Implications of Eligibility Criteria on the Generalizability of Alcohol and Drug Treatment Outcome Research: A Study of Real-World Treatment Seekers in Sweden and in Australia

Background: Clinical studies of alcohol and drug treatment outcomes frequently apply participant eligibility criteria (EC), which may exclude real-world treatment seekers, impairing the representativeness of studied samples. Some research exists on the impact of EC on alcohol treatment seekers. Little is known about drug treatment and country differences. Objectives: We tested and compared the degree to which commonly used EC exclude real-world treatment seekers with problem alcohol and drug use in Sweden and Australia, and compared the impact of EC on outcomes. Methods: Two large naturalistic and comparative service user samples were used. Respondents were recruited in Stockholm County (n = 1,865; data collection 2000–2002), and Victoria and Western Australia (n = 796; in 2012–2013). Follow-up interviews were conducted after 1 year. Cross-tabulations, Chi-square (χ ² ) tests and logistic regressions were used. Results: Percentages of the samples excluded by individual EC ranged from 5% (lack of education/literacy) to 70% (social instability) among Swedish alcohol cases and from 2% (low alcohol problem severity) to 69% (psychiatric medication) among Australian counterparts; and from 2% (age 60+ years) to 82% (social instability) among Swedish drug cases and from 1% (age 60+ years) to 67% (psychiatric medication) among Australian counterparts. Country differences and differences across substances appeared independent of country effect. Co-morbid psychiatric medication, noncompliance, poly drug use, and low education EC caused positive 1-year outcome bias; whereas female sex and old age introduced negative outcome bias. Conclusions/Importance: Commonly used EC exclude large proportions of treatment seekers. This may impair generalizability of clinical research, and the effects of many EC differ by country and drug type.