Predictors for switch from unipolar major depressive disorder to bipolar disorder type I or II: a 5-year prospective study.

OBJECTIVE: In this naturalistic study, we investigated the rate, time course, and predictors of a diagnostic switch from unipolar major depressive disorder (MDD) to bipolar disorder type I or II during a 5-year follow-up.

METHOD: The Vantaa Depression Study included at baseline 269 psychiatric outpatients (82.9%) and inpatients (17.1%) with DSM-IV MDD, diagnosed using structured and semi-structured interviews and followed up at 6 months, 18 months, and 5 years between February 1, 1997 and April 30, 2004. Information on 248 MDD patients (92.2%) was available for analyses of the risk of diagnostic switch. Cox proportional hazards models were used.

RESULTS: Twenty-two subjects (8.9%) with previous unipolar MDD switched to bipolar disorder type II and 7 (2.8%) to type I. Median time for switch to bipolar type I was significantly shorter than to type II. In Cox proportional hazards analyses, severity of MDD (hazard ratio [HR] = 1.08, 95% CI = 1.00 to 1.15, p = .036), obsessive-compulsive disorder (OCD) (HR = 5.00, 95% CI = 2.04 to 12.5, p < .001), social phobia (HR = 2.33, 95% CI = 1.00 to 5.26, p = .050), and large number of cluster B personality disorder symptoms (HR = 1.10, 95% CI = 1.02 to 1.20, p = .022) predicted switch.

CONCLUSION: Among outpatients with MDD in secondary level psychiatric settings, diagnostic switch to bipolar disorder usually refers to type II rather than type I. The few switching to bipolar type I do so relatively early. Predictors for diagnostic switch include not only features of mood disorder, such as severity, but may also include some features of psychiatric comorbidity, such as concurrent social phobia, OCD, and symptoms of cluster B personality disorders.