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Defining Comorbidity: Implications for Understanding Health and Health Services

¹ National Institute for Health Research School for Primary Care Research, The University of Manchester, Manchester, United Kingdom
² Department of Health Policy and Management, Johns Hopkins Bloomberg School of Public Health, Johns Hopkins University, Baltimore, Maryland
³ National Institute for Health Research School for Primary Care Research, University of Bristol, Bristol, United Kingdom

CORRESPONDING AUTHOR: Jose M. Valderas, MD, NIHR School for Primary Care Research The University of Manchester Oxford Road Manchester M13 9PL United Kingdom jose.valderas{at}manchester.ac.uk

Comorbidity is associated with worse health outcomes, more complex clinical management, and increased health care costs. There is no agreement, however, on the meaning of the term, and related constructs, such as multimorbidity, morbidity burden, and patient complexity, are not well conceptualized. In this article, we review definitions of comorbidity and their relationship to related constructs. We show that the value of a given construct lies in its ability to explain a particular phenomenon of interest within the domains of (1) clinical care, (2) epidemiology, or (3) health services planning and financing. Mechanisms that may underlie the coexistence of 2 or more conditions in a patient (direct causation, associated risk factors, heterogeneity, independence) are examined, and the implications for clinical care considered. We conclude that the more precise use of constructs, as proposed in this article, would lead to improved research into the phenomenon of ill health in clinical care, epidemiology, and health services.

Key Words: Comorbidity • multimorbidity • chronic disease, etiology

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