This study shows that more than 40% of disparity in CRC incidence and approximately 20% of disparity in CRC mortality between blacks and whites can be explained by differences in screening uptake. Approximately 35% of the disparity in CRC mortality can be explained by differences in stage-specific relative CRC survival. Together differences in screening and relative survival explain a little over 50% of disparity in CRC mortality between blacks and whites.
Although differences in screening uptake and survival explain approximately half of the racial disparities in CRC incidence and mortality, the other half cannot be explained by these factors. A closer look at the data reveals that this finding would have been anticipated. Where racial disparities in CRC incidence and mortality have been increasing over the years (3
), the disparity in screening has been stable, and even decreasing slightly (). The most recent estimates for colorectal screening uptake report a less than 5% difference in uptake between whites (59.8%) and blacks (55.0%) (40
). If screening would have explained the vast majority of the difference in CRC incidence and mortality, we would have expected the same diverging trend for screening rates as for CRC rates. Moreover, part of the observed disparity, especially in males, is caused by an increase in CRC incidence and mortality rates in blacks over the period 1975–1980 (38
), which was before the introduction or wide dissemination of screening modalities and/or adjuvant chemotherapies. Since this increase in CRC incidence and mortality cannot be a result of screening or survival, part of the disparity could never be explained by these factors.
Other factors that may have contributed to the racial disparity in CRC incidence and mortality rates are differences in susceptibility, quality of care, and lifestyle. The prevalence of polymorphisms associated with CRC risk has been shown to differ between whites and blacks (41
). On the other hand, several studies have shown that most of black/white differences in CRC outcomes such as stage of disease at diagnosis or survival are no longer present when correcting for socioeconomic status, including health insurance status (8
). Furthermore, a follow-up study of Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial participants after a positive sigmoidoscopy revealed a lower uptake of diagnostic colonoscopies among blacks when compared with whites but little differences in the yield of colorectal neoplasia (42
). Finally, CRC incidence and mortality in blacks were lower or comparable to whites in the 1970s (30
), making it unlikely that susceptibility is an important driver of current disparities.
In our primary analysis, we only considered uptake of screening, assuming equal quality of screening among blacks and whites. Quality of endoscopy has been shown to be dependent on the skill of the endoscopist performing the procedure, with adenoma detection rates of high adenoma detectors being double that of low adenoma detectors (43
). Adenoma detection rate is an independent predictor of the risk of interval CRC after screening colonoscopy (47
). Physicians treating black patients are known to be less well trained and have less access to clinical resources than physicians treating white patients (48
). Different quality of screening between blacks and whites is therefore not improbable. The sensitivity analysis indeed showed that if blacks received lower-quality screening, a larger proportion of the CRC disparity would be explained by screening. We have eliminated disparities in quality of stage-specific treatments, by evaluating the impact of assuming the stage-specific relative CRC survival of whites for blacks. However, we have not eliminated potential disparities in timeliness of treatment. Blacks are known to present with more advanced stage of disease than whites (41
). Although part of this difference may be explained by differences in screening uptake, timeliness of care-seeking may also play an important role. In a sensitivity analysis assuming the same stage distribution for blacks as for whites (in the absence of screening) the proportion of disparity in CRC mortality explained by access to care indeed increased.
Known or unknown lifestyle factors are the most likely candidates to explain the remaining 34–46% () of the disparity in CRC incidence and mortality that cannot explained by screening and stage-specific relative CRC survival differences. Several lifestyle factors are known to be associated with CRC risk. Alcohol, smoking, obesity, and meat consumption increase the risk of CRC, whereas physical activity and post menopausal hormone replacement therapy (in women) decrease risk (49
). Smoking prevalence had been higher in black men than white men until the late nineties (50
). Furthermore, obesity has been consistently higher in blacks than in whites since 1970, while rates of physical activity have consistently been lower (50
). We have not explicitly evaluated the effect of these established lifestyle factors. Instead, we decided to focus on the more actionable items of screening and survival, because lifestyle factors might be more difficult to modify.
