Context: There is growing evidence that major depressive disorder (MDD) might be overdiagnosed at the expense of bipolar disorder (BPD).
Objectives: To identify a subgroup of subthreshold BPD among DSM-IV MDD, which is distinct from pure MDD regarding a range of validators of bipolarity, and to examine the pattern of these validators among different groups with affective disorders.
Design: Ten-year prospective longitudinal and family study including 3 follow-up waves. Data were assessed with the DSM-IV Munich Composite International Diagnostic Interview.
Setting: Community sample in Munich, Germany.
Participants: A total of 2210 subjects (aged 14-24 years at baseline) who completed the third follow-up.
Main outcome measures: Cumulative incidence of pure MDD, BPD, and subthreshold BPD (defined as fulfilling criteria for MDD plus having manic symptoms but never having met criteria for [hypo]mania).
Results: Among 488 respondents with MDD, 286 (58.6%) had pure MDD and 202 (41.4%) had subthreshold BPD (cumulative incidence, 9.3%). Compared with respondents who had pure MDD, respondents with subthreshold BPD were found to have a significantly increased family history of mania, considerably higher rates of nicotine dependence and alcohol use disorders, rates of panic disorder that were twice as high, and a tendency toward higher rates of criminal acts. Prospective analyses showed that subthreshold BPD converted more often into BPD during follow-up, with DSM-IV criterion D (symptoms observable by others) being of critical predictive relevance. With increasing severity of the manic component, rates for diverse validators accordingly increased (eg, alcohol use disorders, parental mania) or decreased (harm avoidance).
Conclusions: Data suggest that MDD is a heterogeneous concept including a large group with subthreshold BPD, which is clinically significant and shares similarities with BPD. Findings might support the need for a broader concept and a more comprehensive screening of bipolarity, which could be substantial for future research and adequate treatment of patients with bipolarity.