Blood test results??

Mary
Hey ladies - question for you all. Anyone have an idea how early blood tests can pick up pregnancy? My doc gave me one at 3 dpo and said it could pick up the HCg possibly. Has anyone had this experience? I didn't think this could happen until implantation. 
It came back negative but I'm wondering if I should lose all hope this Round. Any comments are greatly appreciated. 
Thanks :)