With so many false positives and negatives, does the quad marker blood test any real value, beyond random chance ?

Efficiency: It guides the search for down's and grave neural tube defects in women who might want to end such pregnancies; it will miss some, but the alternative is much more problematic testing for all pregnant women. You are playing the odds but unlike in las vegas, "the house" is trying to give the most favorable odds to the patient. But getting a false-positive is most upsetting & we can all relate.

Difficult issue: Trying to diagnose disease in a fetus is a difficult problem. Screening tests are set to pick up as many people as possible with a disease, as a result such screens generate a lot of false positives. Unfortunately there are still a few false negatives.