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A Physiologic Index of Comorbidity: Relationship to Mortality and Disability

¹ Department of Epidemiology, Graduate School of Public Health, and ² Department of Medicine, School of Medicine, University of Pittsburgh, Pennsylvania.
³ Laboratory for Epidemiology, Demography and Biometry, National Institute on Aging, Baltimore, Maryland.

Address correspondence to Anne B. Newman, MD, MPH, Professor of Epidemiology and Medicine, Director, Center for Aging and Population Health, University of Pittsburgh, Graduate School of Public Health, 130 N. Bellefield Ave. Room 532, Pittsburgh, PA 15213. E-mail: newmana{at}edc.pitt.edu

Background. In older adults, there is often substantial undiagnosed chronic disease detectable on noninvasive testing, not accounted for by most comorbidity indices. We developed a simple physiologic index of comorbidity by scoring five noninvasive tests across the full range of values. We examined the predictive validity of this index for mortality and disability.

Methods. There were 2928 (mean age 74.5 years, 60% women, 85% white, and 15% black) participants in the Cardiovascular Health Study (1992–1993) who had carotid ultrasound, pulmonary function testing, brain magnetic resonance scan, serum cystatin-C, and fasting glucose. These were combined into a single physiologic index of comorbid chronic disease on a scale of 0–10. Cox proportional hazard models were used to predict mortality, mobility limitation, and activities of daily living (ADL) difficulty after a maximum of 9 years.

Results. The range of the physiologic index was quite broad, with very few individuals having total scores of either 0 or 10. Those with an index of 7–10 had a hazard ratio of 3.80 (95% confidence interval, 2.82-5.13) for mortality compared to those with scores of 0–2, after adjustment for demographics, behavioral risk factors, and clinically diagnosed conditions. Associations with mobility limitation and ADL difficulty were also significant. The index explained about 40% of the age effect on mortality risk.

Conclusion. Older adults with low levels of markers of chronic disease are rather rare but have remarkably good health outcomes. The ability of such an index to distinguish usual from low risk might provide an opportunity to better understand optimal health in old age.

Key Words: Disability • Mortality • Comorbidity