Normally most laboratores test the common recreatinonal drugs for medical or legal porpuses. For examples some jobs require that an employe most be tested every 4 weeks, so they test the basic. Also if you arrive unconcious to a hospital the basic drug tests are made.
If there is a suspicion of another kind of drug it can be performed under a medical suspicion. Almost every drug can be checked in blood. Since there are hundreds of them you have to be very specific about wha you are testing.
It means one drug = one test at the time. So medical diagnosis and clinical suspicion about a specific drug are mandatory.