Risk factors for conversion from unipolar psychotic depression to bipolar disorder

Objectives Patients with unipolar psychotic depression (PD) are at high risk of developing bipolar disorder (BD). This conversion has important implications for the choice of treatment. This study, therefore, aimed to identify risk factors associated with diagnostic conversion from PD to BD. Methods We conducted a population-based, historical prospective cohort study by merging data from Danish registers. Patients assigned an ICD-10 diagnosis of PD between 1 January 1995 and 31 December 2007 were identified in the Danish Central Psychiatric Research Register and were followed until the development of BD, death, loss to follow-up, or 31 December 2007. Potential risk factors for conversion to BD, also defined through various Danish registers, were tested in multiple logistic regression analyses with risk expressed as adjusted odds ratios (AOR). Results We identified 8,588 patients with PD, of whom 609 (7.1%) developed BD during follow-up. The following characteristics were significantly associated with diagnostic conversion from PD to BD: early onset of PD [AOR=0.99 (per year of increasing age), p=0.044], recurrent depression [AOR=1.02 (per episode), p=0.036], living alone (AOR=1.29, p=0.007), receiving a disability pension (AOR=1.55, p<0.001), and the highest educational level being a technical education (AOR=1.55, p<0.001), short-cycle higher education (AOR=2.65, p<0.001), or medium-cycle higher education (AOR=1.75, p<0.001). Conclusions Diagnostic conversion to BD was prevalent among patients with PD. The following characteristics were significantly associated with this conversion: early onset of PD, recurrent depression, living alone, receiving a disability pension, and the highest educational level being a technical education, short-cycle higher education, or medium-cycle higher education.