title: The impact of temporal variability of biochemical markers PAPP-A and free beta-hCG on the specificity of the first-trimester Down syndrome screening: a Croatian retrospective study.
abstract: BACKGROUND
  The variability of maternal serum biochemical markers for Down syndrome, free beta-hCG and PAPP-A can have a different impact on false-positive rates between the 10+0 and 13+6 week of gestation. The study population comprised 2883 unaffected, singleton, spontaneously conceived pregnancies in Croatian women, who delivered apparently healthy child at term. Women were separated in 4 groups, dependently on the gestational week when the analyses of biochemical markers were performed. The concentrations of free beta-hCG and PAPP-A in maternal serum were determined by solid-phase, enzyme-labeled chemiluminiscent immunometric assay (Siemens Immulite). Concentrations were converted to MoMs, according to centre-specific weighted regression median curves for both markers in unaffected pregnancies. The individual risks for trisomies 21, 18 and 13 were computed by Prisca 4.0 software.
  
  
  RESULTS
  There were no significant differences between the sub-groups, regarding maternal age, maternal weight and the proportion of smokers. The difference in log10 MoM free beta-hCG values, between the 11th and 12th gestational week, was significant (p = 0.002). The difference in log10 MoM PAPP-A values between the 11th and 12th, and between 12th and 13th week of gestation was significant (p = 0.006 and p = 0.003, respectively). False-positive rates of biochemical risk for trisomies were 16.1% before the 11th week, 12.8% in week 12th, 11.9% in week 13th and 9.9% after week 13th. The differences were not statistically significant.
  
  
  CONCLUSIONS
  Biochemical markers (log10 MoMs) showed gestation related variations in the first-trimester unaffected pregnancies, although the variations could not be attributed either to the inaccuracy of analytical procedures or to the inappropriately settled curves of median values for the first-trimester biochemical markers.
authors.affiliation: Laboratory of Endocrinology, Clinics of Oncology and Nuclear Medicine, University Hospital "Sestre milosrdnice", Zagreb, Croatia. tislaric@kbsm.hr.
authors.forename: Dubravka
authors.initials: D
authors.lastname: Tislarić-Medenjak
authors.forename: Ivana
authors.initials: I
authors.lastname: Zec
authors.forename: Ana-Maria
authors.initials: AM
authors.lastname: Simundić
authors.forename: Senka
authors.initials: S
authors.lastname: Sabolović-Rudman
authors.forename: Milan
authors.initials: M
authors.lastname: Kos
authors.forename: Zeljka Bukovec
authors.initials: ZB
authors.lastname: Megla