If they're going a blood test, they're normally just looking to see that you have normal lab values. If you've had a virus, they're looking to see what your white count and antibodies and various blood-related elements of your immune system look like. Unless they do a specific toxicology or drug screen, they're not going to know you've been smoking weed. Same thing if they do a urinalysis, generally. They have to specifically test for the presence of drugs, and that's not normally a part of routine lab testing unless someone has requested a drug screen or unless they have some specific reason to be suspicious. For instance, if they were suspicious that you might have picked up HIV, they might do a drug screen on you to check for the presence of common IV drugs to see if that's where your HIV-risky behavior came in. I'm trusting you don't have HIV and haven't been an IV drug user, though, and that they're not worried about your HIV status. I'm going to move this thread to our Drug Testing area. Gave some consideration to Medical, but I think this fits best in the Drug Testing category.