After the final oral THC dose, no subsequent oral-fluid specimens were THC-positive, while plasma THC remained measurable in all ten participants through discharge. The negative saliva findings did not establish complete elimination.
The compartments gave different answers
The controlled study administered 20 mg dronabinol capsules at increasing frequency to daily cannabis smokers. Participants were discharged 22.5 hours after the last dose. At that time, plasma THC remained quantifiable in every participant, with a median of 3.2 micrograms per liter.
In contrast, the report found no THC-positive oral-fluid specimens after the last dose. This discrepancy occurred despite known administration and simultaneous investigation of both matrices.
A negative result retains its method boundary
The oral-fluid finding means the method did not quantify parent THC in that specimen under its reporting criteria. It does not prove that no drug remained elsewhere. Nor does a positive plasma result establish a particular behavioral deficit at discharge; this comparison was pharmacokinetic.
How to describe cessation evidence
If a case requires a conclusion about elimination, identify the analyte, matrix, reporting limit and collection time. Avoid saying the person was drug-free solely because one saliva sample was negative. The result also cannot establish the sensitivity of every roadside device, since collection procedures and analytical cutoffs differ.
Related reading: cannabis screening and impairment.
Research cited
Milman and colleagues (2011). Oral Fluid and Plasma Cannabinoid Ratios after Around-the-Clock Controlled Oral Δ9-Tetrahydrocannabinol Administration. DOI: 10.1373/clinchem.2011.169490.
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