The relationship between clinical impairment and blood drug concentration: Comparison between the most prevalent traffic relevant drug groups.

Aim: The aim of the present study was to investigate the relationship between blood concentrations of four different drug classes; ethanol, benzodiazepines, amphetamines and tetrahydrocannabinol (THC) and driver impairment as assessed by a clinical test of impairment (CTI).
Methods: Data was retrieved from a national database on CTI assessments and accompanying blood drug concentrations from apprehended drivers. All drug concentrations in blood were quantified using Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS), and compared to the results of the CTI which were categorized as either "not impaired", "mildly impaired", "moderately impaired", or "considerably impaired".
Results: A total number of 15 514 individual mono drug-cases collected over 9 years was included. 89 % were men and the median age was 34 years. In addition, 3 684 individual cases with similar age and gender distribution where no drugs were detected, were included as a reference group. For ethanol and benzodiazepines the percentage of clinically impaired cases increased markedly from lower to higher concentration windows, from 60 % to 97 % for ethanol and from 38 % to 76 % for benzodiazepines. The corresponding increase for amphetamines and THC was modest, from 43 % to 58 % for amphetamines and from 41 % to 55 % for THC. The correlation between drug concentration and degree of impairment was high for ethanol (Spearman´s rho=0.548, p<0.001) and relatively high for benzodiazepines (Spearman´s rho=0.377, p<0.001), but low for amphetamines (Spearman´s rho=0.078, p<0.001) and THC (Spearman´s rho=0.100, p<0.001).
Conclusion: The percentage of impaired drivers increased with increasing blood drug concentration for all four drug classes, most pronounced for ethanol and benzodiazepines and much less for amphetamines and THC. The median blood drug concentration increased with increasing magnitude of impairment for ethanol and benzodiazepines, while this was much less pronounced for amphetamines and THC. The ranges of drug concentrations, however, were wide for all four drug classes in all impairment categories as assessed by individual clinical examination.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)