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

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

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

* The differences are not significant (Data are referring to calculated Down syndrome biochemical risk)

ISSN: 1756-0500