Cumulative index of elderly disorders and its dynamic contribution to mortality and longevity

The composite index constructed from longitudinal survey data as the level of deficits accumulated by an individual (frailty index) captures important systemic aspects of deterioration in a human organism, and is an attractive candidate for studying determinants of aging and longevity. The objective of this paper was to investigate the ability of this index to characterize human aging, mortality, and longevity. We use 32 variables from the National Long Term Care Survey data characterizing health and daily activities deficits to construct the cumulative frailty index. We use the Cox's proportional hazard model to describe its contribution to mortality. The risk of death considered as the function of the frailty index has asymmetric U-shaped form. The asymmetric U-function of the absolute risk is getting steeper (narrower) with age. The asymmetric U-function describing the relative risk exhibits the opposite tendency with age. The narrowing of the absolute risk functions with age reflects the age-related decline in stress resistance. The widening of the relative risk function with age indicates an increase in the relative contribution of other, unobserved, risk factors (including senescence) to the risk of death. This suggests increasing the role of senescence per se in the increasing risk of death with age compared to the role of specific pathology.