This study has two main findings. First, socioeconomic determinants and physician density are key predictors of CRC screening. This result confirms previous findings in the literature.4,5,11,12,20
Second, adjustment for socioeconomic determinants eliminated black-white disparities, and further adjustment for physician density eliminated Hispanic-white disparities. Previous research on race and CRC screening has primarily focused on black-white disparities. Here, we also considered Hispanic-white disparities, an important undertaking given the Hispanic population is projected to steadily rise to 20% of the total US population in the next 20 years.21
For both FOBT and endoscopic screening, socioeconomic status has been previously observed to mostly or fully account for observed black-white racial disparities.3,8,15
Socioeconomic factors such as income, educational attainment, occupation, and health insurance directly affect an individual's access and utilization of screening, the quality of screening, and the potential benefits derived from screening. We reached similar conclusions for the black-white disparity in recent CRC screening. Yet for Hispanics, these socioeconomic factors explain some, although not all, of the observed disparities. Higher densities of GIs may be especially important for Hispanics. Further work is needed to determine if these effects are causal and, if so, why Hispanics are particularly sensitive to GI physician density.
In addition to health care access and usage, the supply and balance of PCPs and GIs have an important effect on CRC screening, detection, and survival. PCPs serve an important role in initiating and overseeing CRC cancer screening.22,23
Over time, colonoscopy has become the more favored method of endoscopic screening compared with sigmoidoscopy. GIs perform two thirds of these procedures,24,25
so it is logical that GI density will affect screening rates. Our results suggest higher GI density may be especially important to higher overall CRC screening and reductions in some disparities. PCP density is also likely important, although its effect may be masked by regional socioeconomic factors and GI density, with which it is correlated. Beyond screening, CRC incidence, late-stage diagnosis, and mortality decreased as PCP and GI densities increased in studies conducted in Florida26,27
Similar benefits of high PCP and specialist physician density have been noted in breast cancer,29,30
Policy initiatives at the community, state, and national levels have attempted to increase access and physician supply as well as reduce financial, physical, institutional, and organizational barriers. Medicare coverage of annual FOBT and quadrennial sigmoidoscopy began in 1998, and coverage for decennial colonoscopy began in 2001 for average-risk beneficiaries. Results of these initiatives have varied, disparities have persisted,12,13
and modest improvements in screening have been observed most often in the highest socioeconomic groups.33
We acknowledge several limitations in this study. First, we were not able to differentiate between sigmoidoscopy and colonoscopy for the entire time period between 1993 and 2008. Current US Preventive Services Task Force guidelines for endoscopic CRC screening recommend sigmoidoscopy every 5 years or colonoscopy every 10 years for average-risk adults age 50 to 75 years. In considering flexible sigmoidoscopy or colonoscopy within the past 5 years, we may have provided conservative estimates of recent endoscopic screening. Second, we were not able to determine whether the underlying purpose of an individual's endoscopic CRC procedure was screening, diagnosis, or surveillance. In 2008, for example, 40.3% of the adults reporting FOBT within the past year also reported colonoscopy within the past year. Many of these colonoscopies were likely conducted to investigate a positive FOBT result for diagnostic purposes rather than screening. However, our outcome of interest was recent CRC screening, which occurred with the recent FOBT. Third, we relied on self-reported CRC cancer screening, which may be subject to recall bias. Fourth, we were not able to differentiate the type of health insurance carried by respondents. Fifth, reporting lags and other inaccuracies have been identified with the American Medical Association Masterfile, which may have affected our knowledge of physician density levels and the geographic and specialty distributions of the physician workforce.34,35
We may also have underestimated the universe of physicians conducting CRC screening by excluding colon and rectal and general surgeons, some of whom perform screening colonoscopy, and overestimated by including all GIs, including those who do not perform lower endoscopy. Sixth, we did not measure the productivity or number of hours spent on patient care among clinical PCPs and GIs. We treated a state as a unit, yet considerable variation exists within states in the distribution of PCPs and GIs as well as CRC screening levels. Additional physicians in a state may locate in areas where density is already high, and diffusion of care may be limited. Additional analysis at the local or regional health care market level may allow greater understanding of physician supply and utilization of colorectal cancer screening. Finally, the associations we observed may not reflect direct causation.
Future work could expand on this analysis of the joint relationship between individual-level socioeconomic determinants and area-level physician capacity and effect on CRC screening. Focus on smaller geographic areas (eg, hospital referral regions) and more refined calculation of physician capacity (eg, based on full-time equivalents of clinical work) would enable a more nuanced understanding of how local supply might best meet local demand. Additional work could also examine the current distribution of specialist physicians who perform screening colonoscopy (GIs, colon and rectal surgeons, and general surgeons) to determine if future capacity will be able to meet future demand.
As the population ages, and as the modality of screening changes, the demand for CRC cancer screening will likely increase faster than the capacity to perform that screening. First, the proportion of adults age ≥ 50 years is projected to increase by approximately 24% over the next 20 years,36
which may far exceed the rate of growth in physicians.37,37
Second, PCPs serve an important role in initiating and overseeing CRC cancer screening,22,38
and great regional shortages of PCPs currently exist or are anticipated. Third, colonoscopy has become the more favored method of endoscopic screening compared with sigmoidoscopy,39
and GIs—also in limited and variable supply—perform the vast majority of these procedures.24,25
Whether the United States is able to meet the future demand for screening and address historical disparities in screening may depend on access, physician supply, interaction of PCPs and specialists, financial reimbursement of services, and organization of the health care system.