The field of rheumatology has shown great interest in comorbidity-related risk among patients with rheumatoid arthritis (RA). A recent paper reported that a person with RA has a similar risk of sustaining a myocardial infarction as a person with diabetes mellitus (DM), and this risk is comparable to that of a healthy person 10 years older.1 Several other papers have also recently reported that people with RA and DM share a similar cardiovascular risk.2 3 These findings are strikingly similar to those of the Charlson Index published 25 years ago.4

The Charlson Comorbidity Index is the most commonly used prognostic measure of illness burden in contemporary clinical research.5 Cited in nearly 7000 studies, it is considered the gold standard to assess comorbid risk in clinical research.6 Over time the Charlson Index has increased in use and relevance, likely related to increased rates of chronic disease, supporting the need to measure and adjust for these conditions in research.