This article demonstrates that an increasing burden of comorbidity tends to have less of an effect on Black than on White survival among elderly men with prostate cancer. Although 3-year mortality rates are relatively low for all groups, the absolute differences in survival between Black and White men are largest for men without comorbidities and narrow with increasing number of comorbidities. This pattern was most pronounced when examining counts of comorbidities but was also seen for individual comorbidities. Although racial differences in receipt of prostatectomy vary by comorbidity burden, differences in treatment do not explain racial differences in the effect of comorbidities on survival.
Despite the importance of comorbidities to outcomes, there have been relatively few prior studies examining racial differences in the relationship between comorbidity and outcome. One analysis of a North Carolina cohort found that the presence of hypertension or diabetes had a smaller impact on the development of disability among Blacks than among Whites (Kelley-Moore & Ferraro, 2004
). Another study demonstrated similar patterns of association between the Charlson comorbidity index and mortality in Black and White women with newly diagnosed breast cancer (West et al., 1996
). Interestingly, Freeman et al. (2004)
in a study of prostate cancer patients also demonstrated a racial difference in the effect of comorbidity on survival. The study included 864 prostate cancer patients from a single city, and the authors collected data using chart review rather than administrative claims. Results demonstrated that racial differences in mortality were greatest for men with no comorbidities (HR = 1.75; 95% CI = 1.33–2.31) and disappeared for men with five or more comorbidities (HR = 0.90; 95% CI = 0.59–1.29), a pattern remarkably similar to that seen in our data (Freeman et al., 2004
). Our study extends these findings to a national cohort of more than 50,000 men with localized disease, demonstrating a smaller racial disparity in survival among men with more comorbidities, a smaller effect of comorbidity on survival among Black men, and racial differences in the relationship between comorbidity and the use of prostatectomy.
What are the potential explanations for this finding? Several studies suggest that Blacks develop chronic illnesses at an earlier age than Whites (Dunlop et al., 2007
). Thus, it is possible that Black men with chronic illnesses who are alive in their 60s and 70s are less “susceptible” to the impact of the illness because individuals who were at high risk of dying from their illness have died at an earlier age (Feinglass et al., 2007
). This type of survivor bias would select for more resilient individuals among Blacks and could explain a lower effect of comorbidity on mortality among elderly Blacks. Another possible explanation relates to differences in severity of prostate cancer. If Black men have more aggressive or advanced prostate cancer than White men, they may be more likely to die from prostate cancer and less likely to die from other causes. A larger competing risk of death from prostate cancer among Black men than White men could result in a smaller effect of comorbidities on overall survival. Although unmeasured severity of prostate cancer among Blacks may explain some of the results, its contribution is likely to be relatively small given the focus on individuals with localized disease. Furthermore, the same effect was seen in the study by Freeman et al. (2004)
that included more extensive information on stage and grade taken from clinical charts. Another possible explanation is that being diagnosed with a chronic illness is a marker of better access to or quality of care among Blacks. If this were true, the higher level of health care access and quality among Blacks with more comorbidities would counterbalance the increased impact of comorbidity on survival. Finally, it is possible that the difference in effect relates to racial differences in the severity of the comorbidities. For the great majority of comorbidities (except diabetes), we have information only on the presence or absence of the comorbidity, not the severity of the disease itself. If Whites had greater severity of disease for any given comorbidity (e.g., worse coronary heart failure), this may lead to a greater impact of the comorbidity on outcome among Whites. However, most evidence suggests that the severity of common comorbidities (e.g., diabetes, coronary heart failure, renal failure) is greater among Blacks than among Whites, suggesting that this level of unmeasured severity is more likely to have minimized than exaggerated the observed effect.
