Preterm labor test (Fetal fibronectin)

So on Friday I was in alot of pain in my back and kept getting period pains every 20 mins or so, I went to the hospital and I got a preterm labour test. To get a positive you need to be 50ml+ and that's when you'll go into labour soon. I had 21ml in my test so it came back negative for going into labour and put me in the 8% likely. After reading up on stuff online at 32 weeks I should be at 0ml so my question is had anyone else had an experience with these or had a reading like mine? I think I'll go into labour early now but I don't know when. 3rd pregnancy, first was term and second was 3 weeks early. Just a bit confused with the 21ml reading when it should be 0 and 50 is a positive for it coming.