title: [Specificity of biochemical markers of pregnancy second trimester].
abstract: BACKGROUND
  During the second trimester of 21 trisomy pregnancies, median of maternal serum concentrations of alpha-fetoprotein, not conjugated estriol, beta fraction, and inhibin A differ from normal values, and it can be used to select high risk patients.
  
  
  OBJECTIVE
  To evaluate specificity of different forms of second trimester biochemical markers association in a group of healthy pregnant women and determine levels considered as low risk.
  
  
  METHODS
  Random and cross-sectional study in 363 pregnant women with prenatal diagnosis to evaluate specificity of different combinations of biochemical markers. Down's syndrome and defects of neural tube rates of detection were defined as the percentage of pregnancies with tests of detection of true negatives and false positives.
  
  
  RESULTS
  Total beta fraction has differences during weeks 13 to 14, with 70% of cases between 23,000 to 70,000 units, with 1.38% of false positives; alpha-fetoprotein fluctuated between 22 to 30 units, with 5.2% of false positives; conjugated estriol increases between 1.2 to 2.2 units during 2nd quarterly, with 6.3% of false positives; and inhibin has an increase, with 70% of cases located between 125 to 486.5 units, with 10.44% of false positives.
  
  
  CONCLUSIONS
  Marker formula with greater specificity was type IV, included alpha-fetoprotein, not conjugated estriol and beta fraction. In the isolated form, alpha-fetoprotein marker demonstrates greater specificity.
authors.affiliation: Centro Médico del Noroeste. drsalazar@hotmail.com
authors.forename: Roberto
authors.initials: R
authors.lastname: Salazar López
authors.forename: Ana Lidia
authors.initials: AL
authors.lastname: Ibarra Gallardo
authors.forename: Manuel
authors.initials: M
authors.lastname: Iduma Meléndrez
authors.forename: Rosario
authors.initials: R
authors.lastname: Leyva Bojórquez
mesh_terms: D000328:Adult; D015415:Biomarkers; D003430:Cross-Sectional Studies; D004314:Down Syndrome / Q000175:diagnosis*; D004964:Estriol / Q000097:blood*; D005189:False Positive Reactions; D005260:Female; D005865:Gestational Age; D006801:Humans; D007265:Inhibins / Q000097:blood*; D009436:Neural Tube Defects / Q000175:diagnosis*; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D011296:Prenatal Diagnosis; D012680:Sensitivity and Specificity; D000509:alpha-Fetoproteins / Q000032:analysis*
chemical_list: D015415:Biomarkers; D000509:alpha-Fetoproteins; D007265:Inhibins; D004964:Estriol