If you're just having a routine blood test at a doctor's office, a drug screen isn't even going to be a part of the labwork anyway, unless someone requests a drug screen be done on you or you give the doctor some reason to suspect you have a drug problem.

This link from Erowid will give you some basic information about drug testing and cannabis, but again, it would be almost unheard of for that to be a part of a routine order for labwork for a routine checkup.
Erowid Cannabis Vault : Drug Tests

If you got your tonsils removed, you'd need to ask your doctor how long you should give up smoking of both cannabis or cigarettes. In my area, most docs these days won't touch someone on the operating table who hasn't been clean of cigarettes for at least three weeks, ideally longer. And you need to not smoke cigarettes after surgery for about that same length of time to allow yourself time to heal. Cannabis-smoking might be a little different, but ideally you should be clean of it, too, both before and after surgery so your lungs will be nice and clear--and afterwards so your healing will proceed as quickly as possible.