Overview: Blood concentrations of THC and its metabolites are not sufficient to prove impairment. Testimony about additional signs of impairment is necessary to prove impairment.
The main reasons for the disconnect between THC blood concentration and impairment is that impairment occurs in the brain, not the blood, and the chemical nature of THC causes it to move throughout the body in a complex manner. Since blood and brain are different tissues, physically separated from each other, blood concentrations of any drug can only estimate the brain concentration and thus impairment.
For alcohol, the estimation of alcohol in the brain from alcohol in the blood is fairly accurate, because alcohol is highly water soluble and spends much of its time in the body within watery blood. However, estimation of brain concentration/impairment is very difficult with THC because it is more soluble in fat/oil than it is in water. This means that THC only spends a short amount of time within watery blood as it disperses throughout the body and deposits into more fatty tissues. High concentrations of THC reach the blood and brain shortly after smoking starts, causing impairment. Blood concentration decreases quickly after smoking stops, as no more THC is coming into the blood and what remains in the blood distributes into fatty tissue. The THC then stays in the fatty tissue for a long time, but small amounts will slowly leach out of fatty tissue and back into the blood over time, causing a continual low blood concentration.
This low concentration of THC in the blood is not enough to cause the high concentration in the brain needed for impairment. Most blood tests show relatively low concentrations of THC. Since this THC in the blood can be from both past use as well as recent use, it is not possible to differentiate between the two or infer impairment. Impairment by THC is currently best detected from the symptoms of impairment.
The information below is quoted from the sources listed at the end of this article. Direct quotes are used where possible to ensure accuracy. The purpose of this post is to provide attorneys with scientific support for challenging the use of toxicology results alone to prove impairment. Attorneys could use this information in cross-examination or present it through testimony of an independent expert.
What does a lab test for when testing blood for marijuana in a DWI case?
When a forensic lab tests blood for marijuana, they test for the presence of delta-9-tetrahydrocannabinol (THC), which is the primary psychoactive component of cannabis. The NC State Crime Lab also tests for the metabolites 11-hydroxy-delta-9-tetrahydrocannabinol (11-OH-THC) and 11-nor-delta-9-tetrahydrocannabinol-9-carboxylic acid. Reports from the State Crime Lab refer to 11-nor-delta-9-tetrahydrocannabinol-9-carboxylic acid with the abbreviation THCA. Many scientific references refer to 11-nor-delta-9-tetrahydrocannabinol-9-carboxylic acid as THC-COOH. THCA also refers to a separate molecule which is a precursor to THC, which has led to confusion by attorneys reading toxicology reports referencing THCA. For more on this topic, see this post. The State Crime Lab quantitates (measures the amount) of these three substances in the blood.
Is impairment correlated with peak THC blood levels?
No. “While fewer studies have examined the relationship between THC blood levels and degree of impairment, in those studies that have been conducted the consistent finding is that the level of THC in the blood and the degree of impairment do not appear to be closely related. Peak impairment does not occur when THC concentration in the blood is at or near peak levels. Peak THC level can occur when low impairment is measured, and high impairment can be measured when THC level is low. Thus, in contrast to the situation with alcohol, someone can show little or no impairment at a THC level at which someone else may show a greater degree of impairment.” Compton at p. 7.
“A number of States have set a THC limit in their laws indicating that if a suspect’s THC concentration is above that level (typically 5 ng/ml of blood), then the suspect is to be considered impaired. This per se limit appears to have been based on something other than scientific evidence. Some recent studies demonstrate that such per se limits are not evidence-based.” Compton at p. 28.
“Peak cannabinoid concentrations were at 1 to 2 hours post vaped dosing and 3 to 4 hours post oral administration dosing [edibles]. Peak metabolite concentrations were at 2 to 4 hours post vaped dosing and 4 to 6 hours post oral administration dosing. None of the targeted analytes in any biological sample correlated well with impairment measures for either route of administration.” Grabenauer at p. 5.
