Princess is right. This is my first IUI round and my RE said that once we do the hCG trigger we need to count a minimum of 12 days before we test. For some women the hCG clears in as little as 3-4 day, but for other women it can hang around for up to a week and a half. Because of this, there's no way to reliably test until a minimum of 12 days past injection. If you have a quick implantation, it is possible to get a true BFP as early as 7dpo, but since you already have hCG in your system from the trigger, it is impossible to tell what's real and what's not.
My RE recommends waiting until 14-16 dpiui to test, so that if you implanted late you don't get a false negative and get needlessly stressed. Of course, my poas-addicted-self will be starting with the daily FRER's as soon as the 12 day wait is over. I'm such a junkie!