Blood THC accompanied by negative urine THCCOOH does not, by itself, establish cannabis use immediately before death. Tascon and colleagues found that combination substantially more often in selected postmortem cases than in separate living-person cases. They identified redistribution as a possible explanation, rather than validating the pattern as a clock for last use.
The comparison used separate case populations
The Quebec laboratory database covered 2018–2019 and included 276 postmortem cases and 351 antemortem cases with THC or its metabolites detected in blood. The living-person cases comprised 196 sexual-assault investigations and 155 driving-under-the-influence-of-drugs investigations. They were not earlier samples from the same people later examined at autopsy.
For autopsies, cardiac blood was pooled from both ventricles. Femoral blood was obtained from the exposed, unclamped femoral vein, with a maximum collection of 7 mL. Blood received sodium fluoride and potassium oxalate, and specimens were kept at 4°C during analysis. Urine was collected when available. The paper does not provide a uniform urine preservative protocol or a case-specific collection interval for every urine–blood pair.
Keep the paired-specimen denominators separate
Among 221 postmortem cases with femoral blood and urine analyzed, 39 had blood THC with no detected urinary THCCOOH—reported as 18%. Femoral blood THC in those cases ranged from 0.5 to 82 ng/mL.
In the antemortem comparison, the pattern occurred in four of 191 sexual-assault cases and two of 11 driving cases with both matrices analyzed: six of 202, approximately 3% overall. Blood THC in those six cases ranged from 1.1 to 1.8 ng/mL.
The relevant denominators are 221 and 202, not all 276 and 351 cases in the database. These selected populations and their different investigation contexts also prevent treating the percentages as general rates among all deceased or living cannabis users.
What “negative urine” meant analytically
The laboratory used protein precipitation followed by LC–MS/MS, with two monitored transitions per analyte. Blood quantification ranges were 0.5–100 ng/mL for THC, 2–400 ng/mL for 11-OH-THC and 5–1000 ng/mL for THCCOOH. Urine was assessed qualitatively with cutoffs of 1.5, 6 and 15 ng/mL, respectively.
No hydrolysis of glucuronides was performed. The urinary THCCOOH result therefore should not be relabeled as a hydrolyzed total-metabolite concentration. Nor should a negative classification at the 15 ng/mL urinary cutoff be equated with proof that no THCCOOH molecules or conjugates were present.
The study reports method validation including accuracy, precision, matrix effects, interference and dilution integrity, but does not supply full numerical performance results for every parameter in the article. Its urine result was a method-specific qualitative finding, separate from quantitative blood measurement and from a broad immunoassay screen.
Why postmortem redistribution changes the inference
In the authors’ interpretation, absent urinary metabolite can accompany very recent exposure, but postmortem release of THC into blood provides another possible explanation. Their broader database showed substantial blood-site variability. It did not directly establish the last-use time or amount of redistribution in each of the 39 discordant cases.
Without matched predeath specimens, the study cannot assign each case to recent use or redistribution with a known error rate. It also did not test whether every negative urine result arose from the same mechanism. A distribution pattern can raise an interpretive question without answering it.
What a defensible report should retain
Describe the actual blood site and THC concentration, the urine analyte, the cutoff and whether hydrolysis was performed. Review both central and peripheral blood where available, specimen histories and independent case information. A urine result measured after death should not be turned into an unsupported number of minutes since cannabis use.
These findings do not establish impairment or a causal role in death. They also do not make the negative urine result uninformative; they define the limits of the timing inference. For the distinct analytical consequences of deconjugation, see our discussion of hydrolyzed postmortem urine and bile cannabinoid results.
Primary source
Tascon I, Côté C, Garneau B, Desharnais B, Gosselin V, Mireault P. Postmortem redistribution of cannabinoids: Statistical analysis of a novel dataset and meta-analysis. Forensic Science International. 2023;353:111873. doi:10.1016/j.forsciint.2023.111873.
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