“[The completed research] indicates that THC is not a reliable marker of cannabis impairment. Many participants had low levels of THC in their blood and oral fluid at timepoints where they exhibited substantially decreased performance on cognitive and psychomotor assessments.” Grabenauer at p. 10.
While alcohol concentration (BAC or BrAC) is an accurate measurement of alcohol impairment of driving, the presence of THC in the driver’s body has not been shown to be a reliable measure of marijuana impairment of driving. Compton at 13-14.
Why isn’t the level of THC predictive of impairment? How long can THC remain present in the blood?
“While ethyl alcohol is readily soluble in water, and hence blood, THC is fat soluble. This means that once ingested, THC is stored in fatty tissues in the body and can be released back into the blood sometimes long after ingestion. Some studies have detected THC in the blood at 30 days post ingestion (Heustis, 2007). Thus, while THC can be detected in the blood long after ingestion, the acute psychoactive effects of marijuana ingestion last for mere hours, not days or weeks.” Compton at p. 4.
“The peak THC level occurs at the end of smoking or immediately after cessation (depending on the rate and duration of inhalations). THC levels drop rapidly after cessation of smoking. In contrast to alcohol, which is metabolized at a relatively steady rate, THC is metabolized at an exponentially declining rate where the THC blood level first drops rapidly, followed by a slower decline as lower THC levels are reached. As seen in Figure 2, within 30 minutes the THC level has dropped to 80 – 90 percent of the peak level. After a few hours only low or no THC can be detected in the blood. Very low THC levels may persist in the blood from a single administration for more than six hours.”
As the figure above depicts, It is not possible to measure the peak amount of THC present in the blood if the blood sample is taken 30 minutes or more after ingestion. A blood sample is typically taken from a suspected impaired driver one hour or more after driving (and probably even longer after ingestion of the substance), so a blood sample will not measure the peak level of THC in the blood, or anything other than a very low level of THC. See Compton at p. 5.
How long can THC be detected in the blood after ingestion of marijuana?
“Low THC levels of a few nanograms per milliliter (ng/ml) in blood can result from relatively recent use (e.g., smoking within 1 – 3 hours) when some slight or even moderate impairment is likely to be present, or it can result from chronic use where no recent ingestion has occurred and no impairment is present.” Compton at p. 7.
Can an expert testify that a certain amount of THC in the blood indicates impairment?
“The use of BAC or BrAC as an indicator of driving impairment has assisted law enforcement and prosecutors in being able to show that an alcohol-impaired driver has a BAC that has been demonstrated to increase crash risk. The use of THC level cannot serve this same role for marijuana-impaired driving (Dupont, Voas, Walsh, Shea, Talpins, & Neil, 2012).” Compton at p. 27.
“Toxicologists are not able to provide expert testimony that a specific amount of THC present in a suspect’s blood (or other specimen) is definitively associated with being impaired by marijuana and render the driver unable to drive safely.” Compton at p. 28.
Does this mean an individual can’t be prosecuted for driving while impaired by marijuana?
“The lack of an ‘impairment standard’ equivalent to BAC level does not prevent the successful prosecution of a marijuana-impaired driver. The lack of toxicological evidence simply means that the officer has to offer other evidence that the driver was under the influence of marijuana and too impaired to drive safely.” Compton at p. 28.
Can SFSTs or the DRE protocol be used to measure impairment by marijuana?
“Finally, an assessment of whether the combination of the physiological, cognitive and psychomotor indicators could reliably predict whether the driver’s THC concentration was above or below 5 ng/ml threshold was conducted. No differences were found except for the finger-to-nose test. Some individual signs, symptoms, and tests had weak correlations with the THC concentration being above or below the threshold, but none of them met basic sensitivity levels for correctly predicting impairment status. The conclusion of the study was that ‘there is no evidence from the data collected, particularly from the subjects assessed through the DRE exam, that any objective threshold exists that establish impairment base on THC concentrations in suspects placed under arrest for impaired driving’ (Logan, Kacninko, & Beirness, 2016).” Compton at p. 29, emphasis added.
