There is growing evidence that major depressive disorder (MDD) might be overdiagnosed at the expense of bipolar disorders (BPD).

To identify a subgroup of subthreshold BPD among DSM-IV MDD, which is distinct from pure MDD regarding validators of bipolarity.

Data come from the ten-year prospective-longitudinal EDSP-Study, a community survey from Munich, and were assessed with the DSM-IV/M-CIDI. Subthreshold BPD was defined as fulfilling criteria for MDD plus presence of manic symptoms, but never having met criteria for hypomania.

Among 488 respondents with MDD, about 60% had pure MDD and 40% subthreshold BPD. Compared to pure MDD, the subthreshold BPD group was found to have

1. (a) an increased family history of mania,

2. (b) considerably higher rates of nicotine dependence and alcohol use disorders,

3. (c) twice as high rates of panic disorder, and

4. (d) a tendency towards higher rates of criminal acts.

5. (e) In prospective analyses, subthreshold BPD converted more often into BPD during follow-up with the criterion D (symptoms are observable by others) being of critical predictive relevance.

Data suggest that MDD is a heterogeneous concept including a large group of 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.