By Leon M. Gussow, MD (Guest Blog)
Tests for marijuana or marijuana metabolites (THC drug tests) frequently become involved in legal proceedings. These tests are commonly introduced as evidence in cases involving pre-employment screening, occupational injury, motor vehicle accidents, and charges of driving under the influence, and in attempts to establish impairment based on post-mortem blood samples.
When cases like these come up, it is important that the attorneys, the judge, and especially the jury understand the science behind these tests, and what exactly they can — and cannot — tell us. Without such understanding it can be difficult, if not impossible, to reach a fair and equitable verdict.
Without a solid understanding of this science, positive or quantitative THC test results can be misunderstood and misrepresented in legal settings.
Interpreting these laboratory tests requires a basic comprehension of the ways in which the body absorbs, metabolizes, and eliminates marijuana. THC (specifically delta-9-THC) is the principal psychotropic (mind-altering) component of marijuana. THC is transformed by the liver into hydroxy-THC, a metabolite with both psychotropic effects and potency similar to those of THC itself. In turn, hydroxy-THC is converted in the liver to carboxy-THC, an inactive metabolite that has no mind-altering effects and is ultimately eliminated in the urine. Urine screening tests for marijuana primarily document the presence of that inactive metabolite, carboxy-THC, if it is above a certain predetermined level. For marijuana, the typical cut-off for reporting a positive screen is a concentration of 50 ng/ml (nanograms per milliliter.) If the test is reported negative, it means either that no marijuana metabolite was present, or that there was metabolite in the urine but at a level below the cut-off.
Because the screening test with a 50 ng/ml cut-off can produce a high number of false positives, a follow-up test with more specific technology can be performed. The cut-off level for this confirmatory test is lower, 15 ng/ml, to minimize the number of false positives.
But here’s the rub. THC is very lipophilic, which means it accumulates in fat tissue. It is then released back into the bloodstream over time. This is a process that can take weeks or, in some cases, even a month or more after the last exposure to marijuana. As it is released, this residual THC is metabolized by the liver to carboxy-THC. Because of this, urine screening or confirmatory tests can be positive long after the last use, especially in those who were chronic users.
Without a solid understanding of this science, the implications of positive or quantitative tests for THC and its metabolites can be misunderstood and misrepresented in legal settings.
The following are three myths about marijuana tests that are often presented as fact in legal arguments:
MYTH #1: A Positive Urine THC Drug Test Proves Impairment or Recent Marijuana Use.
Reality: Urine THC drug tests detect inactive metabolites, not active THC. These metabolites can remain detectable for days or weeks after last use, long after any psychoactive effects have resolved. A positive urine result establishes prior exposure, not impairment at the time of an incident.
A key question that comes up in many cases involving workplace accidents, motor vehicle collisions, or pedestrian injuries is whether one or more of the parties were impaired from the effects of recent marijuana use. It is often argued that a positive urine test proves such impairment. This canard has been thoroughly debunked by science. Research has shown that carboxy-THC can be detected in a person’s urine even after 77 days of documented abstinence.
A recently published position statement endorsed by the American College of Medical Toxicology emphasizes the limitations of urine drug tests for marijuana:
“A positive test for tetrahydrocannabinol (THC) metabolite indirectly indicates that THC, a psychoactive compound in cannabis, has been present in the body. Urine THC metabolite tests are not designed to identify synthetic cannabinoids or CBD. The test results do not identify route of THC exposure, source of exposure, specific timing of exposure, dose, intentional or accidental nature of exposure, or clinical impairment.”
Certainly, in some cases a positive urine test for marijuana can be combined with other evidence, such as documentation of recent use or behavior consistent with THC intoxication, to strengthen the claim of impairment at the time of a certain incident. But by itself a positive test has no value in proving impairment.
MYTH #2: Higher Quantitative Urine THC Levels Mean More Recent Use.
Reality: Quantitative urine THC levels do not reliably correlate with timing of use and should not be interpreted as evidence of recency.
THC is fat-soluble and accumulates in adipose tissue. In chronic users, stored THC metabolites may be released slowly over time, even in the absence of new marijuana use. Physiologic changes such as weight loss or strenuous exercise can increase this release. The potential effects of medications such as GLP-1 agonists on this process remain an area of clinical interest.
As a result, higher numeric values do not establish more recent marijuana use and, in some cases, may suggest the opposite.
MYTH #3: Post-Mortem THC Levels Reflect Impairment at the Time of Death.
Reality: Post-mortem THC levels are affected by redistribution, degradation, and sampling variability after death. Blood levels should not be used in isolation to infer impairment or causation.
I have consulted in cases involving deaths from severe injuries such as multiple fractured ribs and massive chest trauma. In these cases, blood samples taken at autopsy from inside the heart or from pooled blood in the chest cavity showed stratospherically high THC levels, sometimes an order of magnitude greater than the typical legal level used by many states to determine presumed intoxication (5 ng/mg.) One side or the other would try to use this result as representing the actual blood level just before the traumatic event.
The problem with this is obvious. We know that THC is stored in fatty tissue after exposure, either through smoking, vaping, or ingesting edibles. Broken bones will release THC from concentrated fat in the bone marrow. Chest or abdominal trauma will disrupt pericardial fat around the heart or visceral fat in the abdomen. Remember, we are dealing here with units of nanograms per milliliter. A nanogram is a billionth of a gram. A milliliter is a volume equivalent to approximately 20 drops. Clearly, post-mortem specimens can easily be contaminated and measured levels elevated radically by extremely small amounts of residual THC in fat. Once these scientific principles are explained to the members of a jury, they can easily appreciate the fallacy of misusing post-mortem levels in this fashion.
A Checklist for Attorneys Evaluating THC Drug Test Evidence
Here are the key questions an expert in medical toxicology will ask in evaluating the significance of urine, blood, and post-mortem specimen tests for marijuana and/or its metabolites:
- If the initial test was a screening test, was a follow-up confirmatory test performed?
- Is there any evidence as to when the individual in question last used marijuana?
- Was the person an infrequent, occasional user, or a chronic heavy user?
- Was the exposure from smoking? vaping? ingesting edibles?
- Is there any documented observed behavior consistent with impairment?
- Was there a credible source of specimen contamination?
- Was the person on a legitimate prescription THC medication such as dronabinol or Marinol?
If marijuana testing evidence is central to your case, consider consulting a medical toxicology expert early. You can find additional expert witness resources for attorneys here.
Closing Thoughts
Attorneys and toxicologists have used ethanol (alcohol) levels for decades to argue issues of actual or presumed intoxication. But ethanol is water-soluble, making the connections between blood levels and impairment relatively straightforward. Marijuana, however, is fat soluble. The way it is handled by the body is much more complex, and the association between blood or urine levels and impairment much more tenuous. It is crucial that these nuances and differences be carefully explained to and understood by the court and the jury.
About the Author
Leon M. Gussow, MD FACMT is a Medical Toxicologist with over 30 years of experience in the Chicagoland area. He serves as a medical consultant for the Illinois Poison Control Center and has extensive teaching experience with the Toxikon Consortium Medical Toxicology Fellowship Training Program in Chicago. Dr. Gussow is a peer reviewer for several medical journals, including Clinical Toxicology and the Journal of Medical Toxicology. He also writes the monthly “Toxicology Rounds” column for Emergency Medicine News.
Disclaimer: The views and opinions expressed in this article are those of the author and do not necessarily reflect those of High Rock Experts®. This content is for educational and informational purposes only and does not constitute medical or legal advice.