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The page below is a sample from the LabCE course Drug Testing Methods in the Clinical Toxicology Laboratory. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Methamphetamine has a half-life of approximately 10 hours depending on urinary pH and can be detected in urine for about two days after consumption.
Because methamphetamine is metabolized to amphetamine, both methamphetamine and amphetamine are detected in patients taking methamphetamine.
A urine sample that contains amphetamine at a concentration greater than methamphetamine is not consistent with methamphetamine use alone. It could be the result of drugs such as benzphetamine or mixed use of amphetamine and methamphetamine. Some drugs are actually metabolized to amphetamine and methamphetamine. A couple of examples are benzphetamine and selegeline, a drug used to treat Parkinson's disease. Seligiline is converted to l-methamphetamine and l-amphetamine and benzphetamine is converted to d-methamphetamine and d-amphetamine.
To further confound interpretation, methamphetamine exists as two isomers, d-methamphetamine and l-methamphetamine. The d-isomer is the illicit form of the drug while the l-isomer is found in over the counter medications such as Vick's inhaler.
Occasionally, when a patient has taken methamphetamine, the amphetamine metabolite will be present at a level below the cutoff of the laboratories amp confirmation. For example, a patient screens positive for amphetamines by immunoassay and an amphetamine drug confirmation is reflexed. Methamphetamine is detected at 975 ng/mL. and amphetamine at 190 ng/mL. If the laboratory's confirmation cutoff for amps is 200 ng/mL, they will report methamphetamine as positive and amphetamine as negative. This can be confusing to a physician expecting to see the amphetamine metabolite as well as methamphetamine.