Screening aims to detect bowel cancer (or conditions that can lead to bowel cancer) at an early stage, when there is a good chance that treatment will be successful. There are two methods of screening for bowel cancer:

  1. A test to detect traces of blood in your faeces – the faecal occult blood test.
  2. An examination of the inside of the bowel by a procedure called flexible sigmoidoscopy or colonoscopy.

The faecal occult blood test (FOBT) is a biochemical test that can detect tiny amounts of blood in stool samples; the amounts are so small that they cannot be seen by the naked eye (“occult blood” means “hidden blood”). This is the most frequently used method of bowel cancer screening in the European Union. There are two types of FOBT: the guaiac FOBT and the immunochemical FOBT (also called the faecal immunochemical test, FIT). The two types of test differ in how stool samples need to be collected and how they are analysed. You carry out the guaiac FOBT or FIT in the privacy of your own home. The screening kit provides a simple way for you to collect small samples of your bowel movements. The samples have to be analysed in a laboratory.

Flexible sigmoidoscopy and colonoscopy are medical procedures that use a long, flexible, narrow tube with a light and a tiny lens for viewing at one end. The tube is inserted through the anus to look inside your bowel and can show changes caused by cancer and other diseases, such as polyps. Polyps are abnormal growths inside the bowel that in some cases may develop into bowel cancer unless they are removed. If polyps are found, most can be removed painlessly during the screening examination. Colonoscopy enables the entire large bowel to be inspected. Flexible sigmoidoscopy enables examination of only the lower half of the large bowel, but it is quicker and can be done without sedation and with only an enema as bowel preparation (i.e. the preparation required before attending). Because sigmoidoscopy and screening colonoscopy are invasive procedures they may involve serious complications.

If screening with the guaiac FOBT, FIT, or flexible sigmoidoscopy reveals abnormal results, you will be sent for colonoscopy to check the inside of the entire bowel for cancer and polyps.

Most studies of the effect of screening in avoiding death due to bowel cancer have been conducted with healthy people older than 45–50 years and younger than 70–75 years. All of the recommended screening methods are effective in reducing the risk of dying from bowel cancer. In studies, participating in screening with the guaiac FOBT or FIT has reduced the risk of dying from bowel cancer by 20-30%; participating in flexible sigmoidoscopy screening has reduced the risk by about 50%. Participating in colonoscopy screening is estimated to reduce the risk of dying of bowel cancer by about 30-65%, but the evidence for this estimate is limited.

The risk of developing bowel cancer was reduced in study participants, by about 30% for screening with flexible sigmoidoscopy. In other words 3 out of every 10 bowel cancers were prevented in people who participated in screening. With screening colonoscopy the risk reduction is about 50-65%, but the evidence for this estimate is limited.