The relation between the blood benzodiazepine concentration and performance in suspected impaired drivers.

Several experimental studies have shown a negative influence of benzodiazepines on driving skills. The objective of this study is to study the relationship between the blood concentration of benzodiazepines and the influence on performance in field sobriety tests. A retrospective case file evaluation was conducted to select cases of drivers, tested positive for benzodiazepines only in the period from January 1999 to December 2004. Drivers were grouped into the categories sub therapeutic, therapeutic or elevated concentrations. The outcome of the tests (walking, walking after turn, nystagmus, Romberg's test, behavior, pupils and orientation) was binomial. A Chi square test was used to assess differences in proportions of the categorized cases. In total 171 cases were included. Observations of behavior (n=137; p<0.01), walking (n=109; p<0.01), walking after turn (n=89; p=0.02) and Romberg's test (n=88; p<0.05) were significantly related to the benzodiazepine concentration. There was no significant relation between benzodiazepine concentration and effect on pupil size, nystagmus or orientation. The results of our study indicate a relation between the concentration of benzodiazepines and the results of some performance tests. More effort is needed to standardize the tests and to determine the sensitivity and selectivity of the tests for benzodiazepines.