Positive OPK and period late 13 days

Anyone had any experience with getting a BFP after having a positive opk? I am 13 days late for AF and have a blood test scheduled for Monday. For fun today, I tested my opk and it came out positive. I've read this may be a predictor for early pregnancy. I haven't taken a pregnancy test for a few days, but now am curious that if I do, it'll show up positive. Anyone have this be the case before?