Changing Toxicology-Screening Rates Complicated a Cannabis Trauma Trend

forensic toxicology working group members October 31, 2026 Forensic Toxicology

An increase in THC-positive trauma screens occurred within a dataset whose testing coverage also changed. That limits how directly the observed trend can be read as a change in the entire injured population.

The analysis required completed tests

The trauma study included 11,491 patients with the required blood and urine toxicology results. Average screening coverage was about 59%, rather than universal. The authors report that coverage increased from 58.7% in 2010 to 67.0% in 2018, with institutional variation as well.

The tested group can change over time

A positivity rate among screened patients depends partly on who gets screened. If testing practice changes, the included group can change even before considering underlying cannabis use. The study acknowledges selection bias from differences across institutions and years. Its data do not establish exactly how much that bias affected the trend or its direction.

The trend remains an observation with a defined denominator

The findings still describe positive results among the patients tested under the observed practices. They do not automatically describe every crash patient, every driver or every person using cannabis in the county. A stronger trend interpretation examines coverage and testing criteria alongside positivity, rather than assuming stable selection. This limitation is separate from whether a urine result measures current impairment: even a perfectly accurate exposure assay cannot repair an unrepresentative testing denominator. Both the analytical meaning and the sampling process need to accompany the reported trend.

Related analysis: injury severity and population crash risk.