Abstract
Background. Comorbidity is increasingly important in the medical literature, with ever-increasing impacts as populations age. Comorbidity has multiple and complex implications for the processes of diagnosis, treatment, prognosis, management and health care. The objective of this study is to measure casual versus causal comorbidity in primary care in three family practice populations.Methods. This is a longitudinal observational study using the Transition Project datasets. Transition Project family doctors in the Netherlands, Malta and Serbia recorded details of all patient contacts in an episode of care structure using electronic medical records and the International Classification of Primary Care, collecting data on all elements of the doctor-patient encounter, including diagnoses (1,178,178 in the Netherlands, 93,606 in Malta, 405,150 in Serbia), observing 158,370 patient years in the Netherlands, 43,577 in Malta, 72,673 in Serbia. 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.Results. Comorbidity in family practice in the three population databases is 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.Conclusion. After applying accepted criteria for testing the causality of associations, it is reasonable to conclude that most 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.Trial registration.This study was performed on electronic patient record datasets made publicly available by the University of Amsterdam Department of General Practice, and did not involve any patient intervention.Funding. Self-funded.