A while ago I read a fic where Sam figured out something was going on and did some tests to try and find an angel inside him or Dean.

I remember that Sam needed a lot of his blood to cast a spell and he only had a tiny little needle, the kind that diabetics use to administer insulin, and so he had to keep sticking it in and filling it again and again. When the spell didn't work, because Sam thought the angel was in Dean, Sam tries to cast it again and passes out from the loss of blood.

I think that Kevin may have featured in the fic too, I think he may have been the one who found Sam passed out in his room. I also remember the fic being Gen, but I might be wrong on that account.

FOUND: Angel Unware on ff.net