Experts Going Up in Smoke:

Status of Marijuana Post-Mortem Impairment in Texas

Post-Mortem Marijuana in 18-Wheeler Truck Fatalities

Whenever there is a fatality on a Texas highway, it is common for the investigator to order a drug screen on the deceased person. With the liberalization of marijuana law across the country, access to marijuana has never been easier and more common. However, tests, and their results, are still trying to catch-up.

This is no more apparent than in civil cases where one side is arguing impairment based on a drug screen after-death. Few things are more damning to a jury when there is a positive drug test in a crash. Whenever this occurs, the opposing counsel’s best move is to find a way to get this information in front of a jury. The best way to show this information is to use an expert toxicologist.

However, due to the ever-changing research and current climate, marijuana impairment is at best a fool’s errand – few states ever criminally prosecute a driver for driving while intoxicated by marijuana. The reason is simple: no forensic examiner will testify about impairment due to the ever-changing research.

However, there remain some paid experts who are willing to testify about impairment while going against the newest research and the scientific community. Not only is this worry-some for injured parties, but it is also a great concern for the deceased who are unable to provide testimony to counteract this damning evidence. Thus, it is up to an attorney to move to strike this expert and any of their testimony.

Expert Opinions must be Reliable

To be considered reliable, evidence derived from a scientific theory must satisfy three criteria: (1) the underlying scientific theory must be valid; (2) the technique applying the theory must be valid; and (3) the technique must have been properly applied on the occasion in question. TEX. R. EVID. 702.

If the substance of an expert’s testimony raises only possibility, speculation, and surmise then the evidence and testimony is not admissible. LaRock v. Smith, 2010 LEXIS 1272, *17(Tex. App.––El Paso, 2010). An expert must be able to link his conclusions to facts. Jacob v. Int’l Cellulose Corp., Inc., 2007 LEXIS 3314, *14 (Tex. App.-Austin 2007). This is regardless of how qualified the expert may be. Regardless of his qualifications, an expert must still demonstrate scientific indicia that evidences reasonable probability before an opinion is accorded evidentiary value. Brookshire Bros., Inc. v. Smith, 176 S.W.3d 30,37 (Tex. App. ––Houston [1st Dist.] 2004, reh’g denied).

“An expert’s opinion based on speculation is no evidence of causation.” Gray v. Woodville Health Care Ctr., 225 S.W.3d 613, 619 (Tex. App. ––El Paso, 2006, reh’g denied) citing State Office of Risk Mgmt v. Escalante, 162 S.W.3d 619, 625 (Tex. App. ––El Paso 2005, pet. dism.) citing Schaefer v. Tex. Employers Ins. Ass’n, 612 S.W.2d 199, 202-205 (Tex. 1980). The general rule is that expert testimony is necessary to establish causation as to conditions outside the common knowledge of the jury. Guevarra v. Ferrer, 247 S.W.3d 662, 666 (Tex. 2007). “Competent proof of the relationship between the event sued upon and the injuries or conditions complained of has always been required. Id. Additionally, temporal proximity alone does not meet the standard of scientific reliability and does not, by itself, support an inference of causation. Id. at 667. Furthermore, other possible causes of the conduct complained of must be examined before an expert can have an admissible opinion that also establishes causation. Fitzpatrick v. Watson, 2010 Tex. App. LEXIS 631, *9-10 (Tex. App.-Tyler 2010).

a. Expert Opinions Must be Based on Valid Scientific Theory

A toxicologist will typically state a person is impaired by comparing the number found in the post-mortem sample to a “Per Se” limit. All this means is that there is a uniform cut-off point that establishes impairment, much like alcohol and BAC of 0.08. However, there are numerous problems with using a per se limit for marijuana.

i. The State Laws for Per Se Limits are based on Blood, Not Serum

One thing experts love to do is inflate the number found in the blood sample by turning it into a plasma number. This does nothing more than increase the number but fails to give any context. If an expert attempts to do this and uses the plasma number, then they are unable to rely on any state per se marijuana limits.

A closer look at the state statutes indicates that per ser limits are in fact based on THC in the blood as the threshold. See Exhibit “Y” – COLO. REV. STAT. ANN. § 42-4-1301; (ILL. COMP. STAT. ANN. 5/11-501.2); (MONT. CODE ANN. § 61-8-411); (WASH. REV. CODE ANN. § 46.61.502); (OHIO REV. CODE ANN. § 4511.19); (NEV. REV. STAT. § 484.397); (75 PA. STAT. AND CONS. STAT. ANN. § 3802); and Exhibit “P” – Bergamaschi and Huestis et al. 2013. Impact of Prolonged Cannabinoid Excretion in Chronic Daily Cannabis Smokers’ Blood on Per Se Drugged Driving Laws. The Bergamaschi and Huestis specifically states that fifteen (15) U.S. states and twelve (12) European countries established THC concentration limits in blood. Several jurisdictions have adopted 5 ng/mL.

