians versus gestational time are found to show a pattern that is similar to that in the literature. The log-linear equation is observed to give a good fit and can be suggested as a tool for calculating median MoM values for Belgian laboratories that use Access biochemical prenatal markers.
authors.affiliation: Analis NV, Leeuwerikstraat, Gent, Belgium. gvn@analis.be
authors.forename: G
authors.initials: G
authors.lastname: Vranken
authors.forename: T
authors.initials: T
authors.lastname: Reynolds
authors.forename: J
authors.initials: J
authors.lastname: Van Nueten
mesh_terms: D001530:Belgium; D015415:Biomarkers / Q000097:blood*; D002170:Canada; D006063:Chorionic Gonadotropin / Q000097:blood; D004314:Down Syndrome / Q000175:diagnosis* / Q000208:ethnology; D004812:Epidemiologic Methods; D004964:Estriol / Q000097:blood; D005260:Female; D005315:Fetal Diseases / Q000175:diagnosis* / Q000208:ethnology; D005865:Gestational Age; D006801:Humans; D011247:Pregnancy; D011262:Pregnancy Trimester, Second / Q000097:blood; D011296:Prenatal Diagnosis / Q000379:methods*; D012016:Reference Values; D006113:United Kingdom; D014481:United States; D000509:alpha-Fetoproteins / Q000378:metabolism
chemical_list: D015415:Biomarkers; D006063:Chorionic Gonadotropin; D000509:alpha-Fetoproteins; D004964:Estriol