Opk instead of pregnancy test question

Okay, so I know I should use a pregnancy test but I'm out of those but had an ovulation test left over. AF isn't due until tomorrow so I'll buy a pregnancy test tomorrow if AF doesn't show. My question, I know opk's pick up both the LH and HCG hormones so you can technically use one as a back up pregnancy test. Does it have to be positive like it would for Ovulation to be positive for pregnancy or just in general have a line?