A faecal occult blood test (FOBT) is a simple test that takes a sample of your stool and checks for blood, which can be an early sign of bowel cancer. If the FOBT is positive for blood, more tests are done to find out what has caused the bleeding. Colorectal cancer is a leading cause of cancer-related deaths and a FOBT can find colorectal cancer early when treatment is more effective.

What is being tested?

The faecal occult blood test (FOB) checks for small amounts of blood (not visible by eye) in your stool. Normally, there will not be enough blood lost through the gastrointestinal tract (stomach and intestines) to turn an FOB positive or for you to notice it by looking.

A positive FOB will tell your doctor that you have bleeding occurring somewhere in your gastrointestinal tract. This blood loss could be due to ulcers, diverticulosis, bleeding polyps, inflammatory bowel disease, haemorrhoids (piles), from swallowed blood due to bleeding gums or nosebleeds, or it could be due to benign or malignant tumours.

Anything that sticks out into the intestine, like a polyp or tumour, and is rubbed against by the faeces as it passes through, has the potential to bleed now and again. Often this small amount of blood is the first, and sometimes the only, symptom of early bowel cancer, making the FOB a valuable screening tool.

How is it used?

The main use for the FOB test is as a screen for early bowel cancer. We reported in a November 2013 news item that FOB screening tests are highly effective in reducing bowel cancer deaths. Blood in the stool may be the only symptom of early cancer. If the cancer is detected before it metastasises (spreads to other areas), there is a greater chance it will be cured. A positive FOB test does not necessarily 'prove' that you have cancer; any positive tests should be followed up with colonoscopy.

When is it requested?

Faecal occult blood tests are offered  every 1-2 years after the age of 50 for screening for colon cancer, or as directed by your doctor. The test should be performed in patients without symptoms. Patients with symptoms of colon cancer should have more definitive investigations such as colonoscopy.

What does the result mean?

The FOB test is normally negative. About 1 in 13 people have a positive test. A positive result indicates blood in the stool. This does not necessarily indicate colon cancer and may be due other sources of blood such as to haemorrhoids or bowel inflammation. A positive test must be followed up with a further test (usually colonoscopy) to determine the source of the bleeding. 

Common questions

• What kind of procedures might follow a positive FOB test?

Before or after you have an FOB test, you may have a rectal examination. A positive FOB test may be followed by:

• Sigmoidoscopy: an examination of the rectum and lower colon (part of the intestines) with a lighted instrument to look for abnormalities, such as polyps;
• Colonoscopy: a more thorough examination of the rectum and entire colon; and
• A double contrast barium enema: a series of X-rays of the colon and rectum.

• Should I buy and use self testing FOB kits which are available in some chemists?

The devices available in chemists detect blood in faeces using several different methods. Their sensitivity to detecting blood can vary and without the skills of either an experienced laboratory scientist or doctor, the results of these tests can be misleading. Consult your GP if you are concerned and think you need a test performed.

More information

RCPA Manual: Occult blood faeces

Healthdirect Australia: Bowel cancer