Between 2006 and 2010, six population based case-control studies were conducted as part of the European research-project DRUID (DRiving Under the Influence of Drugs, alcohol and medicines). The aim of these case-control studies was to calculate odds ratios indicating the relative risk of serious injury in car crashes. The calculated odds ratios in these studies showed large variations, despite the use of uniform guidelines for the study designs. The main objective of the present article is to provide insight into the presence of random and systematic errors in the six DRUID case-control studies. Relevant information was gathered from the DRUID-reports for eleven indicators for errors. The results showed that differences between the odds ratios in the DRUID case-control studies may indeed be (partially) explained by random and systematic errors. Selection bias and errors due to small sample sizes and cell counts were the most frequently observed errors in the six DRUID case-control studies. Therefore, it is recommended that epidemiological studies that assess the risk of psychoactive substances in traffic pay specific attention to avoid these potential sources of random and systematic errors. The list of indicators that was identified in this study is useful both as guidance for systematic reviews and meta-analyses and for future epidemiological studies in the field of driving under the influence to minimize sources of errors already at the start of the study.

Rijden onder invloed is één van de belangrijkste oorzaken van verkeersongevallen. Tussen 2006 en 2011 vond het Europese onderzoeksproject DRUID (Driving Under the Influence of Drugs, Alcohol and Medicines) plaats. Eén van de voornaamste doelen van dit project was om meer informatie te verkrijgen met betrekking tot rijden onder invloed van drugs en geneesmiddelen en de risico’s ervan. Daartoe is in dertien verschillende Europese landen op een systematische wijze het gebruik van psychoactieve stoffen door automobilisten onderzocht (Houwing et al., 2011). Onder deze dertien landen bevonden zich ook Nederland en België. Ondanks dat Nederland en België buurlanden van elkaar zijn, verschillen zij sterk in de (gemeten) prevalentie van psychoactieve stoffen in het verkeer. Geneesmiddelen en alcohol werden vaker aangetroffen onder automobilisten in België, terwijl drugs weer vaker in het Nederlandse verkeer werden aangetroffen. Wanneer de Belgische gegevens over drugsgebruik in het verkeer worden vergeleken met drugsgebruik in de totale Belgische bevolking en onder ernstig gewonde automobilisten in België, dan is het waarschijnlijk dat het gebruik van drugs door automobilisten in België in werkelijkheid op een hoger niveau ligt dan nu is gemeten. In beide landen is het aandeel ernstig gewonde autobestuurders dat positief test voor alcohol veel hoger dan dat voor andere stoffen. Daarom zou de handhaving op drugs in het verkeer niet ten koste moeten gaan van de handhaving op alcohol.

Objective: The purpose of this study was to compare the prevalence of psychoactive substances in general traffic in The Nether- lands and Belgium. Method: Randomly selected car drivers and drivers of small vans in six police regions in The Netherlands and five police regions in Belgium were included between January 2007 and August 2009. Blood and oral fluid samples were analyzed for 23 substances, including ethanol (alcohol), by means of ultra performance liquid chromatography-tandem mass spectrometry or gas chromatography- mass spectrometry analysis. Samples were weighted according to the distribution of traffic over eight 6-hour periods. Substance groups were categorized in five mutually exclusive classes: single alcohol use, single illicit drug use, single medicinal drugs use, multiple drug use (includ- ing drugs from two or more separate substance groups but excluding alcohol), and drug use (either single or multiple) in combination with alcohol. Results: In total, 7,771 drivers (4,822 in The Netherlands and 2,949 in Belgium) were included in the study. In Belgium, the prevalence of single alcohol (6.4%) and single medicinal drugs (3.0%) was much higher than in The Netherlands (2.2% and 0.6%, respectively), whereas the single illicit drugs were more common in Dutch traffic (2.2%) than in Belgian traffic (0.6%). Compared with the estimated prevalence of psychoactive substances in the general driving public in Europe, the prevalence in Belgium (10.7%) was greater than the European average (7.4%), and the prevalence in The Netherlands was below the European average (5.5%). Conclusions: The observed prevalence of psychoac- tive substances varies largely between The Netherlands and Belgium. Probable reasons for the differences are the higher level of alcohol enforcement in The Netherlands and nonresponse bias in the Belgian study (for illicit drugs in particular). Furthermore, cultural differences and variances in prescription policy could also be influential.

Study objective: To compare the prevalence of alcohol and (il)licit drugs in seriously injured drivers in Belgium (BE) and the Netherlands (NL). Methods: Injured car and van drivers admitted to the emergency departments of five hospitals in Belgium and three in the Netherlands from January 2008 to May 2010 were included. Blood samples were taken and analysed for ethanol (with an enzymatic method) and 22 other psychoactive substances (UPLC-MS/MS or GC-MS). Results: In total 535 injured drivers were included in the study (BE: 348; NL: 187). More drivers were found positive for alcohol and drugs in Belgium (52.6%) than in the Netherlands (33.9%). Alcohol (≥0.1. g/L) was the most prevalent substance in both countries (BE: 42.5%; NL: 29.6%). A similar prevalence was found for amphetamine (BE: 2.6%; NL: 2.2%) and cocaine (BE: 2.3%; NL: 2.1%). In the Netherlands almost no positive findings for cannabis were recorded (0.5%). No driver tested positive for benzodiazepines in the Netherlands compared to 7.3% in Belgium. More injured drivers tested positive for Z-drugs (BE: 1.8%; NL: 0.5%) and medicinal opioids (BE: 3.3%; NL: 0.5%) in Belgium. Conclusions: The prevalence of alcohol in seriously injured drivers was 12% higher found in Belgium than in the Netherlands. The prevalence of drugs was similar in both countries except for THC and medicinal drugs, particularly benzodiazepines, with a much higher prevalence in Belgium. In comparison to previous survey there were differences in the prevalence of THC, benzodiazepines and combinations of drugs. Possible explanations are the different matrix used, a bias in study population, or in case of illicit opiates and benzodiazepines a different consumption pattern in the two countries. Alcohol is still the most prevalent substance among the injured driver population and this increased the last 15 years.