A Living-Subject Cannabis Blood-Fraction Ratio Did Not Validate Postmortem Conversion
A plasma-to-whole-blood relationship observed in living cannabis users did not validate the same conversion after death. Giroud and colleagues found substantially greater variability in postmortem samples and cautioned against applying an exposure-time model in that setting.
The study compared different blood fractions
The investigators examined cannabinoid measurements in eight living regular cannabis users and six deceased people. The living comparison involved plasma and whole blood; the postmortem comparison involved the separated fraction described as serum and whole blood.
Among living subjects, ratios were more consistent within individuals, and the authors discussed an approximate factor of 1.6. Postmortem ratios were more scattered, with a reported mean around 2.4. These are study summaries, not exact conversion constants for an individual specimen.
Conversion and model validity are separate
Even if a blood-fraction ratio were estimated accurately, that would not establish that an exposure-time model remained valid in a different biological setting. Changing the input’s units or matrix does not recreate the conditions under which a model was developed.
The authors did not recommend applying the Huestis exposure-time model to postmortem specimens. Their caution should not be replaced by multiplying a whole-blood result by the postmortem group mean and treating the resulting estimate as validated.
Document the specimen before calculating
A case interpretation should identify whether the laboratory tested whole blood, plasma or serum, and whether the sample was collected before or after death. Those details belong alongside any concentration.
The study was small, and the observed variability limits precision. Its findings support careful specimen-specific interpretation; they do not establish a universal postmortem correction or convert cannabinoid exposure into proof of impairment at an earlier event.
Related: Why the blood fraction matters for THC interpretation.
Research cited
Giroud C; Menetrey A; Augsburger M; et al.; Forensic Science International; 2001. Delta-9-THC, 11-OH-Delta-9-THC and Delta-9-THCCOOH plasma or serum to whole blood concentrations distribution ratios in blood samples taken from living and dead people.
