Down syndrome screening is complicated, because a huge number of different risk factors are taken into account. The lab I used to work for had an entire section devoted to calculating risks based on test results and demographics.
I’m vaguely familiar with this and am just going off what I’m reading, so if someone with more direct expertise shows up, I defer to them. First off, it looks like they’re using the triple test as a screening test, meaning you want to be sure you catch every single Down syndrome pregnancy that comes through (though the reality is that it only catches about 60%, according to that paper). The flip side of this is that a lot of normal pregnancies are also flagged. That’s why, if you look at the scheme in Figure 2, they go on to do a lot more testing to confirm if the triple test is positive. It should be seen as a flag to look further rather than a diagnosis of Down syndrome. 98% of those positive triple tests end up being free of Down syndrome. That’s how a screening test is supposed to work. It sucks that you scare a lot of people, but hopefully you don’t miss very many real cases.
The false positive thing seems confusing, I agree. I think they’re using it in a more technical manner. A “false positive” in this case doesn’t mean “it was positive, but they really didn’t have Down syndrome”. I think it means “we measured the levels of these hormones, and the numbers we saw gave a positive result, but the real hormone levels were within the negative range.” That is, the test itself screwed up due to inherent limitations. All clinical tests are required to measure these rates. You would do this by taking a whole bunch of samples that you know should register as negative and testing them, then seeing how many came up as positive.
What did your doctor talk to you about as far as follow-up testing? Because from what I understand, it definitely should not stop here.