The Charlson Comorbidity Index: problems with use in epidemiological research

The Charlson Comorbidity Index (CCI) is a highly cited and well established tool for measuring comorbidity in clinical research, but there are problems with its use in practice. Like most comorbidity summary measures, the CCI was developed to adjust for prognostic comorbidities in statistical models, particularly those exploring associations between a risk of death or survival time and other patient-related and disease-related factors. Despite this, the CCI is often used in cancer research to measure all comorbidity, or as a multimorbidity measure, and CCI scores are often used to assess the prognostic importance of multiple health conditions. In the latter case, it is not at all surprising that researchers report a significant association between CCI scores and a risk of death or survival times because CCI scores provide a summary of the presence or absence of a set of prognostic comorbidities. Advances in multimorbidity research require specific attention to the methods used to develop relevant indices. Published literature on the association between the comorbidity and risk of death or survival time should be interpreted with caution, especially if the CCI was used to provide a measure of comorbidities.