Common screening tests

Common screening tests for colorectal cancer include faecal occult blood test, colonoscopy and flexible sigmoidoscopy. According to available evidence, there is insufficient information to determine which screening test is superior to another.

• Faecal occult blood test
  A faecal occult blood test is relatively simple and convenient. You need to follow the instructions and collect a small sample of your stool at home for testing. The following are common faecal occult blood tests :

  1. Guaiac-based Faecal Occult Blood Test (gFOBT)
     This test uses a chemical indicator to detect hidden blood in the stool. If blood exists, colour of the testing paper changes. Since some food (for example, pig blood, red meat, broccoli and pumpkin) and drugs (for example, nonsteroidal anti-inflammatory drugs) may affect the result of the test, it is recommended that these food and drugs should not be consumed a few days before the test. In addition, three stool samples are required for the test.
  2. Faecal Immunochemical Test (FIT)
     This test uses antibodies to detect hidden human blood in the stool. It requires fewer (one or two) stool samples than gFOBT. No dietary or drug restrictions are needed before the test. It detects hidden blood in stool samples, even if tiny amount. The presence of hidden blood indicates bleeding somewhere in the bowel.
     
     Comparison of gFOBT and FIT

     	gFOBT	FIT
     Substances detected	Substances other than human blood may be detected	Only sensitive to human blood
     Restrictions on drugs	Necessary	Not necessary
     Dietary restrictions	Necessary	Not necessary
     Number of samples	3	1-2
     Interpretation of test results	The only indication is a colour change, which errors may occur	Standard equipment used for the test, hence more accurate

(FIT Tube)

• Colonoscopy
  Colonoscopy is currently the best method to examine the lower digestive tract. With the scope length of 1.6m, it helps to inspect the entire colon. Usually it takes about 15 minutes to an hour for the procedure. Bowel preparation and low residue diet are needed. Sedative drug and pain-killer are injected right before examination to reduce patients’ anxiety and discomfort. During colonoscopy, tissue samples for laboratory examination can be obtained and polyps can be removed. Potential complications associated with colonoscopy include bowel perforation, significant bleeding and complications related to the removal of polyps, but they are uncommon.

  
  

• Flexible sigmoidoscopy
  Doctor inserts a scope with a video camera, which is about 60 cm in length, into the large bowel through the anus, in order examine the wall of the large bowel. The range of examination is up to the lower portion of the colon. The procedure is similar to that of colonoscopy. Bowel preparation and low residue diet are needed. Sedative drug and pain-killer may be injected right before examination to reduce patients’ anxiety and discomfort. During the procedure, the doctor can also obtain tissue samples for laboratory examination and remove any polyps. Potential complications associated with sigmoidoscopy include bowel perforation, significant bleeding and complications related to the removal of polyps, but they are uncommon.

  
  

  A faecal occult blood test is non-invasive and convenient, but it cannot detect tumours which are not bleeding. Therefore, repeated faecal occult blood tests are needed in CRC screening. The best method to clearly inspect the entire large bowel is through colonoscopy. If polyps are discovered, they can be removed immediately. There is, however, a risk of bleeding and perforation of the bowel. Before you go for screening, consult your family doctor to choose the most appropriate method.