ii. Per Se Limits are Not Science Based

Around the country there has been a push to create a per se marijuana DUI limit for THC like that for alcohol. Alabama has proposed legislation of 5ng/mL THC in the blood as the threshold but as of 2019, such legislation has not been made law. Appendix 1B. Marijuana / THC and DUI: what does the research show? Alabama DUI Handbook Appendix 1B (Updated November 2019). The delay for setting per se limits is summarized by Dr. Jeffrey Michael, Associate Administrator for Research and Program Development at National Highway Traffic Safety Administration when he gave testimony related to this subject to a Congressional committee regarding setting a legal standard for THC: “[s]ix states have adopted per se limits on the amount of active THC allowed in the blood before drivers can be considered impaired. The amounts range from 0.5 nanograms per milliliter in Pennsylvania to 5 nanograms in Washington. But studies by the AAA Foundation and others have found no scientific basis for such numbers. Huestis agreed, saying there’s not a good scientific case right now for per se limits.” [emphasis added]; Id. Another 2015 study conducted by Huestis at the University of Iowa by Huestis, who was at the time, Chief of Chemistry and Drug Metabolism at the Intramural Research Program at the National Institute on Drug Abuse demonstrates the trouble of testing and establishing per se limits. The study generated that 13.1 ng/mL THC (in blood) was equivalent to a BAC of.08% alcohol. Hartman and Huestis et al. 2015. Cannabis Effects on Driving Lateral Control With and Without Alcohol, p. 11. Additionally, in a Congressional Research Service Report regarding the effects of marijuana and highway safety that was relied on by Lindsay, it was determined that the presence of marijuana is inadequate to determine impairment. David Peterman, May 14, 2019, Marijuana Use and Highway Safety, Congressional Research Service.

Id. at 8.

Further, a report conducted by the National Highway Traffic Safety Administration concluded that there is no correlation between THC in the blood and driver impairment. Richard Compton, U.S. Department of Transportation, Marijuana-Impaired Driving—A Report to Congress, DOT HS 812440, July 2017

Id. at 27.

Inconsistent results reveal why, unlike with the established legal threshold of 0.08 percent blood alcohol content, there’s no clear scientific standard to determine when a driver is illegally impaired by marijuana. Simply stated, there is no comparable standard for THC because the available evidence does not support the development of impairment threshold for THC in blood which would be analogous to that of alcohol and must be considered insufficient for deriving a science-based legal limit for THC in blood.

Grotenhennan’s 2007 article ultimately concludes that while experimental studies suggest a THC concentration in serum of 7-10 ng/mL as a reasonably reliable separation of drivers whose driving is in fact impaired by cannabis from those who are not impaired that inadequate evidence from epidemiological studies renders his results preliminary and suggests the need for review and possible revision in the future. There is no conclusive scientific evidence to establish impairment from a post-mortem drug sample. Despite increasing acceptance of progressive marijuana laws and the various regulations that come with it, such as DUI laws, “it remains far from established that the identification of either THC or the carboxy THC metabolite at specific levels may be consistently correlated with behavioral impairment.” Armentano, P., “Are THC Concentrations Appropriate for Presuming Psychomotor Impairment? In Medicolegal Aspects of Marijuana: Colorado Edition, 131-132. As long as science can find no direct correlation between increased blood THC concentrations and driving impairment then any numerical standard starts to become suspect and illegitimate under a jurisprudence that should be based upon actual impairment and dangerousness. The U.S. Department of Transportation put it best:

U.S. Department of Transportation, Marijuana-Impaired Driving—A Report to Congress

iii. Huestis’ “Last-Use” Model Does Not Apply to Post-Mortem Samples

The Huestis’ model to determine last time of use of marijuana is meant to be used only on living subjects and not post-mortem cases. In 2005, Dr. Huestis published an article examining whether the 1992 models maintained their reliability when applied to persons who ingested multiple doses of THC. Marilyn A. Huestis, Estimating the Time of Last Cannabis Use from Plasma delta-9-Tetrahydrocannabinol and 11- nor-9-Carboxy-delta-9 Tetrahydrocannabinol Concentrations, Clinical Chemistry 51:12 (2005). In her 2005 work, Dr. Huestis commented on the applicability of her models to post-mortem subjects:

“A common problem in applying the models to actual situations is that many accident victims die and THC and THCCOOH concentrations are available only for post-mortem blood…. [I]n fatal accident cases, the important variables are the concentrations of the drugs and metabolites in antemortem plasma; estimating these from postmortem blood has not been well documented.”

Id. at 2294.

In 2011, Dr. Huestis and others attempted to measure one variable that had been determined to contribute to the models’ unreliability when applied to postmortem cases. Holland, et al., “Postmortem redistribution of Delta-9-Tetrahydrocannabinol (THC), 11-hydroxy-THC (11-OH-THC, and 11-nor-9-carboxy-THC (tHCCOOH), Forensic Science Int’11212 (2011), 247-251. In doing so, the authors listed several reasons for their hesitation in relying upon Dr. Huestis’ models where the subject had died: (1) available studies examining the levels of THC in a person’s blood and their impairment to drive is done with live participants; (2) those studies use plasma or serum, which is not available in postmortem cases, and the nature of THC makes it difficult to account for the difference between using plasma/serum and whole blood; and (3) researchers had not yet determined whether THC exhibited postmortem redistribution throughout the body. Id. at 251. The study sought to resolve the problems posed by just one of these issues, i.e., the extent that THC experiences postmortem redistribution. The study concluded that even knowing the extent of postmortem redistribution, it was still “currently unknown whether these models would be valid for impairment determinations when using postmortem THC and THCCOOH concentrations.”

The unreliability of Huestis’ mathematical model would not be applicable in either case. Therefore, time of “last use” should not be considered.


These are only a few of the scientific issues that come up when marijuana is positive following a fatality. As most lawyers know, the experts testifying on these issues should be approached with suspicion as there is very few times this has been addressed in the civil court throughout the nation. The research is still on-going, and as marijuana becomes easier to buy, this issue will have to be addressed more often, especially by the Texas Courts.