Pregnyl will leave your body at a rate of about 1000iu per day, so if your trigger was 5000iu, you can test on day 6 or 7 after EPU (though that's waaaaay too early to get a BFP, so why would you?) If you're using pregnyl for luteal support, then I believe it's usually 1500iu per shot, so you can theoretically test on the third day after each shot.

And I know of a former LTer who got a BFN on a HPT the very day she got a +ve on her BT, and she has a gorgeous little boy now from that cycle, so don't always believe that evil pee stick!