Day 21 testing?

Alana

So I've heard that day 21 testing is for women who have regular 28 day cycles who most likely O around dah 14. Well I got my positive on day 17 and still went in for day 21 testing.. so now I'm wondering if I went in to early and that's why my numbers were so low... I was reading that your supposed to go in 7 days after your positive.. Tia