Comorbidity index

Abstract

The impact of comorbidity on the outcomes of interest in rheumatic diseases is noteworthy: comorbidity has proven to be a more significant predictor of premature death than shared epitope, rheumatoid factor, or erosions in inflammatory arthritis. Therefore, controlling for coexisting clinical conditions, or comorbidities, booked its place as a main confounder, both in standard healthcare practice and in research. Comorbidity indices are tools used to quantify the total burden of comorbidity contributing to the patient's overall illness. The simplest method to measure comorbidity is to use the summation of each comorbid illness to generate a total value of comorbidity, often termed a "comorbidity count." However, not all comorbid diseases have the same impact on the outcome of interest. Thus, more complex comorbidity indices were created to select and weight specific comorbid illnesses to measure more accurately the burden and impact of overall comorbidity. Comorbidities can also be categorized based on the way they have been recorded, whether by administrative data, e.g., the International Classification of Diseases, or by self-administered questionnaires. With the substantial impact that comorbidity exerts on health outcomes in patients with rheumatic diseases, and given the lack of a standardized comorbidity index for clinical or research use, there is an obvious need for an accurate tool to measure the burden of comorbidity in patients with rheumatic disease. This chapter discusses the most commonly used instruments, as well as the recently developed comorbidity indices, their implication onto standard rheumatology clinical practice as well as on patients' management.