Colonoscopy and colorectal cancer mortality in the veterans affairs health care system: A case-control study

Background: Colonoscopy is widely used in the Veterans Affairs (VA) health care system for colorectal cancer (CRC) prevention, but its effect on CRC mortality is unknown. Objective: To determine whether colonoscopy is associated with decreased CRC mortality in veterans and whether its effect differs by anatomical location of CRC. Design: Case-control study. Setting: VA-Medicare administrative data. Participants: Case patients were veterans aged 52 years or older who were diagnosed with CRC between 2002 and 2008 and died of the disease by the end of 2010. Case patients were matched to 4 control patients without prior CRC on the basis of age, sex, and facility. Conditional logistic regression was performed to calculate odds ratios (ORs) for exposure to colonoscopy, with adjustment for race, Charlson Comorbidity Index score, selected chronic conditions, nonsteroidal anti-inflammatory drug use, and family history of CRC. Measurements: Exposure to colonoscopy was determined from 1997 to 6 months before CRC diagnosis in case patients and to a corresponding date in control patients. Subgroup analysis was performed for patients who had undergone screening colonoscopy. Results: A total of 4964 case patients and 19 856 control patients were identified. Case patients were significantly less likely to have undergone any colonoscopy (OR, 0.39 [95% CI, 0.35 to 0.43]). Colonoscopy was associated with reduced mortality for left-sided cancer (OR, 0.28 [CI, 0.24 to 0.32]) and right-sided cancer (OR, 0.54 [CI, 0.47 to 0.63]). The results were similar for patients who had undergone screening colonoscopy (overall OR, 0.30 [CI, 0.24 to 0.38]). Sensitivity analyses that varied the interval between CRC diagnosis and colonoscopy exposure did not affect the primary findings. Limitation: Unmeasured confounding. Conclusion: In this study using national VA-Medicare data, colonoscopy was associated with significant reductions in CRC mortality among veterans and was associated with greater benefit for left-sided cancer than right-sided cancer.