Blood tests for drugs, or controlled substances, like blood tests for alcohol, are subject to mistakes, and misinterpretation of the chromatograph used to define the levels of the substance they are looking for, that may cause the intoxication. Sometimes, the vials are mixed up. Blood tests are conducted in mass quantity with possibly fifty different vials going at once, because they are run at the same time. A simple mistake in the loading or in the labeling can cause an invalid test for somebody else. The blood tests are not one-hundred percent reliable.