I was tested at 34 or 36 weeks (sorry cant remember and it was a quick swab taken during my Dr visit. (I did midwife care with a few Dr visits scattered in between). I tested positive and as a result was given anti-biotics every 4 hours during labour to prevent Isabelle getting sick. I was also told 30% of women test positive. Some hospitals dont do the test until later in pregnancy, as you may have tested positive at 36 weeks but may not have 2-3 weeks later, or some people may test negative at this test but still be positive at birth, so the whole timing of when the test is done is questioned here as well.
Not sure if that helps or not!