It's not routinely done because of the expense. (in fact, they way laboratories do drug testing for employment screenings is to take a small amount of blood from all the people they're testing and mix them all together and just do one test on the mixed sample. If the mixed sample is clean, they know everyone is clean and don't test individuals.)
furthermore, no one can test you or your friend's blood for drugs without permission. It's considered an illegal search. Unless your friend has given specific authorization to be tested for drugs, the doctor or lab cannot perform the test. If they do, the avvo legal forum is thataway =====>.