QQbaby1,
Got this from a website...
The blood test measures:
• free beta- hCG (human chorionic gonadotrophin)
• PAPP-A (pregnancy associated plasma protein A)
A woman who is carrying a baby with Down's syndrome will have abnormal levels of these two substances in her blood.
The NT scan alone probably picks up about 75 to 82 per cent of babies with Down's syndrome, and the blood test alone, about 60 to 70 per cent. If you put the two tests together, the detection rate is usually in the range of 90 to 93 per cent with about a 3 to 5 per cent false positive rate.
Hope this helps...
For me, they took many tubes of blood to test for other things as well. The nurse rattled off a whole list of diseases/conditions that she said are mandatory by law for testing, like hepatitis B, HIV, iron levels, immunity to rubella etc etc. Think maybe for others, these tests could have been done earlier in the pregnancy alr?