Comorbidity in Family Medicine - Causal or Casual? What is the Effect of Illness Diversity? A Longitudinal Observational Study in Primary Care

Background: This is a study of casual versus causal comorbidity in family medicine in three practice populations. Methods: Participating family doctors (FDs) in the Netherlands, Malta and Serbia recorded details of all patient contacts in an episode of care structure using electronic medical records based on the International Classification of Primary Care, collecting data on all elements of the doctor-patient encounter, including the diagnostic labels (episode of care labels, EoCs). The databases included 15,318 patients and 158,370 patient years over 11 years in the Netherlands (1995-2005), 9,896 patients and 43,577 patient years of observation over 5 years in Malta (2001-2005), 72,673 patient years over 1 year in Serbia (2003). Comorbidity was measured using the odds ratio of both conditions being incident or rest-prevalent in the same patient in one-year dataframes, as against not. Findings: Comorbidity in family practice in the three population databases expressed as odds ratios between the 41 joint most prevalent (joint top 20) episode titles in the three populations. Specific associations were explored in different age groups to observe the changes in odds ratios with increasing age as a surrogate for a temporal or biological gradient. Interpretation: After applying accepted criteria for testing the causality of associations, it is reasonable to conclude that most of the observed primary care comorbidity is casual. It would be incorrect to assume causal relationships between co-occurring diseases in family medicine, even if such a relationship might be plausible or consistent with current conceptualisations of the causation of disease. Most observed comorbidity in primary care is the result of increasing illness diversity. Funding Statement: This study was self-funded by the authors. Declaration of Interests: All authors have completed the ICMJE uniform disclosure form and declare: “no support from any organisation for the submitted work, no financial relationships with any organisations that might have an interest in the submitted work in the previous three years and no other relationships or activities that could appear to have influenced the submitted work.” Ethics Approval Statement: The study did not involve the collection of new data. Ethical approval was applied for locally, when appropriate, for the original data collection and for individual studies based on these data in the Netherlands, Malta and Serbia.