Mortality prediction with a single question

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“On a scale from 1 to 5, how would you rate your own health?”

Self-reported or self-rated health status (SRH) is a commonly used indicator in both clinical epidemiology and population health. It is a subjective measure of health that is thought to reflect an individual’s integrated perception of the domains of health, including biological, psychological, and social dimensions. The World Health Organization considers SRH to be a reflection of population health and healthy life expectancy within countries1. It is assessed either by a questionnaire or by a single question which asks subjects to rate their own health, usually on a four or five-point scale from poor to excellent. SRH has been used as a health indicator in epidemiological studies since the 1950s, and has been found to predict future health outcomes independent of physical, socio-demographic, and psychosocial indicators2-5. It is widely considered to be a valid indicator of health status.

Relationship with mortality and other health indicators

Studies have demonstrated a strong association between SRH and a number of outcomes, including future mortality, morbidity, health care use, and nursing home entry4, 6-8. These relationships are generally attenuated, but persist even after adjusting for a wide variety of sociodemographic variables and physical and mental health indicators. The most commonly studied association is that between SRH and mortality, where research has shown poor SRH to be associated with increased risk of mortality3, 9-11. Numerous studies have shown a strong and consistent relationship between SRH and risk of all-cause mortality3, 6, 9, 24, 25. This relationship exists in both genders, and all age groups and ethnic groups studied. However, it is strongest in younger age groups, males, and those of higher socioeconomic status21, 26, 27. The odds ratio of mortality for those individuals who rate their health as ‘poor’ compared to those who rate their health as ‘excellent’ has been calculated as being 1.61-1.92, controlling for a variety of other health status indicators and mortality risk factors6, 25. However, there is some evidence to suggest that knowledge of one’s objective health status is an important factor in the relationship between SRH and mortality. Idler et al. showed that in a group of people with circulatory disease, SRH was an independent predictor of mortality only in the subgroup that was aware of their diagnosis28. In the group that was unaware of their disease, the association was not statistically significant. Other studies have also shown that the relationship between SRH and mortality is strongest when the cause of death is a condition which the person is likely to have been aware of, for example, diabetes29.

Studies have also illustrated associations between SRH and other health indicators. Latham et al. found that SRH is a significant predictor of the onset of coronary heart disease, diabetes, stroke, lung disease, and arthritis, but not cancer4. Additional studies have also found statistically significant relationships between SRH and chronic diseases, including stroke and type II diabetes30, 31. One possible explanation for these findings is that the predictive relationship between SRH and mortality works through morbidity. Other studies have found associations between SRH and health service use, including physician service use and overall healthcare expenditure5, 32. Finally, SRH is known to be related to decline in functional health status3, 33. Specifically, studies have found that poor SRH is predictive of decline in gait speed, for example34. Thus, it makes sense that it is also predictive of nursing home entry35.

Studies looking at correlates of SRH have found that various sociodemographic variables, as well as indicators of physical and mental health and functioning are associated with SRH. For example, Borim et al. found that educational attainment, income, physical health, and an indicator of mental health/happiness were associated with SRH in an older population36. In a study of two large occupational cohorts in Europe, Singh-Manoux et al. found that physical indicators of health such as number of recurring health problems, symptom score, physical mobility, and measures of minor psychiatric morbidity explained the majority of the variance in SRH20. Differences in SRH have also been reported with regards to sex, marital status, ethnicity, employment, smoking status, and mental health indicators, such as depression 3, 37-40.

Theories of Self-rated health

The reasons underlying the relationship between SRH and mortality are unclear. This is partly due to the fact that the construct of SRH is poorly understood. Attempts to illuminate the construct of SRH have included studies on its determinants, as well as presentations of conceptual models.

