Concepts of comorbidities, multiple morbidities, complications, and their clinical epidemiologic analogs

Abstract

The proportion of older people in the world population is expected to increase rapidly during the upcoming decades. Consequently, the number of patients with multimorbidity will increase dramatically. In epidemiologic research, the concepts of multimorbidity, comorbidity, and complications have been confusing, and some of these concepts are used interchangeably. In this commentary, the authors propose a clear terminology for clinical concepts describing different aspects of multimorbidity and elucidate the relationship between these clinical concepts and their epidemiologic analogs. Depending on whether a study uses causal or predictive models, a proper distinction between concepts of multimorbidity is important. It can be very difficult to separate complications of the index disease under study from comorbidity. In this context, use of comorbidity indices as confounding scores should be done with caution. Other methodologic issues are type, duration, severity, and number of comorbidities included in the ascertainment methods, as well as sources included in the research. Studies that recognize these challenges have the potential to yield valid estimates of the comorbidity burden and results that can be compared with other studies.

Keywords: comorbidity; complications; diagnosis-related groups; epidemiologic methods; epidemiology; risk adjustment.

Figure 1

Simple epidemiological models illustrating the association between the exposure variable and the outcome under study. Notes: (A) Illustrates the confounding pathway from the exposure to the endpoint. (B) Illustrates effect modification of the association between the exposure and the endpoint, and (C) Illustrates an intermediate step from the exposure to the endpoint.