Screening tests cannot find every bowel cancer. The number of cancers not detected by screening depends on the type of test performed. Up to 5 out of 10 cancers may be missed with the guaiac faecal occult blood test (FOBT). With the newer faecal immunochemical test (FIT), fewer cancers are missed. With colonoscopy, about 1 out of 10 bowel cancers may be missed. Flexible sigmoidoscopy performs similarly to colonoscopy in the lower part of the bowel, but it does not reach the upper part of the bowel; therefore, approximately 4 out of 10 bowel cancers will not be detected with flexible sigmoidoscopy.

Cancer is a slow developing disease, so many of these cancers can be detected at the next screening test or examination, hence the importance of regular participation in screening. Repeat screening increases your chance of having a bowel cancer detected.