THC-COOH in Chest-Cavity Fluid: High Concentrations Are Not Blood Equivalents

forensic toxicology working group members October 28, 2026 Forensic Toxicology

THC-COOH in chest-cavity fluid can provide evidence of cannabinoid exposure when ordinary autopsy specimens are difficult to obtain. Its concentration cannot simply be substituted for a blood result or interpreted as a measure of intoxication.

Fifteen postmortem cases

The study examined fluid from the chest cavity in 15 cases, comparing it with available blood, other fluids and tissues. Ten cases showed some degree of decomposition, and the median postmortem interval was 48 hours. This was an observational autopsy series, not a controlled study of living users.

THC-COOH was detected in the cavity fluid, whereas parent THC and 11-hydroxy-THC were not the positive targets reported there. The median cavity-fluid THC-COOH concentration was 480 ng/mL, with a range of 80–3,010 ng/mL.

A large number can reflect the specimen

Cavity-fluid concentrations were higher than those in many paired specimens. The authors discuss redistribution from surrounding organs and concentration changes associated with decomposition and water loss. Contamination from nearby tissues could not be excluded.

These observations do not provide a reliable multiplier for reconstructing a particular individual’s blood concentration. A median ratio across selected cases describes that series; it does not validate interchangeable matrices.

Descriptive patterns are not universal predictors

Longer postmortem intervals and decomposition were associated with higher observed cavity-fluid concentrations in the reported groupings. The study did not find significant differences by drug-use history, and it did not establish an individual relationship between the metabolite concentration and premortem impairment.

The small sample, variable case circumstances and use of nonhydrolyzed cavity fluid constrain interpretation. Blood-based calibration was used, which also makes matrix performance an appropriate subject for laboratory review.

Use as supplementary evidence

The useful claim is that an alternative specimen may retain exposure information. An opinion should identify the actual fluid, target analyte, calibration approach and decomposition status. Calling the result a “high THC blood level” would change both the specimen and the compound, and would overstate what this study measured.

For context, see our discussion of THC concentrations and the separate question of impairment.