Challenges in the Interpretation of Isolated THC‐COOH Detection in Hair: Forensic and Clinical Implications

Cannabis detection in hair typically relies on Δ9-tetrahydrocannabinol (THC), but interpretation is limited by its lipophilicity, external contamination, and washout. The metabolite 11-nor-9-carboxy-THC (THC-COOH) is considered a more specific biomarker, though often present at much lower concentrations than THC. Cases with THC-COOH detected without THC present an interpretative challenge. This study investigated the prevalence, segmental distribution, and significance of hair samples positive for THC-COOH in the absence of THC in a forensic cohort monitored for driving license regranting. Hair (n = 639), blood (n = 198), and urine (n = 221) were collected. Hair was segmented (0-3 cm proximal, 3-6 cm distal) and analyzed using validated UPLC-MS/MS for THC, CBN, CBD, and THC-COOH. Cutoffs were THC ≥ 50 pg/mg and THC-COOH ≥ 0.2 pg/mg. Self-reported use was recorded and compared to biological results using phi (φ) coefficients. Of 639 hair samples, 276 were negative, 207 positive for both THC and THC-COOH, 76 positive for THC-COOH only, and 52 positive for THC only. THC-COOH-only cases included 29 with both segments positive. Median proximal hair concentrations (pg/mg) were THC-COOH 3, THC 340, CBD 270, CBN 170, higher than distal segments. Concordance with self-report improved when THC-COOH was considered when detected alone (φ up to 0.54). Wash tests of THC-COOH-only hair (n = 100) were negative, confirming endogenous origin. THC-COOH detection in hair, even without THC, is valid evidence of cannabis use. Multi-matrix analysis and recognition of THC-COOH as a primary marker improve abstinence monitoring and support updates to forensic guidelines.