Blood drug test is a less popular drug testing method for pre-employment drug testing
as it is able to detect blood usage for a largely limited prior period of time. Blood test is rather used to check if a person is actively using a suspected drug or not. Since, drug substances are quickly metabolized and transferred to various parts of the body through blood and then eliminated from the bloodstreams, a blood tests is only able to detect drug usage for a brief time frame.
Drug tests through blood are expensive than the usual urine and saliva tests, hence they are only preferred by employers when blood tests are mandatory. On top that, there are chances of false positives because of which additional tests are required to confirm in blood tests.