Researchers have noted an increased risk of colorectal cancer and its precursors in people with diabetes (1-5). This increased risk is hypothesized to be secondary to several factors, such as increased circulating insulin and insulin-like growth factor (IGF-1), and to effects of treatment for diabetes (6-8). Awareness of this increased risk may encourage health care providers to promote colorectal cancer screening to patients with diabetes. Primary care physicians are the most likely candidates to recommend screening, since they provide care to the majority of diabetes patients nationally (9-11) and are primarily responsible for screening and prevention.

Rates of colorectal cancer screening in many states were below 50% between 1998 and 2005 (12), although more recent data indicate rates of 50% to 60% in most states and 60% to 70% in others (13). The US Preventive Services Task Force recommends colonoscopy every 10 years, sigmoidoscopy every 5 years, or annual fecal occult blood testing (FOBT) starting at age 50 and continuing until at least age 75 (14). Efforts are under way to increase rates of colorectal cancer screening among racial/ethnic minorities but have not targeted people with diabetes, even though their risk for colorectal cancer may be higher than the risk for the general population (15). People with diabetes may be less likely to pursue any cancer screening (16,17), even with more primary care visits per year than patients without diabetes (18).

Glycemic control may be associated with colorectal cancer screening in people with diabetes for several reasons. Many diabetic patients with good glycemic control (hemoglobin A1c [HbA1c] <7.0%) visit their physician regularly and adhere to lifestyle recommendations regarding diabetes; therefore, they would have more frequent opportunities to hear about screening and may be more likely to accept colorectal cancer screening recommendations. People with fair (HbA1c 7.0%-8.5%) or poor (HbA1c >8.5%) glycemic control may visit their physician less regularly and be less likely to adhere to prescribed dietary and medication regimens, and may be less likely to accept colorectal cancer screening recommendations.

However, even when receiving regular care, patients with poorly controlled diabetes may not be screened for colorectal cancer. They may have multiple medical and social issues for the physician to address at each visit, and a substantial amount of the visit may be devoted to improving glycemic control, leaving the physician unable to accomplish all preventive goals (19). These patients may also be more medically complex, with more diabetic complications, leading the physician to defer colorectal cancer screening until they are more stable.

The existing research on low rates of screening for other cancers in people with diabetes and on competing priorities in the office visit prompted us to examine associations between glycemic control and rates of colorectal cancer screening in people with diabetes. The objective of this study was to examine the association between glycemic control (average HbA1c) and colorectal cancer screening rates (colonoscopy or sigmoidoscopy within 5 years or FOBT within 1 year) in a large urban hospital with robust numbers of racial/ethnic minorities and large numbers of patients with poorly controlled diabetes. The findings from this study should add to physician awareness about colorectal cancer screening for people with diabetes by identifying their unique factors associated with lack of screening and generate hypotheses for future research.

The study population was 58% female, 57% black, and 14% Latino (Table 1). The colorectal cancer screening rate overall was 38%. Most patients (59%) were covered by some form of government insurance; 22% had no listed insurance and 19% had private insurance. Thirty-six percent of patients had good glycemic control, 37% fair control, and 27% poor control.

Average HbA1c was strongly associated with screening; the highest rates of screening occurred in patients whose HbA1c was 8.5% or less (Table 1). We observed significant associations for race/ethnicity; patients with end-stage renal disease also had higher screening rates than those without. Higher numbers of primary care visits were strongly associated with screening, as expected. However, more frequent ED visits also appeared to be strongly associated with screening.

On average, patients with poor (>8.5%) HbA1c values tended to be younger, uninsured, nonwhite, and have higher rates of no-shows compared with patients exhibiting good and fair HbA1c values (Table 2). The number of primary care visits did not appear to be lower in the group with high HbA1c values.

Correlation coefficients indicated that primary care visits were weakly correlated with ED visits (ρ = 0.19) and no-shows (ρ = 0.34). Although the Pearson and Spearman coefficients were similar, we report the Spearman coefficients because the numbers of primary care visits, ED visits, and no-shows were not normally distributed.

We also performed sensitivity analyses to examine different types of colorectal cancer screening individually by HbA1c level. We found that for all screening groups (5-year colonoscopy, 5-year sigmoidoscopy, 1-year FOBT, and 10-year colonoscopy) the rates of colorectal cancer screening decreased as the glycemic control worsened.

In the final logistic regression model, poor average HbA1c was predictive of not receiving colorectal cancer screening, as was having had fewer than 20 primary care visits in a 5-year period (Table 3). Patients with either government or private insurance were more likely to have been screened than those with no insurance.

Poor glycemic control was associated with not being screened for colorectal cancer, even after adjusting for number of primary care visits and for insurance status. This finding suggests that the issue for these patients goes beyond simply not being seen in primary care offices; there may also be issues of competing priorities (19,20) in the office visit when they do go. Either physicians occupied with helping their patients to gain better glycemic control may be less likely to recommend colorectal cancer screening, or patients struggling to gain better glycemic control may be less likely to follow through on the recommendation. Other studies on cancer screening in people with diabetes (16-18) have also noted lower rates of screening and proposed similar hypotheses (both patient and physician factors) for them. Further research, with primary data collection, would be ideal to define barriers to colorectal cancer screening specifically for people with diabetes.

The emerging research on colorectal cancer risk for people with diabetes suggests that diabetic patients with poor glycemic control may have even higher risk because they exhibit high levels of circulating insulin and IGF-1 and are often receiving multiple treatments (21-25). Therefore, people with poorly controlled diabetes are an important group to target for colorectal cancer screening, even if the care providers are spending most of their time on diabetes management.

This study had several limitations. Although the database's information was extracted from electronic medical records and was complete for most variables, we could not obtain reliable data on several variables of interest: years since diabetes diagnosis, type of diabetes, other diabetic complications beyond end-stage renal disease, reason for colonoscopy (screening vs diagnostic), and physician recommendation for colorectal cancer screening. These are all variables that should be examined further for associations with colorectal cancer screening among people with diabetes. The data also came from a large, urban, safety-net hospital with large numbers of black and Latino patients. We considered this a strength in terms of obtaining a robust sample of racial/ethnic minorities for analyses, but the association of both groups with higher colorectal cancer screening rates (compared with white patients) may not be generalizable to other settings.

We hope the findings of this study will help to provide context for the recent studies associating diabetes with a higher risk of colorectal cancer and polyps; this information is more valuable if more is known about the colorectal cancer screening patterns of people with diabetes and the specific factors associated with lack of colorectal cancer screening for this group. Further studies are needed to explore this finding and to determine the association of other variables with colorectal cancer screening for people with diabetes. Future research could specifically examine variables related to the hypotheses we generated about low colorectal cancer screening rates in people with poor glycemic control but adequate numbers of primary care visits, to define whether patient factors or physician factors are more closely related to the lack of colorectal cancer screening. This information might help health care systems to promote screening in this group of patients, which appears to be more at risk for colorectal cancer and more at risk for not being screened.