Baseline Breath THC and Post-Use Results Told Different Stories

A relationship between blood and breath THC before smoking did not persist in the same way at the study’s post-smoking peak assessment. This finding cautions against treating one correlation as a universal conversion between specimens or proof of impairment.

Different subsets answered different questions

Wurz and DeGregorio analyzed data from a human cannabis-use study that recruited 74 participants, predominantly chronic daily users. Forty-four provided both blood and breath, while baseline comparisons were available for 34. Ten lacked the baseline samples needed for those analyses.

Among the 34 with baseline data, 23 had detectable breath THC before the experimental smoking session despite being classified as not impaired. Participants were asked to abstain before arrival, but the study did not independently establish a precise last-use time for everyone.

The correlation depended on when it was calculated

Baseline blood and breath THC were correlated. In the unstratified peak post-smoking comparison involving 27 paired specimens, the correlation was not statistically significant (p = 0.2297). A baseline association therefore could not simply be carried over to the post-use period.

The paper’s impairment classifications relied substantially on participants’ self-assessments, with horizontal gaze nystagmus also examined. These are not interchangeable with a validated driving-performance endpoint. The article’s use of an impairment label should not be mistaken for direct evidence that every classified participant could or could not safely drive.

Limits on independent application

Participants used cannabis with variable THC content, and the sample included many frequent users. Missing specimens further restricted particular analyses. The authors also disclosed patents and commercial interests in recent-use testing, which makes independent validation especially relevant.

This report reanalyzed data related to an earlier breath-testing publication; the two papers should not automatically be counted as independent cohorts. Its useful contribution is the contrast between sampling contexts, not a single number that identifies impairment.

For a case-specific interpretation, retain the specimen type, collection time and classification method alongside every concentration. Our discussion of THC blood levels explains why a chemical association cannot substitute for evidence of functional effects.

Primary research

Gregory T. Wurz and colleagues. Scientific Reports (2022). Indeterminacy of cannabis impairment and ∆9-tetrahydrocannabinol (∆9-THC) levels in blood and breath.