You can potentially test as early as 9DPO and get a positive result, however if you test and it's negative you could be getting a false negaitve as for many women, this would be too early for HCG levels to be detectable. Many women don't have implantation until Day 10, and its a few days after implantation occurs that HCG becomes detectable on a HPT.
For a reliable results, its best to wait until at least the day your period is due.