BACKGROUND Bipolar disorder is complex and can be difficult to diagnose. It is often misdiagnosed as recurrent major depressive disorder. AIMS We had three main aims. To estimate the proportion of primary care patients with a working diagnosis of unipolar depression who satisfy DSM-IV criteria for bipolar disorder. To test two screening instruments for bipolar disorder (the Hypomania Checklist (HCL-32) and Bipolar Spectrum Diagnostic Scale (BSDS)) within a primary care sample. To assess whether individuals with major depressive disorder with subthreshold manic symptoms differ from those individuals with major depressive disorder but with no or little history of manic symptoms in terms of clinical course, psychosocial functioning and quality of life. METHOD Two-phase screening study in primary care. RESULTS Three estimates of the prevalence of undiagnosed bipolar disorder were obtained: 21.6%, 9.6% and 3.3%. The HCL-32 and BSDS questionnaires had quite low positive predictive values (50.0 and 30.1% respectively). Participants with major depressive disorder and with a history of subthreshold manic symptoms differed from those participants with no or little history of manic symptoms on several clinical features and on measures of both psychosocial functioning and quality of life. CONCLUSIONS Between 3.3 and 21.6% of primary care patients with unipolar depression may have an undiagnosed bipolar disorder. The HCL-32 and BSDS screening questionnaires may be more useful for detecting broader definitions of bipolar disorder than DSM-IV-defined bipolar disorder. Subdiagnostic features of bipolar disorder are relatively common in primary care patients with unipolar depression and are associated with a more morbid course of illness. Future classifications of recurrent depression should include dimensional measures of bipolar symptoms.
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