Geriatric Index of Comorbidity: validation and comparison with other measures of comorbidity

Background: the debate about measures of chronic comorbidity in the elderly is mainly due to the lack of consensus on pathogenetic models. Objective: the aim of the present study was to compare the concurrent validity of a number of measures of chronic comorbidity assuming different pathogenic models, versus disability in elderly patients. Setting: the Geriatric Evaluation and Rehabilitation Unit for subacute and disabled patients. Participants: 493 new and consecutive elderly patients (mean age 79 years, 71% females) admitted to the Geriatric Evaluation and Rehabilitation Unit. Measurements: we evaluated age, gender, cognitive status, depressive symptoms, functional status, somatic health, and nutritional status on admission. Functional status was assessed by the self- or proxy reported Katz's BADL scale and by the performance-based Reuben's Physical Performance Test. Somatic health was assessed as presence and severity of diseases according to standardized criteria. Comorbidity was measured as number of diseases, sum of disease severity, and with a composite score (Geriatric Index of Comorbidity) which takes into account both number of diseases and occurrence of very severe diseases. Mortality was assessed after 12 months. Results: specific diseases and their severity were found to be associated with disability measures. All measures of comorbidity were significantly correlated with disability, but only the Geriatric Index of Comorbidity was independently associated after adjustment for severity of individual diseases. In addition, increasing severity of comorbidity as defined by Geriatric Index of Comorbidity was associated with greater disability while this was not true for the other comorbidity measures (F statistics for the regression model including the Geriatric Index of Comorbidity=19.9). The Geriatric Index of Comorbidity, but not the other comorbidity measures, predicted mortality (relative risk of death 2.3, 95% confidence interval 1.7-3.1). Conclusion: the Geriatric Index of Comorbidity, a measure of comorbidity assuming that both number of diseases and occurrence of very severe diseases are determinants of health, has the greatest concurrent validity with disability and is the best predictor of mortality.