“One leg stand, walk and turn, and modified Romberg balance field sobriety tests, which are part of a battery of tests administered to detect alcohol impairment, were not sensitive to cannabis intoxication. Each field sobriety test is scored based on whether pre-defined clues are observed during the test. For example, stepping off the line or taking an incorrect number of steps for the walk and turn, or hopping or putting foot down during the one leg stand. There was no apparent difference in the rate of clue detection between oral administration and vaporized cannabis for any of the field sobriety tests.” Grabeauer at p. 4.
What about the Drugs and Human Performance Fact Sheets from 2004 and 2014?
The NHTSA publications contain additional information, but the take-home message stays the same; it is not possible to determine impairment from interpreting blood concentrations of THC and metabolites. The 2004 and 2014 editions contain the same information quoted below.
“Interpretation of Blood Concentrations: It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects. Concentrations of parent drug and metabolite are very dependent on pattern of use as well as dose. THC concentrations typically peak during the act of smoking, while peak 11-OH THC concentrations occur approximately 9-23 minutes after the start of smoking. Concentrations of both analytes decline rapidly and are often < 5 ng/mL at 3 hours. Significant THC concentrations (7 to 18 ng/mL) are noted following even a single puff or hit of a marijuana cigarette. Peak plasma THC concentrations ranged from 46-188 ng/mL in 6 subjects after they smoked 8.8 mg THC over 10 minutes. Chronic users can have mean plasma levels of THC-COOH of 45 ng/mL, 12 hours after use; corresponding – 9 – THC levels are, however, less than 1 ng/mL. Following oral administration, THC concentrations peak at 1-3 hours and are lower than after smoking. Dronabinol and THCCOOH are present in equal concentrations in plasma and concentrations peak at approximately 2-4 hours after dosing. It is inadvisable to try and predict effects based on blood THC concentrations alone, and currently impossible to predict specific effects based on THC-COOH concentrations. It is possible for a person to be affected by marijuana use with concentrations of THC in their blood below the limit of detection of the method. Mathematical models have been developed to estimate the time of marijuana exposure within a 95% confidence interval. Knowing the elapsed time from marijuana exposure can then be used to predict impairment in concurrent cognitive and psychomotor effects based on data in the published literature.” Couper 2014 at p. 8-9.
Why does a lab test for and quantitate the amounts of other marijuana metabolites such as 11-hydroxy-Δ-9-tetrahydrocannabinol (11-OH-THC) and 11-nor-Δ-9-tetrahydrocannabinol-9-carboxylic acid? Can those metabolites be used to show whether an individual is impaired or when marijuana was ingested?
Per the 2004 Drugs and Human Performance Fact Sheets, since 11-OH-THC levels peak slightly after THC levels (9-23 minutes after the start of smoking vs during the act of smoking), it is possible that a high level of 11-OH-THC could indicate recent usage, but 11-OH-THC levels decline rapidly, like THC does, and 3 hours after use, both levels would likely be < 5 ng/ml. Couper 2004 at p. 8.
What specific tests or procedures do DRE’s follow to test for marijuana impairment?
“Horizontal gaze nystagmus not present; vertical gaze nystagmus not present; lack of convergence present; pupil size normal to dilated; reaction to light normal to slow; pulse rate elevated; blood pressure elevated; body temperature normal to elevated. Other characteristic indicators may include odor of marijuana in car or on subject’s breath, marijuana debris in mouth, green coating of tongue, bloodshot eyes, body and eyelid tremors, relaxed inhibitions, incomplete thought process, and poor performance on field sobriety tests.” Couper 2014 at p. 11.
