I can't answer from a doctor's perspective, but I have been to the phlebotomist quite a few times and have had my blood work done. No one ever mentioned THC or cannabis and I am quite certain that there was a good amount of THC in my blood during the blood draw. I would imagine, if they are not looking for it, they will not pull it up in a standard test.
No, the only way THC will show up in a blood test is if the doctor is testing for it specifically. Since a standard blood test is not looking for the presence of any THC none will be detected. Hope this helps!
In certain circumstances, you may be required to take a THC blood test - particularly if you are in a rehabilitation program or have an employer with strict anti-cannabis rules. However, please keep in mind your "standard blood test" will often not test for THC. It is usually a Complete Blood Count (CBC) panel that will provide diagnostic information on your liver, kidney, and blood cell status. The primary function of this test is to detect diseases.
Typically cannabis will stay in your system for about 1-2 days after consumption. However, if you are a heavier user, it can stay for up to 7 days. With that said, variables that can affect how long THC is present in the blood include metabolism, body mass index as well as frequency and amount of cannabis consumed. If you are going to have a THC blood test soon, I suggest abstaining from cannabis consumption for at least one week - two weeks to be safe! Please check out this article below that provides more information.
I hope this helps! :-)
Like others have mentioned here, THC from cannabis won’t show up in a “standard” blood test, so your employer, doctor, or whoever is requesting the THC blood test would have to be looking for it specifically. "Unlike urine tests, blood tests detect the active presence of THC in the bloodstream. In the case of smoked marijuana, THC peaks rapidly in the first few minutes after inhaling, often to levels above 100 ng/ml in blood plasma. It then declines quickly to single-digit levels within an hour. High THC levels are therefore a good indication that the subject has smoked marijuana recently. THC can remain at low but detectable levels of 1-2 ng/ml for 8 hours or more without any measurable signs of impairment in one-time users. In chronic users, detectable amounts of blood THC can persist for days."
Check out this article for more information re: THC blood tests. http://www.canorml.org/healthfacts/drugtestguide/drugtestdetection.html#blood