It depends when you implant, even if you are pregnant if you haven't implanted you wouldn't have produced enough HGC for the test to pick up,(even a blood test) Most women implant on day 9 but the range is from 8 - 12dpo. I implanted on the 9th day with all 3 pregnancy and got a reading on 9do, and a stronger reading at 10dpo. Everyone is different, another sign is that you usually don't get any symptoms until you have implanted although alot of people get sore boobs immediately.
Good luck!!!