The study was based on data from a prospective study of the course of buprenorphine treatment in a highly structured clinic. Patients in the clinic received maintenance treatment for opiate dependence, either buprenorphine alone or buprenorphine/naloxone formulation tablets to be taken sublingually.
The subjects in the study were consecutively admitted for treatment between August 2004 and November 2009. At intake to treatment, patients were informed of the conditions of treatment and, after both verbal and written consent, were requested to provide a urine specimen, and were seen by a senior consultant psychiatrist who initiated and supervised the buprenorphine treatment continuously. The treatment staff comprising nurses and a social worker supervised the daily activities as well as the structure and contacts with other authorities responsible for the treatment. There is a continuous and close contact between the patients and the staff. Work activities and education were organized through joint collaboration between representatives from the social insurance, social welfare, employment agency and the psychiatric unit at the hospital. This type of collaboration in a maintenance programme is unique in Sweden.
All subjects who completed at least 4 weeks of treatment and who agreed to be included in the firm structure of the programme were enrolled. According to the regulations from the National Board of Health and Welfare (2007)  the exclusion criteria for opioid substitution treatment, and thus for the study were as follows: being younger than twenty years of age, less than one year of frequent opiate use, florid symptoms of psychosis/history of psychosis or ongoing compulsive treatment within psychiatry .
After completing detoxification the subjects went through a phase of psychological testing and psychiatric assessment including psychiatric screening for psychiatric symptoms and personality disorders: (SCID-II), SCL-90, AUDIT, SSP, SOC and a standardized clinical interview. ICD-10 diagnoses of substance disorder were issued for all patients admitted. In addition diagnoses of psychiatric disorders were issued in relation to additional pharmacological treatment interventions.
The subjects were followed from their admission to treatment and until they were involuntarily discharged, or until January 1, 2010. In addition to the supervised urine samples interviews and tests were repeated every third months up to one year after admission.
The Buprenorphine clinic is part of the St. Lars psychiatric hospital in Southern Sweden, Scania County with an uptake area of the entire Southern region of Sweden. Treatment is free for the patients. Patients first attend a meeting with the unit psychiatrist (PT), the clinic social worker (LÖ), and a clinic attendant or nurse. Patients are then offered treatment at the clinic on the basis of mutual agreements during this meeting and are encouraged to begin tapering their use of substances before admittance for treatment.
The clinic employs abstinence-oriented Buprenorphine maintenance treatment, in the sense that no illegal drug use is tolerated after admission to the program. Patients in Buprenorphine treatment are discharged from treatment if the rules are violated. Violence of all kinds in the unit, directed at staff or fellow patients, is prohibited, as well as purchasing or dealing drugs during treatment. Criminal activities result in discharge from the program. The patients must adhere to the ongoing social and medical case management within the clinic. This includes participating in drug counselling at their home town's counselling services, mostly case-management or cognitive behavioural therapy or a Twelve Steps approach. The amount of counselling is decided by the home town services.
Being discharged from the program requires that the positive urine screen at the unit is verified by an independent laboratory finding. Urine samples are collected under surveillance and sent to Lund University Hospital's chemical laboratory. If tests are positive for drugs, they are sent to a second laboratory for a confirmatory analysis. Urine samples are analyzed using Gas chromatography-mass spectrometry (GC-MS) 
Discontinuation of treatment is always decided jointly by the senior consultant psychiatrist and the staff after informing the other authorities and the patient. After three months of suspension the patient may apply for a renewed treatment. During the suspension period the patient is seen on an outpatient basis. The aim of that particular strategy is to maintain contact with the patient in order to reduce the risk of drug overdose. The patient is also allowed to continue in his work/education.
The staff, outpatient counsellors and officials from social services and from the regional social insurance office together help the patients to find work, and to coordinate their work with treatment adherence. All patients submit three tests per week, and maintain a fulltime job or fulltime study. After 4 months of treatment, the required urine tests are reduced to one per week.
All patients who are admitted are administered self-report tests at intake (see measures below). When patients score two standard deviations above the age and gender adjusted norms on the Alcohol Use Disorder Identification Test (AUDIT), they are routinely offered pharmacotherapy for alcoholism, generally disulfiram or acamprosate. Patients scoring above T = 70 on Symptom Checklist 90 (SCL-90) at any time are referred for a full psychiatric assessment and may be offered pharmacotherapy indicated.
During the ongoing treatment patients with non-treatable adverse reactions to buprenorphine are referred to the general opioid agonist maintenance unit at the same hospital, where methadone is an alternative intervention.
After one month of treatment, patients undergo assessment for personality disorders with the SCID-II and SSP (see below). Thus, all patients who are administered the SCID-II have been drug free for one month.
