It pays to do your homework, or have your own expert who can do the homework for you. Celaya v. Hankook Tire Am. Corp. was a product liability case involving a tread detachment and the driver’s death. The driver’s post-mortem blood sample found, among other things, remnants of marijuana. Defendant disclosed a toxicological expert who concluded the driver was impaired when the accident occurred. This then supported an argument that he was comparatively negligent.
The toxicologist apparently relied upon “mathematical models created by Dr. Marilyn Huestis to determine how long before [driver’s]death he last ingested marijuana.” However, Plaintiff found that Dr. Huestis had written that her models were not standardized for post-mortem testing.
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.
Another toxicologist “concluded that the evidence ‘preclude[d] any attempt to evaluate the time of Cannibis use from cannabinoids concentrations measured after death,’ and that use of Dr. Huestis’s models in postmortem cases ‘is not advocated.’” Subsequent studies on the topic noted other uncontrolled variables in the analysis that had not yet been resolved if applied to post-mortem samples. The analysis was ultimately excluded. “Without some evidence that the models have been tested and proven to be reliable in postmortem cases, the Court must conclude that [the toxicologist’s] impairment opinion is based upon an unreliable methodology and is therefore inadmissible.”
What does this mean? Using a methodology that is reliable in one application does not mean the methodology is reliable if used in a different application. Secondarily, do you homework.
 No. CV-11-00429, 2016 U.S. Dist. LEXIS 192856 (D. Ariz. Mar. 29, 2016).
 Marilyn A. Huestis, Estimating the Time of Last Cannibis Use from Plasma Δ9 – Tetrahydrocannabinol and 11-nor-9-Carboxy-Δ9 – Tetrahydrocannabinol Concentrations, Clinical Chemistry 51:12 at 2294 (2005).