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Table 6 Comparison of the false positive rates (FPR) in calculated biochemical risk in particular weeks

From: 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

Group Week of gestation Risk for trisomy 21
(Cut-off ≤1:300)
N/Total (%)
Risk for trisomy 18
(Cut-off ≤1:100)
N/Total (%)
Between-weeks' difference *
1 10 to 10+6/7 15/96 (16.1%) 2/96 (2.2%) /
2 11 to 11+6/7 109/853 (12.8%) 17/853 (2.0%) Group 1 vs. Group 2 χ2=0.402, p = 0.52
3 12 to 12+6/7 158/1326 (11.9%) 15/1326 (1.1%) Group 2 vs. Group 3 χ2=0.313, p = 0.58
4 13 to 13+6/7 60/608 (9.9%) 10/608 (1.6%) Group 3 vs. Group 4 χ2=0.813, p = 0.37
  1. * The differences are not significant (Data are referring to calculated Down syndrome biochemical risk)