The use of designer benzodiazepines appears to be increasing in many countries, but data concerning blood concentrations is scarce, making interpretation of concentrations difficult. The aim of this study was to report blood concentrations of clonazolam, diclazepam, etizolam, flualprazolam, flubromazepam, flubromazolam and phenazepam, and to investigate the relationship between blood concentrations and impairment. The concentration data are from blood samples collected from living cases (apprehended drivers and other drug offences) and medico-legal autopsies. The blood samples were analysed for the seven designer benzodiazepines mentioned above by ultra-high-performance liquid chromatography-tandem mass spectrometry. Positive cases from between June 1st 2016 and September 30th 2019 were included. Blood concentrations and the conclusion from a clinical test of impairment (when available) are reported. The presented seven benzodiazepines were detected in a total of 575 cases, where 554 of these cases concerned apprehended drivers or other criminal offenders. The number of findings and the median (range) concentrations were as follows: clonazolam, n = 22, 0.0041 mg/L (0.0017-0.053 mg/L); diclazepam, n = 334, 0.096 mg/L (0.0016-0.25 mg/L); etizolam, n = 40, 0.054 mg/L (0.015-0.30 mg/L); flualprazolam, n = 10, 0.0080 mg/L (0.0033-0.056 mg/L); flubromazepam, n = 5, 0.037 mg/L (0.0070-0.70 mg/L); flubromazolam, n = 20, 0.0056 mg/L (0.0004-0.036 mg/L) and phenazepam, n = 138, 0.022 mg/L (0.0018-0.85 mg/L). A designer benzodiazepine was the only drug detected with relevance for impairment in 25 of the 554 living cases. The physician concluded with impairment in 19 of the 25 cases. Most of the concentrations in these cases were relatively similar to or higher than the median reported concentrations. The most frequent other drugs detected were amphetamine, tetrahydrocannabinol, clonazepam and methamphetamine. The presented blood concentrations can be helpful with the interpretation of cases involving one or more of these seven benzodiazepines. The results indicate that concentrations commonly observed in forensic cases are associated with impairment.
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