A Review of the Nature and Impact of Exclusion Criteria in Depression Treatment Outcome Research

Background: Depression treatment outcome research is typically designed to maximize internal validity. However, many studies utilize extensive exclusion criteria that reduce the extent to which study samples resemble clinical populations. A better understanding of exclusion criteria and their use in depression treatment outcome research is necessary to generalize accurately from studies. This review identifies the most commonly-used exclusion criteria, the proportion of potential participants excluded, and differences between participant patient samples and patient samples excluded from studies. Methods: Eighteen studies of exclusion in depression treatment research were identified through PubMed and reviewed by both authors. Results: A typical study of depression utilizes approximately 8-11 exclusion criteria and excludes between 75% and 85% of depressed individuals. Insufficient depression severity and comorbid Axis I disorders excluded the most potential participants. Excluded individuals tended to be younger and male, and to have poorer mental health status in the form of longer depressive episodes and more psychiatric comorbidities. Treatment response and remission were generally worse for excluded patients than enrolled subjects. Conclusion: Most depressed individuals would be excluded from a typical clinical trial of depression. These excluded individuals differ significantly in baseline characteristics and outcomes from study participants. More inclusive samples and more thorough reporting of exclusion criteria are needed to safely generalize from depression treatment outcome studies.