The triple and quad screens are notoriously inaccurate. Something like 60% right? The screen will tell you out of x number of women what the likelihood is that you are the one that has a baby with a genetic defect. It does not mean that there is anything wrong. Your age and other factors can influence the test. The only way to know for sure of a genetic defect is amnio.
I had a quad screen with my first and the test came back with a very high number - meaning that my chances were very slim there was anything wrong. The Drs didn't even say anything about it because it was considered normal for a 26 year old. My son was born with Down Syndrome. I had many ultrasounds and nothing was ever detected. With my second I never requested the test knowing that even if it came back "negative" it didn't mean anything. I have heard soooo many stories about the test being "positive" for a defect and people worrying like crazy only to find out there was absolutely nothing wrong. I think it causes a lot of unnecessary stress and shouldn't be used.