Diagnostic conversion from unipolar to bipolar affective disorder: a population-based study.

It is essential to clinically distinguish bipolar affective disorder from unipolar affective disorders. However, patients previously diagnosed with unipolar affective disorder are sometimes later diagnosed with bipolar affective disorder, known as diagnostic conversion. Here we investigated diagnostic conversion using data from a nationwide population-based register.We obtained claims data from 2007 to 2020 in Korea's Health Insurance Review Agency database and identified a cohort of patients who were diagnosed with unipolar depression in 2009 without prior psychiatric diseases within the previous 2 years. We studied the rate of diagnostic conversion and risk factors, especially antidepressants.About 6.5% of patients underwent diagnostic conversion during the observation period. Younger age at disease onset and usage of antidepressants increased the relative risk for diagnostic conversion. Patients using serotonin-norepinephrine reuptake inhibitors (SNRI) showed more than twice the risk compared to no usage of antidepressant.First, this study was based on the population-based register data. Thus, we defined the patient cohort diagnosed with unipolar depression with strict inclusion criteria. Second, the exposure time differed between different antidepressants. Third, we estimated the relative risk for diagnostic conversion compared to no use of antidepressants. Moreover, we could not rule out the potential influence of antidepressant polypharmacy.We confirmed diagnostic conversion in some patients and identified younger age or usage of antidepressants, especially SNRI, as risk factors. Because unipolar and bipolar affective disorders show different disease courses or prognoses and have different treatment strategies, clinicians should be mindful of diagnostic conversion.