Repeat maternal serum testing for down's syndrome screening using multiple markers with special reference to free α and free β-hCG

To determine the effect of routine repeat testing in serum screening for Down's syndrome, we compared estimates of the detection and false-positive rates. Five serum markers were measured-alpha-fetoprotein (AFP), unconjugated oestriol (uE₃), human chorionic gonadotrophin (hCG), and its two subunits, free α and free β-hCG. First and repeat test marker levels were available from 142 women whose samples had been routinely collected and stored in an antenatal serum bank. Different repeat testing policies were compared for various combinations of the markers. If all women had repeat tests using the four markers AFP, uE₃, and free α and free β-hCG, the detection rate for a 5 per cent false-positive rate was 69 per cent compared with 65 per cent if no women were repeated. Policies of repeating selected women gave similar results. The small gain in screening performance with repeat testing performed routinely is not worthwhile. If a woman does happen to have a repeat test, her risk estimate should, however, be based on both results, not just the second.