Five limitations are noteworthy. First, we assumed black-white differences in adenoma onset, CRC location and stage distribution between blacks and whites, but not in types of cancers or tumor aggressiveness. This assumption is supported by the fact that CRC incidence and mortality used to be lower in blacks than in whites (30
). Second, we did not incorporate risk factors into the model. As a result of that limitation and because of other potential exogenous factors, the simulated CRC incidence and mortality levels will not correspond with the observed. Instead, we assumed that the simulated relative benefit of white screening and stage-specific relative CRC survival patterns over black would be applicable to the observed CRC incidence and mortality. This approach mirrors a relative-risk approach, where it is assumed that the relative risk of e.g. screening is constant irrespective of the background incidence and mortality level. This assumption seems reasonable: all three randomized controlled trials on biennial guaiac FOBT screening found similar percent mortality reductions ranging from 15–21% despite being performed in populations with a different background incidence level (51
Third, screening uptake in the model was based upon estimated test rates from multiple waves of the National Health Interview Survey. These tests may have been performed for other reasons than screening. Furthermore, the estimates from these surveys may be biased because they are based on self-report; the data are not longitudinal so we had to make assumptions for screening patterns within individuals in the model; the questions on CRC screening have changed from survey to survey; and the respondents may not be representative for the US population as a whole. This last bias in particular may have influenced our results if the bias differs for whites versus blacks. If we have overestimated screening rates in blacks, the contribution of screening differences to the observed disparities in CRC rates may be higher than the estimated 40–50%.
Fourth, CRC incidence and mortality data for blacks are sparse. For this analysis, we therefore used 3-year pooled estimates for CRC incidence and mortality rates over time. However, even with this pooling, an interesting discrepancy exists between the racial disparities in CRC incidence and mortality for women. The absolute racial disparity in CRC incidence is larger than that for mortality, where the opposite would have been expected (as can be seen for men). As a result, the proportion of CRC incidence disparity that can be explained by screening is much smaller for women than for men, whereas the results for CRC mortality are similar. Fifth, we restricted our analysis to the simulated population aged 50 years and older, because this is the group for whom screening is recommended (5
) and incidence and mortality in this group accounts for almost 90% of total CRC incidence and mortality (38
). CRC incidence and mortality is disproportionally higher in blacks among people younger than 50. Racial disparities in survival could have played a role in this difference.
Finally, we have not explicitly considered racial differences in treatment but used racial differences in stage-specific relative CRC survival as a proxy. Data on use and quality of CRC treatment by race are sparse, especially for the population below 65 years of age. There is some information on chemotherapy use (9
), but data on other types of treatment such as surgery and radiotherapy are hard to come by. If part of the racial differences in survival cannot be explained by differences in (quality of) treatment, we have overestimated the potential for reducing disparities in CRC mortality. A systematic review of cancer-specific survival differences between blacks and whites showed that only modest cancer-specific survival differences are evident for blacks and whites treated comparably for similar-stage cancer. For CRC, no difference was found (54
). Therefore, differences in cancer biology between racial groups are unlikely to be responsible for a substantial portion of the observed discrepancy in stage-specific relative CRC survival (54
). We explored the impact of our assumption in a sensitivity analysis and found that the effect was limited.
Although differences in screening and stage-specific relative CRC survival do not explain all of the observed racial disparities in CRC incidence and mortality, they do explain roughly half. Measures should therefore be taken to eliminate the gaps in screening use and survival between blacks and whites. The National Institutes of Health’s State-of-the-Science conference has concluded that elimination of financial barriers should be the first priority area to enhance the use of CRC screening (55
). The Affordable Care Act may be an important step towards this elimination. The Act aims to improve access to quality health care for all Americans (56
). Furthermore, all new health plans must cover certain preventive services including CRC screening without charging a deductible, co-pay or coinsurance. Several studies have shown that in situations with equal access to care, such as military medical centers, Department of Defense facilities, Medicare, Medicaid or clinical trials, no differences in screening uptake or CRC treatments between blacks and whites exist (57
In conclusion, this study shows that approximately half of the disparities in CRC incidence and mortality rates between blacks and whites can be explained by differences in screening and survival. Enabling blacks to achieve equal access to care as whites could therefore substantially reduce the racial disparities in CRC burden.