Associations between blood ethanol concentration, clinical impairment test results, and traffic accident involvement...

Driving under the influence of drugs is a major cause of road traffic accidents, and alcohol markedly increases crash risk. A clinical test of impairment (CTI) is in some countries performed alongside blood sampling in apprehended drivers, yielding a conclusion of either "impaired" or "not impaired". While the relationship between CTI results and blood drug concentrations has been examined, no previous studies have evaluated CTI outcomes in relation to traffic accidents. This study assesses whether ethanol-positive drivers judged as "impaired" have a higher risk of traffic accident involvement than those assessed as "not impaired", and whether accident involvement increases with impairment severity and ethanol concentration.

Drivers positive for ethanol alone were included if a valid CTI conclusion was available and blood sampling occurred 0.25-3 h after the incident. Cases were categorized as either traffic accident group or non-accident controls, the latter including police stops unrelated to accidents (e.g., routine checks, license issues, erratic driving). Age, sex, blood ethanol concentration and CTI results were compared between groups. Associations between impairment level and accident risk were analyzed using multivariable logistic regression.

The accident included group 5,290 cases and the non-accident group included 17,596 cases. Accident-involved drivers were younger (median 30 vs. 34 years, p < 0.001), had higher ethanol concentrations (median [IQR] 1.60 g/kg [1.06, 2.05] vs. 1.29 g/kg [0.63, 1.89], p < 0.001) and were more often assessed as "impaired" (94.8% vs. 90.3%, p < 0.001) compared to non-accident drivers. After adjusting for age, sex, ethanol concentration and driving time, CTI-assessed impairment remained significantly associated with accident involvement (OR 1.47; 95% CI 1.28, 1.69). Accident odds increased progressively with both impairment severity and ethanol concentration (p_trend < 0.001).

Ethanol-positive drivers assessed as "impaired" by the CTI had 47% higher odds of being involved in traffic accidents compared with drivers apprehended for other reasons, independent of ethanol concentration and demographic factors. Being assessed as "impaired" by the CTI therefore appear to be independently associated with accident risk among alcohol-positive drivers.