Comorbidity

Comorbidity refers to the presence of one or more additional diseases or conditions that co-occur with a primary disease or condition. The term is often used in contrast to multimorbidity; comorbidity implies a focus on a primary condition with other coexisting conditions affecting its course, while multimorbidity generally describes multiple conditions without specifying a primary focus. The relationships among conditions can be causal, coincidental, or bidirectional.

Comorbidity influences prognosis, symptom burden, treatment choices, drug interactions, and health service utilization. It can complicate diagnosis due to overlapping symptoms and may reduce adherence to treatment plans. Common combinations include cardiovascular disease with diabetes, cancer with cardiovascular disease, and psychiatric disorders with chronic illnesses. In older adults, comorbidity is common and is associated with higher mortality, greater disability, and increased healthcare costs.

Measurement and research often use indices such as the Charlson Comorbidity Index, which assigns weights to conditions to predict mortality, or the Elixhauser comorbidity index. These tools rely on administrative data or medical records and have limitations, including imperfect capture of disease severity and the temporal sequence of conditions. Studies may adjust for comorbidity to reduce confounding when examining outcomes.

Clinical implications emphasize integrated, patient-centered care that accounts for multiple conditions and potential polypharmacy. Guidelines increasingly address comorbidity and deprescribing, while care coordination, multidisciplinary teams, and palliative approaches can improve management and quality of life for patients with complex health needs.