OP23.02: Contingent testing, an alternative for regions lacking first trimester screening for Down's syndrome

late samples. Age standardised detection and false positive rates for different screening protocols were produced. Results: We collected two blood samples in 27 pregnancies affected by trisomy 21 and in 3891 control pregnancies. The early samples were taken between gestational week 8+0 up to week 13+6, and the late samples between week 11+3 up to week 14+6. The median interval between the samples was 17 days (range 1–40 days). We found a significantly better (P 0.05). Conclusions: Using early sampling with measurement of PAPPA and free β-hCG in the combined first trimester screening can optimise screening performance for trisomy 21. Using a double set of the maternal serum markers has the potential to further improve screening performance.