Postmortem THC Ratios Are Not Universal Correction Factors
A median ratio between central and peripheral postmortem blood cannot safely be used as a fixed multiplier to reconstruct an individual’s earlier THC concentration. A human case series found substantial variation, including ratios below and above one.
What was compared
Holland and colleagues selected 19 adult death cases with a positive postmortem urine cannabinoid screen. Cardiac blood represented the central specimen and iliac-vein blood the peripheral specimen. They measured THC, 11-hydroxy-THC and THCCOOH and separately used cannabinoid-negative postmortem cases for method validation.
This was an observational human postmortem study. It did not administer cannabis at a known time or dose before death. Three cases had available antemortem blood, a useful but very small subset for evaluating changes across the death interval.
A median concealed wide ranges
The median central-to-peripheral ratio was 1.5 for THC, with a range of 0.3–3.1. The corresponding median ratios were 1.7 for 11-hydroxy-THC and 1.8 for THCCOOH. Ratios below one demonstrate that central blood was not invariably higher.
Central–peripheral differences were not statistically significant for THC or 11-hydroxy-THC in the reported comparisons. THCCOOH did show a significant difference. Trends involving postmortem interval did not reach statistical significance. These results do not support treating all three analytes as interchangeable redistribution markers.
Why reconstruction requires restraint
A selected urine-positive case series is not a representative sample of every death investigation. Some specimens also had analytical interferences, and the antemortem and postmortem samples used different preservatives. Sampling site, storage and interval therefore remain relevant.
Dividing a central THC result by 1.5 would substitute a group median for the unknown behavior of one case. It would not establish the concentration at an earlier crash, the last-use time, or a causal role in death.
A defensible report identifies the precise sampling site and the limits of any comparison. The distinction between measured exposure and functional effects, discussed in our THC impairment analysis, remains necessary even before adding the further uncertainty introduced by postmortem changes.
Primary research
Michael G. Holland and colleagues. Forensic science international (2011). Postmortem redistribution of Δ9-tetrahydrocannabinol (THC), 11-hydroxy-THC (11-OH-THC), and 11-nor-9-carboxy-THC (THCCOOH).
