Measuring THC in postmortem brain tissue does not, by itself, establish impairment before death. Meneses and Hernandez analyzed blood and brain specimens in 43 selected cases and found no clear distribution pattern that could translate a chemical concentration into a behavioral conclusion. Brain testing added useful analytical information, with substantial sampling and interpretation limits.
Selection and specimen availability shaped the study
The study included 21 fatally injured drivers and 22 people who were not driving at death. Overall there were seven females and 36 males, aged 17–76 years. Selection required THC confirmation in central blood and availability of peripheral blood and/or brain. This design could not measure how frequently brain contained THC when central blood was negative.
Brain was unavailable in cases receiving only an external examination. For analysis, portions from the supplied brain specimen were homogenized 1:1 by weight with water. The original anatomical regions, lobes and proportions of white and grey matter were unspecified. These measurements therefore were not standardized samples from one defined brain region.
Blood and brain did not maintain a fixed relationship
Table 2 reports quantifiable brain THC in 14 driver cases, ranging from 1.5 to 343.7 ng/g, and 17 non-driver cases, ranging from 1.4 to 50.2 ng/g. These are smaller specimen-specific counts within the total of 43 people.
The authors reported no correlation between blood and brain THC concentrations and no discernible overall distribution pattern. Their comparisons sometimes found a numerically higher brain concentration, sometimes a higher blood concentration. Blood was expressed in ng/mL and brain in ng/g; these are different matrix units, not directly interchangeable measurements.
The active metabolite 11-OH-THC was also measured. In the driver group, the authors observed higher brain than blood concentrations in the available comparisons. That finding did not establish an impairment threshold, and they considered possible information about recency of use a question for further research.
Collection and storage complicate the comparison
Central blood came from the heart-cavity area. The methods describe peripheral collection from femoral or clavicle arteries, but the exact site was not recorded on individual specimens. External examinations used blind collection. It would be inaccurate to relabel all peripheral results as anatomically verified femoral venous blood.
Blood was preserved with sodium fluoride and potassium oxalate. Central blood was refrigerated at 2–4°C after receipt; peripheral blood and tissue were frozen at −20°C. The overall postmortem interval averaged three days, with a median of two and a range of 0–27 days. All but one case were within seven days. Differing collection and storage conditions prevent assigning every blood–brain difference to redistribution alone.
What the analytical method demonstrated
Supported liquid extraction and LC–MS/MS were used for brain cannabinoids, with two monitored transitions per compound. The stated quantitative range for THC, 11-OH-THC, CBN and CBD was 1–100 ng/mL, with detection and quantification limits administratively set at 1 ng/mL. For THCCOOH the corresponding figures were 5–500 ng/mL and 5 ng/mL. Brain results were reported per gram after preparation.
The paper states that blood and brain methods had been validated, but does not reproduce all numerical validation results. Homogenized liver failed reproducibility requirements for bias and precision during validation and was excluded. A method working in one tissue did not automatically work in another.
From tissue detection to an opinion
The study lacked behavior and performance information needed to determine impairment, and cannabis dose, route and use history were unknown. The non-driver group was not a matched population of unimpaired drivers. The data therefore cannot establish crash risk or a brain concentration causing a particular driving error.
Brain can be a supplemental specimen when blood is limited, but its interpretation must retain the sampling and method limits. Compare the separate issue of central-to-peripheral blood ratios; neither type of comparison directly measures behavior before death.
Primary source
Meneses V, Hernandez M. Cannabinoid distribution in postmortem samples. Forensic Chemistry. 2021;23:100303. doi:10.1016/j.forc.2020.100303.
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