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. 2008 Jun;63(6):603-9.
doi: 10.1093/gerona/63.6.603.

A physiologic index of comorbidity: relationship to mortality and disability

Anne B Newman  1 , Robert M Boudreau, Barbara L Naydeck, Linda F Fried, Tamara B Harris
Affiliations

A physiologic index of comorbidity: relationship to mortality and disability

Anne B Newman et al. J Gerontol A Biol Sci Med Sci. 2008 Jun.

Abstract

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.

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Figures

Figure 1
Figure 1
Distribution of physiologic index of comorbidity versus comorbidity count in 2928 Cardiovascular Health Study participants, 1992–1993. (Lower scores indicate less disease burden for both measures.)
Figure 2
Figure 2
Crude mortality rates by physiologic index of comorbidity versus comorbidity count.
Figure 3
Figure 3
a, Survival curves according to physiologic index of comorbidity, grouped by 0–3, 4–5, 6–7, 8–10. b, Mobility limitation according to physiologic index of comorbidity, grouped by 0–3, 4–5, 6–7, 8–10. c, Incident activities of daily living (ADL) difficulty according to physiologic index of comorbidity, grouped by 0–3, 4–5, 6–7, 8–10.
Figure 4
Figure 4
a, Association of age with mortality (i) unadjusted, (ii) adjusted for physiologic index of comorbidity, and (iii) adjusted for age, sex, race, and physiologic index of comorbidity. b, Association of age with mortality (i) unadjusted, (ii) adjusted for comorbidity count, and (iii) adjusted for age, sex, race, and comorbidity count.
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