Challenges associated with a polythetic diagnostic system: criteria combinations in the personality disorders

Converging research on the diagnostic criteria for personality disorders (PDs) reveals that most criteria have different psychometric properties. This finding is inconsistent with the PD diagnostic system according to the Diagnostic and Statistical Manual of Mental Disorders (4th ed.; American Psychiatric Association, 1994), which weights each criterion equally. The purpose of the current study was to examine the potential effects of using equal weights for differentially functioning criteria. Data from over 2,100 outpatients were used to analyze and score response patterns to the diagnostic criteria for 9 PDs within an item response theory framework. Results indicated that combinations that included the same number of endorsed criteria yielded differing estimates of PD traits, depending on which criteria were met. Moreover, trait estimates from subthreshold criteria combinations often overlapped with diagnostic (at-threshold or higher) combinations, indicating that there were subthreshold combinations of criteria that indicated as much or more PD than did some combinations at the diagnostic threshold. These results suggest that counting the number of criteria an individual meets provides only a coarse estimation of his or her PD trait level. Implications for the assessment of polythetically defined mental disorders and for the PD proposal for the 5th edition of the Diagnostic and Statistical Manual of Mental Disorders are discussed.