Interestingly, we also find a racial difference in the effect of comorbidity on the use of prostatectomy, although these differences do not mediate the relationship between race, comorbidity, and survival. Here the pattern is not linear, with the greatest racial differences seen in patients with no comorbidities and the smallest differences seen in patients with one or two comorbidities. This pattern may reflect the combined effect of several forces: Having one or two comorbidities may be a marker for better access to care and a more “level playing field” for Black and White men. Above this number of comorbidities, the overall burden of disease may overwhelm the equalizing effect of access to care, and prostatectomy is likely to become a relatively rare intervention, with greater use among individuals with greater access and resources.
Individual comorbidities were heterogeneous in terms of their association with race and the absolute risk of mortality. Despite this heterogeneity, examination of the patterns across individual comorbidities suggested that the results seen for the comorbidity count largely reflected a broad underlying pattern among the individual comorbidities rather than a pronounced impact of a small set of individual comorbidities. With the exception of hypothyroidism, hypertension, and complicated diabetes, individual comorbidities tended to be associated with increased risk of mortality for both Blacks and Whites. In general, the risk for Blacks versus Whites with a particular comorbidity tended to be smaller than for Blacks versus Whites without comorbidities, although the CIs for these risk estimates were broadly overlapping. Beyond this finding, the results for the individual comorbidities did not provide any particular insight on the mechanism for the underlying pathway. We caution that because the sample sizes for individual comorbidities, particularly for Blacks, were considerably smaller than for the comorbidity count, these comparisons had more limited power, and for this reason, we focused on broad patterns rather than specific categories.
The finding that the effect of comorbidities varies by race has important implications for studies of racial disparities in health and health care. Many studies adjust for comorbidity when examining racial differences, but few, if any, consider the interaction between comorbidity and race. We have previously demonstrated that racial differences in physician trust vary substantially by geographic location and socio-demographic characteristics (Armstrong, Ravenell, McMurphy, & Putt, 2007
). It seems likely that this type of variation in disparity is more the rule than the exception—a reality that has important consequences for efforts to reduce disparities. If disparities are not uniform within a specific clinical situation but vary by the characteristics of the patient or even of their city of residence, it becomes critically important to understand which groups are at greatest risk before developing interventions to reduce disparities. For example, although programs to reduce racial disparities in prostate cancer survival might naturally focus on Black and White men with the greatest contact with the health care system, those men are also most likely to have multiple comorbidities and, as demonstrated in this study, the smallest disparity in survival. Similarly, we have previously demonstrated that disparities in prostate cancer treatment may vary widely between hospitals or geographic regions (Asch & Armstrong, 2007
). Given the recent evidence that relatively little progress has been made in reducing cancer disparities over the last decade (Gross, Smith, Wolf, & Andersen, 2008
), it is possible that an inadequate appreciation of the complex variation within any given disparity has limited the effectiveness of our efforts to reduce these disparities. It is hoped that increasing our understanding of this variation can lead to more effective interventions and a different story for the next decade.
This study has several limitations. We used administrative data to define the cohort and assess comorbidities, treatment, and outcome. Administrative data have limited accuracy in some settings, although considerable support exists for their use in defining surgical treatment, comorbidities, and survival (Fisher et al., 1992
). The ability to identify prostate cancer patients from administrative data is less certain; however, we triangulated several approaches to maximize the accuracy of the case ascertainment. For several states, we were also able to compare the number of cases in our data with the number in the state SEER registries and found very similar numbers of cases. We studied a single disease, and the relationship between race, comorbidity, and survival in prostate cancer may not be generalizable to other diseases. As mentioned earlier, the number of Blacks in our study, while large, was substantially smaller than the number of Whites, so our ability to detect effects in Blacks was smaller than in Whites. Although the use of comorbidity number in addition to individual comorbidities reduced analytical issues with small numbers, comorbidity number is an imperfect estimate of comorbidity burden because all comorbidities are weighted similarly. Our study focused on men diagnosed with localized prostate cancer, and the generalizability of the finding to men with metastatic disease is unknown.
In summary, comorbid conditions have differential effects on survival after a diagnosis of localized prostate cancer between Black and White men. Racial disparities are most pronounced between Black and White men with no or few comorbidities and are not evident at higher levels of comorbidity.