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The impact of temporal variability of biochemical markers PAPP-A and free β-hCG on the specificity of the first-trimester Down syndrome screening: a Croatian retrospective study

Background: The variability of maternal serum biochemical markers for Down syndrome, free β-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 β-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. Findings: 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 β-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.

Down syndrome screening; PAPP-A; free β-hCG; specificity; false-positive rate

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