Objective: It is often important to adjust for the effect of comorbid diseases on patient outcomes. This study compares the association between physical function in acute respiratory distress syndrome patients with scores on two comorbidity indices, the Charlson Comorbidity Index, designed to predict mortality, and the Functional Comorbidity Index (FCI), which was designed to predict physical function.
Design: This is a prospective, longitudinal, observational study. A total of 73 survivors of acute respiratory distress syndrome were contacted at 3, 6, and 12 mos. Patient comorbidity was evaluated with the Charlson Comorbidity Index and the FCI. Physical function was measured using the Medical Outcomes Study 36-Item Short Form Health Survey Physical Function Subscale and the Physical Component Subscale scores.
Result: Mean FCI and Charlson Comorbidity Index scores correlated fairly strongly (Spearman rho = 0.62, P < 0.001). FCI, but not the Charlson Comorbidity Index, scores correlated with the Physical Function Subscale and Physical Component Subscale scores. After controlling for other potentially confounding variables such as age and severity of illness through regression analysis, the FCI score was still significantly associated with both Physical Function Subscale and Physical Component Subscale scores.
Conclusions: The FCI is a better method of measuring comorbidity with physical function as the outcome. This study illustrates the importance of choosing the most appropriate comorbidity index for the outcome of interest.