Preterm labor question

I took a blood screening in second trimester after an accident for preterm labor, it was neg thank goodness. I'm now 33 weeks and having symptoms that I'm not really sure if it's the beginning of things starting. Does anyone know what time span that blood test is for? Does it predict the entire pregnancy or just a few weeks from the time the test was taken? I go see my high risk dr tomorrow but was curious and can't find anything when I google it.