In order to help determine whether a methamphetamine positive may be due to an illicit source, it is helpful to determine the percentage of "d" and “l” isomers. For example, following an individual's use of Vick's Vapor Inhaler, the expected result would be nearly 100% "l" methamphetamine. Based on current laboratory guidelines, if the laboratory report indicates more than 80% "l" methamphetamine, the results are consistent with Vick's Vapor Inhaler use or one of the prescription medications metabolized to l-methamphetamine. If the laboratory reports more than 20% "d" methamphetamine present, the result indicates the use of "d" methamphetamine as either a prescription or illicit preparation. Illegally-produced methamphetamine and amphetamine may contain mixtures of "d" and "l" isomers. LabCorp recommends use of a medical review officer (MRO) to review all non-negative test results.1