Postmortem “serum” should not automatically be treated as the serum or plasma collected from a living patient. Hoffman and colleagues studied the supernatant produced by centrifuging postmortem central blood and found cannabinoid ratios that differed from clinical blood-to-plasma relationships. Their results caution against importing a living-person conversion factor into a postmortem report.
What the quotation marks around “serum” mean
In this 25-decedent observational study, central blood was collected from the cardiac vein into glass tubes, including serum-separator tubes without preservatives. The centrifuged supernatant from the latter was designated central “serum.” The label describes how the specimen was produced; it does not demonstrate that its composition matches antemortem plasma.
The series was selected using cannabinoid screening. The detailed methods specify peripheral-blood ELISA with a THCCOOH cutoff of 30 ng/mL. Specimens were obtained at autopsy, on average 23.4 hours after death, with a range of 9.08–49.3 hours, and stored at 4°C for up to six weeks. Only “serum” specimens with low visual hemolysis scores, 1 or 2 on a five-point scale, were included. Results should not be extended without qualification to heavily hemolyzed or decomposed specimens.
The ratios differed by analyte
The median central blood-to-“serum” ratio was 1.0 for THC, with an interquartile range of 0.8–1.2, and 0.8 for THCCOOH, with an interquartile range of 0.7–0.9. A ratio below one means that the denominator, “serum,” had the higher concentration. Reversing the ratio would reverse the intended interpretation.
THC’s ratio was not significantly different from 1.0 (P = 0.82). THCCOOH’s ratio was significantly below 1.0 (P < 0.001) and below the THC ratio (P = 0.02). Individual variability was greater for THC: the reported coefficients of variation were 38% for THC and 16% for THCCOOH. A median of one is therefore not a guarantee of equality in a particular pair.
Strong correlation is not a validated conversion rule
Central blood and “serum” concentrations were strongly correlated, with R² values of 0.953 for THC and 0.994 for THCCOOH. Correlation describes how values varied together in the sample. It does not eliminate systematic specimen differences or uncertainty in an individual prediction.
The quantitative method used solid-phase extraction and GC–MS selected-ion monitoring after screening. Blood-method detection and quantification limits were 1 ng/mL for both compounds. THC was above the lower quantification limit in 20 cases, whereas THCCOOH was quantified in all 25. Precision ranged from 1.27–8.11% CV and accuracy from 95.6–105%. Those analytical characteristics do not themselves validate conversion between postmortem “serum,” peripheral blood and antemortem plasma.
Storage and container effects complicate equivalence
In the accompanying preserved-versus-unpreserved central-blood comparison, preserved THC averaged 12.5% higher; preserved THCCOOH averaged 18.3% lower. The authors described changes below 20% under the observed refrigerated conditions. This was not proof of stability under every temperature, storage duration or decomposition condition.
The authors also identified possible retention of cannabinoids in separator gel and the effects of hemolysis as limitations. All collection tubes were glass, so the study did not establish equivalent results for every container material. Additional cannabinoids, including 11-hydroxy-THC and glucuronidated metabolites, were not included.
How to evaluate a proposed conversion
First identify the actual material, anatomical source, additives, processing and storage. Then ask whether the proposed conversion was validated for that combination and analyte. This study does not support a universal multiplication factor for reconstructing antemortem THC. It also does not establish impairment or cause of death from a cannabinoid concentration.
The broader specimen principle appears in our discussion of why measurements from different biological compartments are not interchangeable. The cannabinoid ratios here remain specific to the postmortem study.
Primary source
Hoffman MA, Trochta A, Gary RD, Fitzgerald RL, McIntyre IM. An evaluation of postmortem concentrations of Δ9-tetrahydrocannabinol (THC) and 11-nor-9-carboxy-Δ9-tetrahydrocannabinol (THCCOOH). Forensic Science International. 2020;315:110414. doi:10.1016/j.forsciint.2020.110414.
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