At intake to treatment patients in the study were asked to complete the Alcohol Use Disorder Identification Test (AUDIT), the Sense of Coherence scale (SOC), and the Symptom Checklist 90 (SCL-90). After one month of treatment, patients were administered the Structured Clinical Interview for the DSM-IV-TR (SCID-II) and the Swedish universities Scales of Personality (SSP). The SOC and SCL-90 tests were repeated every third month and AUDIT twice during the first year of study.
The SCID-II and SCID Screen
The Structured Clinical Interview for the DSM-IV- TR, Axis II (SCID-II) is a widely used semi-structured interview designed to assess personality disorders . The interview covers the eleven DSM-IV Personality Disorders (including personality disorders not otherwise specified) and the appendix categories Depressive Personality Disorder and Passive-Aggressive Personality Disorder. Patients first complete the self-report questionnaire and in a subsequent interview the interviewer asks follow-up questions about items that are endorsed on the questionnaire. For antisocial personality disorder the SCID-II screen contains questions about conduct disorder before age 15. If patients satisfy criteria for conduct disorder, they are asked questions about all criteria for adult antisocial personality disorder.
For the present study the symptom count from the SCID screen for conduct disorder was used as indicators of personality disorder-related traits. While there are advantages with the full interview data for clinical use (the ability to have a dialogue with the patient and understand the subjective meaning of the problems reported), the SCID-questionnaire is less susceptible to interviewer bias and has been shown to be highly correlated with symptom counts from the interview with a correlation of 0.86 between the questionnaire and interview , and to be highly stable in drug abusers, with a test-retest correlation of 0.76 over one year .
The Symptom Checklist 90 - SCL-90
The Symptom Checklist-90 (SCL-90) is a self-report measure of psychiatric symptoms, covering nine different symptoms relating to psychiatric conditions. Symptoms are rated on a 5 point Likert scale . The patient responds to each statement (e.g., "nervousness or shakiness") to what degree of severity the symptom has been present in the past week on a 5-point scale (0 "not at all", 1 "a little bit", 2 "moderately", 3 "quite a bit", or 4 "extremely"). For the calculations only the Global Severity Index, the mean of all items, was used.
The Swedish SCL-90 version was translated and back-translated into English, and standardized on a nationally representative sample of 5,000 community residents and validated against psychiatric samples with relevant diagnoses and substance abusers (total n = 1,800). On the basis of the representative sample gender-adjusted T-scores have been developed. T-scores have a normal mean of 50 and a standard deviation of 10 . The cut-off level indicating clinically significant problems was set to T≥70. These are reported in the descriptive statistics for the sample.
The Sense of Coherence Scale (SOC)
The Sense of Coherence Scale is a 29-item self-report scale designed to measure Antonovsky's construct of sense of coherence . It is designed to measure a basic attitude to life, or a personality dimension, hypothesized to facilitate the ability to cope with stress. The Swedish standardization and validation is based on Hansson and Olsson .
The Alcohol Use Disorder Identification Test (AUDIT)
The AUDIT is a 10-item scale designed to measure alcohol related disorders  used in a very large number of both epidemiological and clinical studies. For this study we report age- and gender-adjusted T-scores based on a Swedish standardization study . However, for statistical analyses, we used the unadjusted scores, since the subjects' age and gender were also included as co-variates.
The Swedish universities Scales of Personality (SSP)
The Swedish universities Scales of Personality (SSP) is a revision of the Karolinska Scales of Personality (KSP). SSP is published in Sweden but has been translated into English . The personality profile is presented in T-score format (mean 50 and standard deviation 10). It has 91 items and yields 13 personality scales: somatic trait anxiety, psychic trait anxiety, stress susceptibility, lack of assertiveness, impulsiveness, adventure seeking, detachment, social desirability, embitterment, trait irritability, mistrust, verbal trait aggression and physical aggression.
All statistics were calculated on Stata 11 for Windows. Cox Proportional Hazard Regression was used to assess predictors of cumulative retention. All selected predictors (age, gender, number of drugs in urine at baseline, AUDIT score, criteria count for conduct disorder from the SCID Screen and SCL-90 global severity index) were entered in a multivariate analysis. Two patients who dropped out within the first two days of treatment were treated as censored observations. We controlled for age and gender, because two of our covariates are known to vary substantially by age and gender, namely psychiatric symptoms  and antisocial behaviour [30, 31]. We first estimated a model for each covariate to describe the univariate relationship between the covariate and retention. Further, the proportional hazards assumption for each covariate was tested. The test is a χ2 statistic with one degree of freedom, where rejection of the null hypothesis indicates that the effect of a covariate is not constant over time.
Because there is evidence that dimensional models of antisocial personality pathology are superior to taxonomic ones, we chose to enter the criteria count rather than a categorical predictor based on a rationally derived cut-off for diagnosis that would result in loss of information on either side of the cut-off [32–34]. For the statistical predictor analysis raw scores were used.
Ethics approval was obtained from the Regional Ethical Review Board in Lund (# 847/2004).