Although it may be tempting to assume that SRH simply reflects objective health status, empirical research has shown it to be more complex. Even after accounting for a wide range of physical and mental health indicators, including physiological measures, some variance in SRH remains unexplained. One theory is that SRH is a more inclusive and accurate measure of health status than the covariates used3, 12, 13. It is simply not possible to collect information on every single disease, disability, and functional impairment that a person may have. In addition, surveys generally do not gather information on disease severity and prognosis, and individuals may weigh their self-ratings of health based on these factors13. Similarly, SRH may capture physical health risk factors that are undetectable or not known14. For example, it has been demonstrated that people may not be aware of diagnosable risk factors – one study showed that 17.4% of Canadians with high blood pressure were not aware of it15.

Another theory is that SRH reflects one’s self-concept and lifestyle factors, including social and material conditions that are known to have adverse effects on health16. Although studies have attempted to control for various factors, residual confounding is still a possibility, and this theory postulates that uncontrolled-for psychosocial and socio-demographic factors at least partially explain the relationship between SRH and mortality. It is important to note that SRH represents an individual’s own perception, so cognitive processing plays a role. Some personality factors, such as a weak sense of mastery or fatalism, have been shown to be correlated with SRH17. Hypochondria and preoccupation with health are also associated with SRH18. This theory also suggests that SRH reflects dispositional optimism or pessimism, and that it can influence future health behaviours3, 6. However, it is important to note that while health behaviour, social resources, and optimism are associated with mortality, their effects are not independent of their influences on the individual. Therefore, these factors could explain the effect of SRH on mortality risk only if they reflected change in objective health status that takes place between the self-rating and death. This is not supported by the majority of findings that show the association between SRH and mortality is strongest for shorter rather than longer follow-up periods19-21. Finally, contextual factors, such as culture, affect how individuals rate their own health22. In truth, SRH probably captures and is influenced by many factors. To this end, Jylha has proposed a model that integrates both social and biological pathways to explain how individuals rate their own health13.

In her paper, Jylha theorizes that “the basis of self-rated health lies in the biological and physiological state of the individual organism,” and that contextual factors influence the processing of information used to inform this rating13. She proposes that individuals consider information on formal health status, such as number of physician-diagnosed diseases and prescription medications. Additional input information includes subjective experiences of symptoms and body sensations, which is reflective of overall functioning and may indicate sub-clinical disease. Contextual influences include cultural and historical definitions of ‘health,’ comparative groups (for example, how healthy are age-group peers?), previous health experiences, health expectations, and cultural conventions in expressing positive and negative opinions13. In addition, individuals may incorporate family history into their self-ratings of health by using this information to judge how likely they are to suffer from heritable diseases. Jylha emphasizes that SRH is a dynamic evaluation, in which individuals may judge the trajectory of their health and not only their current level of health.

Jylha proposes that sociodemographic factors correlated with SRH influence it by acting through pathways which affect objective health status and functioning. She proposes that factors such as education and social status do not directly describe health status, but affect the likelihood of physical and mental health conditions that individuals use as the basis for self-ratings of health23. She further suggests that SRH may reflect the presence or absence of resources that can attenuate decline in health, such as social support or personal characteristics and resources. Finally, Jylha argues that ‘non-health’ individual and social characteristics may influence SRH by shaping the framework of evaluation13.

Research on this topic has primarily been carried out as secondary data analyses conducted on existing datasets that were not designed to answer questions specifically related to SRH. However, these datasets tend to have large, representative samples, which reduces the risk of selection bias. In addition, the variable of SRH is not subject to misclassification. The relationship between SRH and mortality has been studied in a number of different populations, including young adults, the old-old, occupational cohorts, emergency department patients, and in people with mild to moderate cognitive decline20, 41-44. Although there are relative differences by subgroup, the consistency of the association provides strong support for the idea that SRH is indeed predicative of mortality, and is therefore an acceptable proxy for overall health status.

 

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Nicole Haywood

Associate editor for the IJHS. Bachelor of Health Sciences, class of 2014, University of Ottawa.

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