Effect of colonoscopy on colorectal cancer incidence and mortality: an instrumental variable analysis

BACKGROUND: Using population-based health services information to estimate the effectiveness of colonoscopy on colorectal cancer (CRC) outcomes is prone to selection bias. OBJECTIVE: To determine the effect of colonoscopy on CRC incidence and mortality. DESIGN: Population-based retrospective cohort study. SETTING: Ontario provincial health data information. PATIENTS: This study involved average-risk persons aged 50 to 74 years from 1996 to 2000 who were alive and free of CRC on January 1, 2001. INTERVENTION: Colonoscopy between 1996 and 2000. MAIN OUTCOME MEASUREMENTS: CRC incidence and mortality from 2001 to 2007. RESULTS: The study cohort contained 1,089,998 persons, 7.9% of whom had undergone a colonoscopy between 1996 and 2000. Using primary care physician rate of discretionary colonoscopy as an instrumental variable, the receipt of colonoscopy was associated with a 0.60% (95% confidence interval [CI], 0.31%-0.78%) absolute reduction in the 7-year colorectal cancer incidence and a 0.17% (95% CI, 0.14%-0.21%) absolute reduction in the 5-year risk of death caused by CRC. This corresponds to a 48% relative decrease in CRC incidence (risk ratio [RR] 0.52; 95% CI, 0.34-0.76) and 81% decrease in mortality caused by CRC (RR 0.19, 95% CI, 0.07-0.47). In subgroup analyses, the reduction in the risk of death due to CRC was larger in women than men. The reduction in CRC incidence was larger for complete colonoscopies and for left-sided cancers. LIMITATIONS: Instrumental variable methods estimate only the marginal effect on the population studied. CONCLUSION: Increased use of colonoscopy procedures is associated with a reduction in the incidence and mortality of CRC in the population studied.