Background. Everything known about the roles, relationships, and repercussions of comorbidity in cardiovascular disease is shaped by how comorbidity is currently measured. Objectives. To critically examine how comorbidity is measured in randomized controlled trials or clinical trials and prospective observational studies in acute myocardial infarction (AMI), heart failure (HF), or stroke. Design. Systematic review of studies of hospitalized adults from MEDLINE CINAHL, PsychINFO, and ISI Web of Science Social Science databases. At least two reviewers screened and extracted all data. Results. From 1432 reviewed abstracts, 26 studies were included (AMI , HF , stroke ). Five studies used an instrument to measure comorbidity while the remaining used the presence or absence of an unsubstantiated list of individual diseases. Comorbidity data were obtained from 1–4 different sources with 35% of studies not reporting the source. A year-by-year analysis showed no changes in measurement. Conclusions. The measurement of comorbidity remains limited to a list of conditions without stated rationale or standards increasing the likelihood that the true impact is underestimated. 1. Introduction Heart disease and stroke, common cardiovascular diseases, are the third and fourth leading causes of disease burden and the primary causes of death worldwide [1, 2]. Cardiovascular disease (CVD), a systemic disease, rarely occurs alone so it is common to find multiple comorbid conditions in the setting of CVD, particularly in the older adult population who bear a disproportionate share of the comorbidity burden [3]. Comorbidity, at this time, is generally understood to be the presence of other disease entities in the setting of an index disease or condition [4]. However, everything known about the roles, relationships, and repercussions of comorbidity in CVD is shaped by how comorbidity is currently measured. The actual burden of comorbid conditions and the impact on outcomes in CVD may not be fully realized as a result of methodologic limitations in prospective studies completed to date. A brief overview of the history of comorbidity measurement will set the stage for understanding these methodologic limitations (Table 1). During the 1970s Kaplan and Feinstein [5] investigated taxonomic problems with classifying comorbidity which they defined as “any distinct additional clinical entity that has existed or that may occur during the clinical course of a patient who has the index disease under study” [6, page 456-7]. According to their conceptualization, comorbidity played one of
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