A central-to-peripheral blood ratio describes a difference between two postmortem sampling sites. It does not directly measure the change from life to death. In a 25-decedent study, Hoffman and colleagues found modest median ratios for THC and THCCOOH, but individual variation and the absence of paired antemortem measurements prevent a universal correction back to a living-person concentration.
Which cases and collection sites were studied?
The methods describe selection by a positive peripheral-blood cannabinoid ELISA screen with a THCCOOH cutoff of 30 ng/mL. Although the paper’s introductory description refers to central-blood screening, the detailed inclusion method specifies peripheral blood. This was a selected cannabinoid-positive series, not a random sample of deaths.
Peripheral blood came from the iliac vein; central blood was collected from the cardiac vein. Glass tubes with sodium fluoride and potassium oxalate and tubes without additives were collected. Samples were stored at 4°C for up to six weeks. The mean death-to-collection interval was 23.4 hours, ranging from 9.08 to 49.3 hours. For unwitnessed deaths, the time found and pronounced could substitute for the actual death time, limiting precision of the postmortem interval.
Screening and quantitative measurement were separate
The ELISA result selected cases. Subsequent solid-phase extraction and GC–MS in selected-ion monitoring measured THC and its inactive metabolite THCCOOH separately. The validated blood method reported detection and quantification limits of 1 ng/mL for both analytes, precision coefficients of variation of 1.27–8.11%, accuracy of 95.6–105%, and average recoveries of 90% for THC and 93% for THCCOOH.
THC exceeded the lower quantification limit in 20 of the 25 cases; THCCOOH was quantified in all 25. Those different denominators matter when comparing the two analytes.
Median ratios were modest, but findings were not identical
The median central-to-peripheral ratio was 1.1 for THC, with an interquartile range of 0.9–1.4. For THCCOOH it was 1.3, with an interquartile range of 1.2–1.5. THC ratios were not significantly greater than 1.0 (P = 0.13); THCCOOH ratios were significantly greater than 1.0 (P < 0.001). Thus, a nonsignificant result for THC should not be generalized to the metabolite.
Central concentrations averaged 23% higher for THC and 22% higher for THCCOOH than peripheral concentrations. These are group averages, not corrections applicable to every case. One tabulated THC ratio was 4.05, illustrating how an individual pair can differ from the median.
The interval did not provide a reliable redistribution clock
The relationship between ratio and postmortem interval was weak: R² was 0.02 for THC and 0.21 for THCCOOH. These data do not justify dating exposure or calculating a case-specific redistribution amount from elapsed hours. Nor do they establish behavior beyond the approximately 9–49-hour collection window studied, including advanced decomposition.
The authors interpreted the results as indicating minimal potential for redistribution in this series while recognizing inconsistent findings in the literature and substantial individual variability. Because both specimens were postmortem, a ratio near one cannot establish that neither site changed after death.
From chemical findings to a defensible conclusion
The paper expressly cautions against multiplication factors for estimating antemortem THC concentrations and against determining impairment from blood concentrations alone. THC detection and THCCOOH detection also are different findings; the inactive metabolite should not be treated as a direct measure of current psychoactive effect.
For a death investigation, identify collection sites, timing, preservation, analytical limits and other case evidence before attributing impairment or causation. The distinction between a confirmed chemical result and a behavioral inference is also explained in our laboratory-records discussion, using a different drug-specific example.
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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