Carolien Zeetsen

114 to GHB, location of use, frequency of use, dose, duration of use, number of comas, number of hospital admissions and experienced withdrawal symptoms. For this study we included five questions to assess the frequency of GHB use, the dose of GHB used (in millilitres), the duration of GHB use (in months), the duration of daily GHB use (in months) and how often participants experienced a coma due to GHB use in their lifetime. Montreal Cognitive Assessment The Montreal Cognitive Assessment (MoCA; Nasreddine et al., 2005) was used to screen for substance–induced cognitive impairment (Bruijnen, Jansen, et al., 2019). It consists of 12 items measuring: executive and visuospatial functioning; attention, concentration and working memory (referred to as ‘attention’ from now on); language; abstract reasoning; memory; and orientation. For this study the Dutch MoCA version 7.1 and alternate form 7.2 were used at T1 and T2, respectively. Administration of the MoCA takes approximately 15 minutes. The MoCA Total Score (MoCA–TS) is calculated by summing scores on all items with a maximum score of 30 points. A higher score represents better cognitive performance. An adjustment for level of education is applied in which participants with a low level of education receive two extra points, and participants with an average level of education receive one extra point to their total score, while maintaining a maximum score of 30 points (see Bruijnen, Jansen, et al., (2019) for a description of how level of education was measured). Originally, a cut–off score of 25 or lower was used as an indicator of cognitive impairment (Nasreddine et al., 2005). The MoCA is widely used in clinical practice for screening for cognitive impairment in various populations and has a moderate to excellent inter–rater reliability between κ = 0.46 and κ = 0.94 (Cumming et al., 2020). Treatment outcome Three months after detoxification all patients were contacted either in person (when the patient was still in treatment) or by phone (when patients where no longer in treatment). During this interview patients were asked whether they had relapsed in GHB use in the past three months. Patients were considered non–relapse if they had used GHB less than five times in the past three months. This self–reported abstinence was not confirmed using systematic urine or blood tests, due to the narrow timeframe in which GHB can be detected as a result of its short half–life (Abanades et al., 2007). When patients could not be reached, a predetermined close contact of the patient was approached about treatment outcome. In cases where nobody was available, patient records were examined for treatment outcome or (former) counsellors were approached. The last clinical observation was carried forward in this case.