generally a blood test should be taken 12-14 DPO at the least, but considering the time from ovulation to implantation (on average of course) waiting 3-5 days from implantation would have a detectable level of hCG.
Of course, one should take into consideration what the lab values are in terms of at what level do they consider the test to be positive. Some labs have it as low as 5 while others are at 30 (that's the line I have more experience with). Some will simply give a yes/no answer.
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