Around a third of patients initially diagnosed with major depressive disorder (MDD) have their diagnosis changed to bipolar (BD), Polish researchers report.
Dominika Dudek (Jagiellonian University, Cracow) and team also found that diagnostic conversion from MDD to BD was significantly associated with an earlier age at depression onset, treatment-resistant depression, a higher number of depressive episodes, and a higher number of hospitalizations.
"The variables above may be a useful predictor of bipolar diathesis," comment the researchers in the Journal of Affective Disorders.
The findings come from a retrospective chart review of 122 patients, aged at least 18 years, initially diagnosed with MDD between 1979 and 2009. The mean age at initial diagnosis was 39.8 years and 75.9% were women.
Overall, 32.8% of the patients had a diagnostic conversion to BD over a mean follow-up period of 18.5 years, with a mean time to diagnostic conversion of 9.3 years.
The team found a significant negative correlation between age at depression onset and time to diagnostic conversion. And patients with an earlier age at depression onset (<30 years) had higher conversion rates than those with a later age at onset (≥30 years), at 69% versus 28%.
Furthermore, treatment-resistant depression, defined as lack of significant improvement following at least two adequate antidepressant trials, was 3.6 times more common among converters than nonconverters, at 40% versus 11%.
The researchers also found that, compared with nonconverters, converters had a significantly higher number of previous depressive episodes (7.1 vs 8.5), a higher number of hospitalizations (1.8 vs 3.7), and had spent more time in psychiatric wards (10.7 vs 26.3 weeks).
However, there were no significant differences between the groups regarding years of illness duration, duration of the first depressive episode, number of drugs and/or dosage changes during treatment, or the time elapsed before the occurrence of a second episode of depression.
"The main finding of our study is that about one-third of our subjects, diagnosed with depression and primarily outpatients, had their diagnosis converted into BD," conclude Dudek et al.
They add: "As the risk of a diagnostic switch is a linear function of time, the relatively long follow-up period seems to be an important advantage of our study."