When Teen Drivers’ Cannabis Reports and Biological Tests Disagree

A disagreement between self-reported cannabis use and a THC test is not automatically proof that either measure is useless. A national roadside study illustrates how reporting windows, specimen differences, and study participation affect interpretation.

What was compared?

Pressley and colleagues examined drivers aged 16–19 in the 2013–2014 National Roadside Survey. Of 718 teen participants, 666 provided oral fluid, blood, or both. The researchers compared reported use with biological results and evaluated predictors of positivity.

The study presented unweighted findings. Its percentages therefore describe the analyzed sample and should not be silently promoted to precise national prevalence estimates.

Agreement was substantial, but incomplete

Some teens denying past-year use had positive THC tests, while not everyone reporting recent use tested positive. Among 305 teens with both oral-fluid and blood testing, 17 had discordant results. The reported agreement statistic was substantial, rather than perfect.

A past-year question covers a much longer interval than a specimen’s detectable exposure window. Conversely, a positive biological result cannot independently establish when use occurred or why someone answered a question as they did.

Keep exposure separate from crash risk

The analysis identified associations with factors including reported cannabis use and early alcohol use. Those associations do not establish that the factor caused a positive result, nor do they measure individual impairment. This was not a controlled driving challenge or a collision-culpability study.

For a disputed test, identify the exact analyte, cutoff, collection time, and specimen. For a disputed statement, identify the question’s wording and timeframe. Comparing those details is more informative than treating any discrepancy as definitive evidence of deception or current inability to drive.

Related reading: why urine and blood results are not interchangeable.

Primary research

Joyce C. Pressley; Arushi Arora; Raina Sarmah. Marijuana use in U.S. teen drivers: a comparison of a road-side survey of reported use and fluid tests for tetrahydrocannabinol (THC). Injury Epidemiology (2019). DOI: 10.1186/s40621-019-0204-0.