sounds like you could be experiancing inplantational bleeding, I had that with my daughter. I did not test positve with her until I was 3 days late...this time around I got a positve a few days before my af was due so it really varies. I say hold off as long as you can then test. For most of us that is before af :)
It depends on the baby. From pregnancy to pregnancy and woman to woman you can have different levels of hcg (what the baby gives off) at the same time in a pregnancy. I took a first response at 11 dpo and got a faint positive. I have heard first response (the hpt with the pink lines) is the most accurate. If I were you I'd wait until the day or so after my missed period. If you go too soon to the doctor, you'll worry yourself silly like I did with the hcg quantitative blood tests (esp if you aren't a 28 day cycle, which I wasn't) They take blood a few days apart to measure the hcg hormone, and that will tell them if the baby is growing right. Wait until after your missed period and take a first response!
Welcome!! The earliest you can take a hpt is whenever you want :) BUT the closer to AF arriving the more accurate. Some women don't get a positive until a week after AF was due. Good luck and keep us posted :)