Multiple axis-II diagnoses of personality disorder.

Background: Simultaneous diagnosis of more than one personality disorder (PD) has been termed 'comorbidity' or 'co-occurrence' implying that single diagnoses are the norm and multiple diagnoses interesting exceptions. Surveys of PD subjects in fact show 1.5-5.6 diagnoses per subject. Our study explores the hypothesis that multiple PD diagnosis is common and increases with increasingly personality disordered populations.
Method: The PDQ-R questionnaire was administered to three UK samples: referrals for specialist PD in-patient treatment (n = 275); high tariff offenders attending a probation centre (n = 57); and undergraduate students (n = 274).
Results: Means of 6.0 (95% CI 5.7-6.3), 4.0 (3.1-5.0) and 3.4 (3.0-3.8) PDQ-R diagnoses per subject were found respectively. High rates of PD diagnosis in individual subjects suggest that multiple diagnosis is the norm rather than the exception.
Conclusions: Multiple diagnosis of PD is better construed as 'breadth' of psychopathology rather than comorbidity and is a function of sampling frame. High rates of multiple diagnoses question the interpretation of studies of any single PD. The graded construct of 'breadth' of axis-II pathology may further our understanding of PD.