Changes in the prevalence of bipolar disorders between 1998 and 2008 in an Australian population

Objective: To identify any changes in the prevalence of bipolar disorder (BD) between 1998, 2004, and 2008. Method: Cross-sectional population-based surveys were conducted involving random and representative samples of South Australian adults aged ≥ 15years. BD was assessed using the mood module of the Primary Care Evaluation of Mental Disorders instrument (PRIME-MD), a single question related to doctor-diagnosed BD and the Mood Disorder Questionnaire (MDQ), which defines bipolar spectrum disorder. Results: The PRIME-MD-derived prevalence of BD increased significantly from 0.5% [95% confidence interval (CI): 0.27-0.79] in 1998 to 1.0% (95% CI: 0.61-1.31) in 2004 and 1.5% (95% CI: 1.05-1.91) in 2008, demonstrating a significant increased linear trend (χ²=13.91, df=2, p=0.002). Similarly, reported doctor-diagnosed BD increased significantly from 1.1% (95% CI: 0.75-1.51) in 1998 to 1.7% (95% CI: 1.26-2.18) in 2004 and 2.9% (95% CI: 2.28-3.48) in 2008 (Linear trend test χ²=24.55, df=2, p<0.001). The MDQ-derived diagnosis of bipolar spectrum disorder changed from 2.5% (95% CI: 1.96-3.08) in 2004 to 3.3% (95% CI: 2.66-3.94) in 2008 (χ²=3.22, df=1, p<0.10), but this difference did not attain statistical significance. Confining the analysis to those positive for BD on all three methods, there was a significant increase in the prevalence of the detection of BD using all three measures (χ²=4.43, df=1, p=0.03) between 2004 and 2008. Conclusions: There has been an increased prevalence of BD in South Australia over the last decade, but this may be related to changing diagnostic practices rather than a true increase.