A Cochrane review included 3 observational studies (one nested case-control study and two cohort studies) with a total of 6900 subjects. No RCTs were found. In the case-control study (n=4664), only 2/40 of the patients dying of colorectal cancer had undergone surveillance colonoscopy on at least one occasion compared with 18/102 of the controls (RR 0.29, 95% CI 0.06 to 1.31). In the first cohort study (n=2050) carcinoma was detected at a significantly earlier stage in the surveillance group; 15/19 had Duke's A or B carcinoma in the surveilled group compared to 9/22 in the non-surveilled group (P = 0.039). The 5-year survival rate was 77.2% for cancers occurring in the surveillance group and 36.3% for the no-surveillance group (P = 0.026). Four of 19 patients in the surveillance group died from colorectal cancer compared to 11 of 22 patients in the non-surveillance group (RR 0.42, 95% CI 0.16 to 1.11). The third study included only 186 patients.
Comment: The quality of evidence is downgraded because data is based on observational studies only.