In a population-based sample of US adults aged 50 to 99 years, we found that chronic diseases were more predictive of all-cause mortality among younger participants (aged 50–59 years), and functional limitations were more predictive of mortality among older participants (aged 80–99 years). Our results suggested that for participants older than 80 years, functional status was a more important predictor of mortality than their disease diagnoses.
Our results have several implications. Because many of the current comparisons of risk-adjusted outcomes for health care quality measurement and performance incentives rely solely on diagnosis codes,21,22
our results raise concerns about the accuracy of these methods in participants older than 80 years. Our results, coupled with previous research suggesting that the use of functional data can significantly improve mortality prediction, suggest that systematic collection of functional status data for persons aged 80 years or older could lead to improved risk adjustment in this population.2-4
Our results also highlight the importance of accounting for function in observational studies of older persons.
Our results also validate geriatricians' long-standing focus on function. Maybe the time has come to teach our students and ourselves to consider functional limitations to be essential clinical information. For example, instead of teaching medical students to start presentations with statements such as “an 87-year-old male with a history of coronary artery disease and hypertension,” we could teach them that the statement “an 87-year-old male with difficulty in toileting and showering independently” may be more informative and may present a clearer picture of the patient and his issues.
There are several possible explanations for our results. First, because of survivor effect, chronic diseases may, on average, behave differently among older participants. Second, the number of diseases associated with mortality may increase with increasing age, decreasing the impact of any single disease on mortality.41
In that case, our finite list of chronic conditions would have accounted for more of the observed mortality in our younger patients than in our older patients. Third, the difference between nondisease and disease may be less clear-cut in the elderly. For example, older patients may be more likely to have prehypertension (classified as normal) and mild hypertension (classified as hypertensive) than younger patients, who are more likely to be either normotensive or clearly hypertensive.16,18
This would cause hypertension to be a weaker predictor of mortality among older participants because of the relative similarity of the hypertensive and nonhypertensive groups. Regardless of the underlying explanation, many risk-adjustment methods also use a finite number of chronic conditions, which means our results still suggest that commonly used risk-adjustment methods may be less accurate in the oldest old.
Our results do not suggest that chronic conditions are unimportant in predicting death in the oldest old. Given the higher absolute rates of mortality among older participants, the lower relative hazards from chronic conditions in this age group still represented a substantial increase in absolute predicted risk because of chronic conditions. However, this distinction between relative and absolute predicted risk has an even greater effect on functional limitations, with higher relative hazard ratios translating into very large increases in absolute predicted risk because of functional limitations.
Previous research has shown that the importance of various risk factors changes with age.7
Epidemiological studies of the oldest old in Israel5
suggested that well-established risk factors for mortality in younger populations, such as history of disease and disease count, were less important among the oldest old. Furthermore, observational studies among older participants have shown that well-established cardiovascular risk factors, such as hypertension and hypercholesterolemia, are relatively less important in predicting mortality among the elderly.8,11,12
We have extended those findings by incorporating measures of disease severity along with disease diagnoses and by examining a cohort that includes younger and older participants, allowing for direct comparisons of the importance of various risk factors across the age spectrum. In addition, by examining function along with chronic conditions, we were able to show that the relative importance of chronic conditions and functional limitations differs markedly at different ages.
We found that the weaker risk factors added little to the predictive power of the model for every age group. Thus, for younger participants, chronic conditions were most important and functional limitations added relatively little to mortality prediction. Conversely, for older participants, functional limitations were most important and chronic conditions added little. These findings suggest that a primary benefit of including both chronic conditions and functional limitations in mortality prediction models is to enable the model to predict more accurately over a wider age range.2
We also found that regardless of the risk factors considered, mortality prediction was less accurate in our oldest participants. Previous researchers have suggested that this may be because of the rapidly changeable nature of health in the elderly, making any mortality prediction more difficult.42
Some have even suggested that survival in the oldest old is a random process, independent of individual characteristics and primarily a function of external chance events.9,43
Although our results suggest that survival in the oldest old is a predictable, nonrandom process, mortality prediction does appear to be more difficult for this population, and optimal prediction may require consideration of risk factors not considered in our study.
Our findings should be considered in the context of the limitations of this study. First, we relied on patient self-report to measure chronic conditions and function, leaving open the possibility that more objective measures of these concepts could lead to different results. However, previous research suggests that patient report of both function and chronic conditions are reliable and have predictive validity.44,45
Although some studies have suggested that the accuracy of self-reported chronic conditions decreases with age, others have concluded that self-reports are valid among both younger and older participants.46
Thus, our findings highlight the need to examine whether the decline in the prognostic value of chronic conditions with increasing age is replicated when chronic conditions are measured through administrative diagnosis codes and chart review.
Second, our measures for chronic conditions and function are not exhaustive, leaving open the possibility that more-comprehensive measures of these concepts could lead to different results. However, we believe our measures are robust, encompassing both difficulty and dependence among ADLs, IADLs, and mobility for function
and the most common causes of death for chronic conditions.31
Third, our observational cohort study design can provide evidence of predictive association but not of causation. Thus, although many of our risk factors have been shown in other studies to be etiologic causes of mortality, our goal in the present study was strictly prediction. Finally, because our study sample was a community-dwelling cohort, it is unclear whether our results can be extrapolated to other populations, such as nursing home residents or hospitalized patients.
In summary, our results suggest that chronic conditions are a stronger predictor of mortality among younger participants, whereas functional limitations are a stronger predictor of mortality among older participants. Therefore, mortality indexes and risk-adjustment methods that only consider chronic conditions may be suboptimal for comparing outcomes among persons older than 80 years. Overall, our findings suggest that comorbidity, as it is usually measured, has different prognostic implications for oldest old and that further research is needed on the interactions between age and chronic conditions.