How representative are clinical studies?
A Dutch study has evaluated whether findings from bipolar disorder (BD) cohort studies apply to everyday clinical practice by comparing 258 outpatients with BD-I to participants from four clinical cohorts and a general population sample.
The research, published in the International Journal of Bipolar Disorders, reveals both alignment and disparities, underscoring the need to scrutinise selection biases in mental health research.
Key findings: Similarities and differences
The study found that many sociodemographic, clinical, and treatment characteristics overlapped between outpatients and cohort participants, supporting the generalisability of research findings to real-world settings. For instance, all groups showed high rates of childhood adverse events and recurrent mood episodes. However, notable differences emerged:
- Age and illness duration: Outpatients were younger (mean age 41.3 vs. 45.8–49.8 years) and had shorter illness durations (16.5 vs. 23–25 years) than clinical cohorts.
- Education and relationships: Outpatients were more likely to hold university degrees (40.7%) and less likely to be married or cohabiting (46.1%) compared to the general population sample.
- Comorbidity: Alcohol/drug use disorders and anxiety were more prevalent in outpatients (14.3% and 19%) than in some clinical cohorts, likely due to differences in diagnostic methods.
Why do these differences matter?
The discrepancies suggest potential selection biases in cohort studies. For example, older patients with longer illness durations in clinical cohorts may reflect recruitment from specialised or long-term care settings. Similarly, lower comorbidity rates in some studies could stem from underreporting in routine clinical assessments versus structured interviews used in population research.
These biases risk skewing treatment guidelines and public health strategies if studies overrepresent specific subgroups.
Implications for clinicians and researchers
The study advocates for:
- Diverse recruitment strategies to include younger patients and those with complex comorbidities.
- Standardised diagnostic tools to improve consistency in measuring variables like suicide risk or childhood trauma.
- Critical evaluation of staging models, as most participants across studies had advanced illness (recurrent episodes), limiting insights into early-stage BD.
Conclusion
While cohort studies largely reflect real-world BD-I populations, this research highlights gaps that could affect quality of care. By addressing biases and prioritising inclusive methodologies, future studies can better inform personalised, evidence-based treatments for diverse bipolar disorder patients. For clinicians, these findings reinforce the value of contextualising research outcomes within individual patient profiles.
Renes, J.W., Kupka, R.W., Nolen, W.A. et al. Generalizability of findings from four clinical cohort studies and a general population study to patients with bipolar I disorder in outpatient treatment in the Netherlands. Int J Bipolar Disord 13, 6 (2025). https://doi.org/10.1186/s40345-025-00375-w