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Reference Letter#: L06-0111
Submit responseWe appreciate the comments of Drs Shishehbor and Litaker. We agree that socioeconomic status (SES) is a multidimensional construct, where income serves as only one of many social measures. While such limitations were acknowledged in our paper, our study did adjust for individual education, employment status, ethnicity, and social support. (1) Adjustment for such variables will have partially accounted for some of the heterogeneity comprising SES. Finally, the inclusion of more elaborative social measures, while intriguing, would not have mitigated the importance of exploring the causal pathway factors mediating income- mortality associations - - associations which themselves have been consistently observed in the literature and require explanation. (2)
The exclusion of very high-risk patients (i.e., those on ventilators or dying prior to enrollment) may have indeed introduced bias and attenuated the association between SES and mortality. Unfortunately, the exclusion was unavoidable given that income was ascertained using self- administered surveys. Enrollment into SESAMI required patient consent, which also likely contributed to selection bias. (3) Consequently, the magnitude of association between income and mortality following acute myocardial infarction (AMI) might have been less than otherwise expected had we been able to examine a more representative ‘real-world’ population. Nonetheless, the extent to which such limitations altered our results remains speculative. For example, available evidence suggests that wealth- health gradients diminish, not widen among elderly as compared to younger subgroups - - subgroups which disproportionately comprise higher-risk ‘real-world’ populations. (2,4) More importantly, the objective of our study was not to quantify the true magnitude of association between income and mortality after AMI, but rather, to quantify the extent to which income-mortality associations were explained by traditional atherogenic or vascular factors, non-cardiac comorbidities, and health service use. Based on our results and those of others, (5) there is no reason to believe that age and cardiovascular risk-factors would have not exerted similar explanatory effects on income-mortality associations had higher-risk populations been examined.
To what extent if any, can SES-mortality disparities be modified through intensive secondary prevention strategies? Alternatively, are socially disadvantaged AMI patients pre-destined to die regardless of the provision of intensive secondary prevention initiatives, given their baseline cardiovascular risk profiles at the time of AMI hospital presentation? These remain the pertinent questions for future study. Social-epidemiological and health service research must now explore the impact of secondary prevention interventions to determine whether outcomes can be improved effectively and efficiently among high-risk populations in the real-world.
David A. Alter, M.D., Ph.D., for the SESAMI study group. Institute for Clinical Evaluative Science, Toronto, Ontario, Canada. David.alter@ices.on.ca
References:
1. Alter DA, Chong A, Austin PC, Mustard C, Iron K, Williams JI, Morgan CD, Tu JV, Irvine J, Naylor CD for the SESAMI study group. Socioeconomic status and mortality after acute myocardial infarction. Ann Intern Med. 2006;144: 82-93. 2. Kaplan GA, Keil JE. Socioeconomic factors and cardiovascular disease: a review of the literature. Circulation. 1993;88:1973-98. 3. Tu JV, Willison DJ, Silver FL, Fang J, Richards JA, Laupacis A, et al. Impracticability of informed consent in the registry of the Canadian Stroke Network. N Engl J Med. 2004;350:1414-21. 4. House JS, Lepkowski JM, Kinney AM, Mero RP, Kessler RC, Herzog AR. The social stratification of aging and health. J Health Soc Behav. 1994;35:213 -34. 5. Lynch JW, Kaplan GA, Cohen RD, Tuomilehto J, Salonen JT. Do cardiovascular risk factors explain the relation between socioeconomic status, risk of all-cause mortality, cardiovascular mortality, and acute myocardial infarction? Am J. Epidemolog. 1996;144:934-42.
Conflict of Interest:
None declared
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TO THE EDITOR: Alter et al. address an important question regarding mediating factors that potentially account for the contribution of socioeconomic status (SES) to health care disparities (1). Given the potential social and political implications of these results, careful consideration should be given to several key issues limiting the authors’ interpretations, however. First, it has been previously suggested that SES is a multidimensional construct. While operational definitions are numerous, most incorporate aspects of educational attainment, occupation, and social class. Use of self-reported income as a single measure to represent this construct therefore has the potential to markedly reduce strength of the intended “signal” and thus under-estimate its association with the outcome of interest (2). Second, exclusion criteria applied in this study also have the potential to further attenuate an association between SES and mortality and introduce bias. The authors observe, for example, that patients with lower income had a significantly higher prevalence of cardiac risk factors and were less likely to receive specialty care. Eliminating patients dying within 24 hours of admission or those with “very severe illness”, therefore, has the effect of removing patients with the greatest “exposure” to the potential health effects of socioeconomic status. In short, we find the work by Alter et al. potentially informative, yet failure to attend to the multi-dimensional nature of SES leads us to conclude that they may have under-represented the importance of this construct on health. References
1. Alter DA, Chong A, Austin PC, et al. Socioeconomic status and mortality after acute myocardial infarction. Ann Intern Med. 2006;144(2):82-93.
2. Kaplan GA, Keil JE. Socioeconomic factors and cardiovascular disease: a review of the literature. Circulation. 1993;88(4 Pt 1):1973-98.
Conflict of Interest:
None declared