I'm going to a medical consultation early tomorrow (circa 5:45 AM) where might be a blood test. I was invited for a party tonight where I would drink and smoke marijuana (marijuana is illegal in my country). Is it a bad decision to go? Can alcohol and THC be detected in my blood? Thanks for the answers!
There are a variety of blood tests a medical provider could use. Your medical provider isn't going to do every blood test in existence on your blood, so if your only concern is about THC detection, unless the consultation is a drug screening (for example, if it were mandated by a court due to previous drug convictions, or part of an employment screening that includes drug testing), they have no reason or time to bother testing for THC.
However, partying of any type, even staying up late eating potato chips and watching movies, could influence the results of blood tests (things like blood sugar and cholesterol levels, for example; see some guidance from the UK's NHS), as well as other tests (your blood pressure could be elevated post-drinking, for example) and make those tests less useful for diagnosis. Usually your physician will provide instructions prior to such tests (if they don't, ask them, those of us here can only guess what tests they might be running), such as "no food for ___ hours" before - that's one of the reasons for the early morning hours, because it is typically easiest to fast overnight.
In conclusion, of course you are safest not consuming illegal substances, but it is important to know the reasons for the medical exam and consider those reasons ahead of the tests.
This is not a medical diagnosis, however though; Yes it can be detected. Generally THC can be detected in blood, urine, and hair. The length of detection depends on the method used to determine what substances are in your system and how long you have been consuming them. If you smoke it, detection for single time use is 12-24 hours via blood and 1 - 7 days or more via urine. A hair test could be used for detection of up to months after use. Regular use is A MINIMUM of 6 - 7 days via urine and 2 - 7 days for a blood test.