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Toxicology Rounds

Toxicology Rounds

Five Myths about Marijuana Tests

Gussow, Leon MD

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doi: 10.1097/01.EEM.0000522212.53581.92

    Job applicants are failing pre-employment drug screenings in record numbers, and some Colorado companies say 90 percent of their prospective employees at times have to be rejected because they fail drug tests. The most rapid increase involved positive tests for marijuana that went up 25 percent between 2012 and 2016. (Washington Post. May 17;

    This explosion in positive marijuana tests is important for emergency practitioners from clinical and occupational medicine perspectives. The prerequisite for making medically and scientifically sound decisions based on these tests is detailed knowledge of how cannabinoids are metabolized and eliminated by the body.

    THC is the primary psychotropic component of cannabis. It is converted by the hepatic P450 system into 11-OH-THC, an active metabolite similar to THC in its psychotropic effects, potency, and kinetics. 11-OH-THC is a significant contributor to the mind-altering effects of cannabis, especially after ingesting edible products when THC gets absorbed from the gastrointestinal tract and a large proportion goes directly to the liver.

    11-OH-THC then is oxidized to 11-carboxy-THC (THC-COOH), an inactive metabolite excreted in the urine. Confirmation tests for marijuana specifically measure urinary 11-OH-THC levels and generally have a cutoff of 15 ng/ml; a lower level is reported as negative. Immunoassay screening tests, on the other hand, detect different cannabinoid metabolites and usually have a cutoff of 50 ng/ml.

    It is quite common in medical and legal settings to misunderstand how urine marijuana tests are interpreted and what inferences can be drawn from a positive test.

    Myth #1: A positive urine test for marijuana establishes impairment. This mistaken conclusion is often argued in legal and worker compensation cases. Remember, the screening test detects a number of inactive metabolites, and the confirmation test solely targets the inactive metabolite THC-COOH. As Marilyn A. Huestis, PhD, of the National Institute on Drug Abuse pointed out: “[A] positive result in a urine test for cannabinoids indicates only that drug exposure has occurred. The result provides no information on the route of administration, the amount of drug exposure, when drug exposure occurred, or the degree of impairment.” (Principles of Forensic Toxicology. Washington, DC: AACC Press, 2010.)

    Myth #2: The actual quantitative urine THC-COOH level can be used to determine recent exposure. This is wrong-headed for the same reasons as the previous myth. THC is taken up primarily by fatty tissues in the body and then slowly released and eliminated. THC-COOH can appear in the urine weeks or even months after the last exposure, often at surprisingly high concentrations. There is no scientific basis for attempting to correlate specific quantitative THC-COOH concentrations with impairment or time of exposure.

    Myth #3: A negative urine test for marijuana followed by a positive test proves exposure to cannabis in the time interval between the two tests. This is somewhat tricky, and I've seen arbitrators in occupational hearings get the science completely wrong. Consider this case: Joe was injured on the job and has been on disability for six months. He is ready to return, and his employer's medical office sends a urine drug screen, which comes back positive for marijuana. Joe admits he was using marijuana for pain. Joe is placed on a “never again” program, and promises not to use marijuana in the future. The medical office arranges to have Joe come in twice a week for a urine marijuana screening test, followed by quantitative confirmation if positive.

    Joe's screening test remains positive for three more weeks while the quantitative THC-COOH level consistently decreases. His next urine test is negative, and no quantitative level is done. Joe returns for a final test two days later, which comes back positive at a level of 18 ng/ml. The employer's occupational medicine “expert” argues that Joe used again between the two tests, and Joe is fired.

    The problem with this reasoning is that a negative test does not mean that no THC-COOH was present, only that the level was less than the cutoff of 15 ng/ml. The precise concentration varies with a person's fluid status, so Joe could have been well hydrated and his urine dilute when the negative sample was obtained and then had concentrated urine 48 hours later, producing a positive test with a level just over 15 ng/ml. Dr. Huestis again: “[I]n almost all cases after smoked or oral THC administration and near the end of the drug's elimination in the urine, negatively and positively testing samples become interspersed. ... The presence of a positive result in a urine test after one or more negative test results may be misinterpreted as new cannabis use.”

    Myth #4: Ingestion of legal hemp seed oil will not cause a positive urine test for marijuana. Hemp seed oil contains a small amount of THC. A 31-month-old child given two teaspoons of hemp seed oil daily for three weeks demonstrated that this exposure could produce a positive test and possibly cause manifestations of toxicity. (Pediatr Emerg Care 2017;33[5]:344.)

    Myth #5: Passive exposure to second-hand marijuana smoke will not produce a positive urine test. This may have been true in the past, but as cannabis has become increasingly potent in recent years, there is concern that passive exposure might result in a positive test. (J Med Toxicol 2017;13[1]:106.)

    As use of legal medical and recreational marijuana becomes more prevalent, we will see more and more positive urine drug tests. It is crucial to know what these mean medically and legally and what they don't mean. Understanding the science can help avoid being trapped by common misconceptions.

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