An 11-Hydroxy-THC-to-THC Ratio Did Not Reliably Identify Oral Use

A plasma 11-hydroxy-THC-to-THC ratio below one did not exclude oral THC administration in a controlled dosing study. Metabolite ratios can reflect route, but they are not automatically reliable labels for how a particular person consumed cannabis.

Oral dosing followed chronic smoking

Ten daily cannabis smokers, all male, lived on a secure research unit and received increasing-frequency 20 mg dronabinol capsules over eight days. Total daily dosing ranged from 40 to 120 mg. Their earlier smoking history and residual cannabinoids formed part of the experimental context.

Researchers measured parent THC, active 11-hydroxy-THC and THC-COOH in plasma, alongside oral-fluid measurements. Plasma is not whole blood, and a ratio involving the active hydroxylated metabolite is different from a parent-to-inactive-carboxy-metabolite ratio.

The proposed threshold did not hold consistently

Following the first oral dose, some plasma 11-hydroxy-THC/THC ratios were below one at collections extending through five hours. Ratios above one were not observed until day three, and many ratios below one still occurred during subsequent repeated oral dosing.

The authors explicitly found this pattern inconsistent with a proposed rule assigning oral intake from ratios above one within two hours or above 1.5 thereafter. Controlled knowledge that a capsule was swallowed is stronger evidence of route than a classification rule that mislabels those observations.

Route contributes to a profile without uniquely determining it

First-pass metabolism helps explain why oral and inhaled exposures can produce different relative metabolite profiles. Yet prior exposure, repeated dosing, collection time and individual pharmacokinetics also influence the numerator and denominator. This selected, high-dose repeated-administration protocol cannot establish a replacement threshold for every edible or occasional user.

When a forensic opinion assigns route from a ratio, ask for validation under the actual dosing and sampling conditions. The ratio may support a hypothesis alongside history and other measurements. It does not independently establish a dose, exact last-use time or impairment level.

Related: interpreting THC concentrations separately from impairment.

Primary research

Oral Fluid and Plasma Cannabinoid Ratios after Around-the-Clock Controlled Oral Δ9-Tetrahydrocannabinol Administration. DOI: 10.1373/clinchem.2011.169490.