Urine test results cannot be used to prove that a person was under the influence of the drug at the time of testing. With regard to urine samples positive for THC, the National Highway Traffic Safety Administration (NHTSA) states that “a positive screening test cannot be taken as evidence that the drug is present in the specimen as these tests lack high specificity, are subject to cross-reactivity and may, on occasion, produce a false positive result. Many of the THC immunoassay screening tests can give a positive response to the presence of THC metabolites even though THC is not present in the sample.” Drugs and drug metabolites, including THC, are detectable in urine for at least several days, and sometimes for weeks, after the drug has been used. Accordingly, detection of THC or other cannabinoids in urine does not necessarily reflect recent use.
Screening tests for drug abuse medical expert witness specialties include toxicology and laboratory medicine.