Association of a Modified Physiologic Index with mortality and incident disability: the Health, Aging, and Body Composition study.

BACKGROUND: Indexes constructed from components may identify individuals who age well across systems. We studied the associations of a Modified Physiologic Index (systolic blood pressure, forced vital capacity, Digit Symbol Substitution Test score, serum cystatin-C, serum fasting glucose) with mortality and incident disability.
METHODS: Data are from the Health, Aging, and Body Composition study on 2,737 persons (51.2% women, 40.3% black) aged 70-79 years at baseline and followed on average 9.3 (2.9) years. Components were graded 0 (healthiest), 1 (middle), or 2 (unhealthiest) by tertile or clinical cutpoints and summed to calculate a continuous index score (range 0-10). We used multivariate Cox proportional hazards regression to calculate risk of death or disability and determined accuracy predicting death using the area under the curve.
RESULTS: Mortality was 19% greater per index unit (p < .05). Those with highest index scores (scores 7-10) had 3.53-fold greater mortality than those with lowest scores (scores 0-2). The unadjusted index (c-statistic = 0.656, 95% CI 0.636-0.677, p < .0001) predicted death better than age (c-statistic = 0.591, 95% CI 0.568-0.613, p < .0001; for comparison, p < .0001). The index attenuated the age association with mortality by 33%. A model including age and the index did not predict death better than the index alone (c-statistic = 0.671). Prediction was improved with the addition of other markers of health (c-statistic = 0.710, 95% CI 0.689-0.730). The index was associated with incident disability (adjusted hazard ratio per index unit = 1.04, 95% CI 1.01-1.07).
CONCLUSIONS: A simple index of available physiologic measurements was associated with mortality and incident disability and may prove useful for identifying persons who age well across systems.