Revisiting the concept of 'chronic disease' from the perspective of the episode of care model. Does the ratio of incidence to prevalence rate help us to define a problem as chronic?

Background: This is a study of the epidemiology of acute and chronic episodes of care (EoCs) in the Transition Project in three countries. We studied the duration of EoCs for acute and chronic health problems and the relationship of incidence to prevalence rates for these EoCs.

Method: The Transition Project databases collect data on all elements of the doctor-patient encounter in family medicine. Family doctors code these elements using the International Classification of Primary Care. We used the data from three practice populations to study the duration of EoCs and the ratio of incidence to prevalence for common health problems.

Results: We found that chronic health problems tended to have proportionately longer duration EoCs, as expected, but also a lower incidence to prevalence rate ratio than acute health problems. Thus, the incidence to prevalence index could be used to define a chronic condition as one with a low ratio, below a defined threshold.

Conclusions: Chronic health problems tend to have longer duration EoCs, proportionately, across populations. This result is expected, but we found important similarities and differences which make defining a problem as chronic on the basis of time rather difficult. The ratio of incidence to prevalence rates has potential to categorise health problems into acute or chronic categories, at different ratio thresholds (such as 20, 30 or 50%). It seems to perform well in this study of three family practice populations, and is proposed to the scientific community for further evaluation.