“HGN in DRE evaluations likewise indicates impairment associated with select categories of drugs, e.g. alcohol, CNS depressants, dissociative anesthetics, inhalants, and/or medical conditions affecting driving ability, but is not typically associated with cannabis in these protocols (Couper and Logan, 2014; Kosnoski et al., 1998; McLane and Carroll, 1986; Richman and Jakobowski, 1994). Thus, the lack of significant HGN differences in our study was expected.” Hartman at p. 226.
“In 302 correctly identified cannabis-only DRE cases, the most reliable impairment indicators included elevated pulse, dilated pupils, lack of convergence (LOC), rebound dilation, and documented impairment in 2 of 4 psychophysical tasks,” Hartman at p. 219, (one leg stand [OLS], walk and turn [WAT], finger to nose [FTN], Modified Romberg Balance [MRB]). “The frequently-debated 5 g/L blood THC per se cutoff showed limited relevance. Combined observations on psychophysical and eye exams produced the best cannabis-impairment indicators.” Hartman at p. 219.
“Working memory (PSAT), psychomotor functioning (DSST), and divided attention (DAT) were all negatively impacted after use of the high oral (25 mg THC) and vaporized (20 mg THC) doses. Oral dosing of 10 and 25 mg, and 20 mg vaporized THC doses impaired cognitive and psychomotor performance, but 5 mg vaporized cannabis produced discriminative drug effects with minimal impairment. After vaping, working memory (PSAT) and balance were affected immediately, whereas psychomotor functioning (DSST) and divided attention (DAT) performance were not significantly impacted until 1 hour after dosing. Peak effects were generally seen between 0 and 2 hours post dosing and performance returned to baseline levels by the 4 hour timepoint. After oral administration, cognitive and psychomotor performance were not impacted until 1 hour after dosing. Peak effects were generally seen around 5 hours post dosing (except for balance – which had a peak effect at 3 hours), and it remained elevated at the 6 hour timepoint, and returned to near baseline performance levels by 8 hours post dosing.” Grabenauer at p. 4.
The information provided in this article is from the following publications:
- R. Compton, Nat’l Highway Traffic Safety Admin., Marijuana-Impaired Driving – A Report to Congress (DOT HS 812 440) (July 2017), https://www.nhtsa.gov/sites/nhtsa.dot.gov/files/documents/812440-marijuana-impaired-driving-report-to-congress.pdf. The Compton publication was prepared as a report to Congress in response to the Fixing America’s Surface Transportation Act (FAST Act), Pub. L. 114-94.
- Megan Grabenauer, RTI Int’l, Differences in Cannabis Impairment and its Measurement Due to Route of Administration (Nat’l Crim. Just. Reference Serv., 2020), https://www.ojp.gov/pdffiles1/nij/grants/255884.pdf.
- Fiona J. Couper and Barry K. Logan, Nat’l Highway Traffic Safety Admin., Drugs and Human Performance Fact Sheets (DOT HS 809 725) (2004), https://www.wsp.wa.gov/breathtest/docs/webdms/DRE_Forms/Publications/drug/Human_Performance_Drug_Fact_Sheets-NHTSA.pdf.
- Fiona J. Couper and Barry K. Logan, Nat’l Highway Traffic Safety Admin., Drugs and Human Performance Fact Sheets (DOT HS 809 725) (2014), https://www.nhtsa.gov/sites/nhtsa.dot.gov/files/809725-drugshumanperformfs.pdf.
- J.G. Ramaekers et al., Marijuana, alcohol and actual driving performance, 15 Human Psychopharmacology 551 (2000). https://pubmed.ncbi.nlm.nih.gov/12404625/
- Rebecca L. Hartman et al., Drug Expert (DRE) examination characteristics of cannabis impairment, 92 Accident Analysis and Prevention 219 (2016), https://www.theiacp.org/sites/default/files/all/3-9/302-Marijuana-DRE-Evaluations-Study.pdf.
Thank you to Dr. Andrew Ewens for his assistance with this post.