Faecal Occult Blood Test

The faecal occult blood test helps to diagnose bleeding disorders of the gut

What is a faecal occult blood test?

The faecal occult blood test (FOB test) detects small amounts of blood in your faeces which you would not normally see or be aware of. (Faeces are sometimes called stools or motions. It is the waste that you pass out from your anus. Occult means 'unseen' or 'invisible'.)

Why is the test done?

There are several disorders which may cause bleeding into the of the guts (intestines). For example: ulcers, colitis, polyps, and bowel (colon) cancer. If these bleed heavily then your faeces would be obviously 'bloody' or a very black colour. However, sometimes these disorders only bleed with a trickle of blood. If you only have a small amount of blood in your faeces then the faeces look normal. However, the FOB test will detect the blood. So, the test may be done if you have symptoms in the abdomen such as persistent pain. It may also be done to screen for bowel cancer before any symptoms develop (see below).

Note: the FOB test can only say that you are bleeding from somewhere in the gut. It cannot tell from which part. If the test is positive then further tests will usually be arranged to find the source of the bleeding. For example, endoscopy, colonoscopy, barium enema, various scans, etc.

How is the faecal occult blood test done?

A small sample of faeces is smeared onto a piece of card. You obtain a sample by using a small scrapper to scrape some faeces off toilet tissue which you have just used after going to the toilet. A chemical is added to the sample on the card. If there is a change in colour after adding the chemical, it indicates that some blood is present.

A doctor may do this test in the GP surgery, or send a sample to the 'lab' for testing. Also, if required, there are test kits that you can get at pharmacies which enable you to do the test at home. Some people are issued with test kits to do testing at home.

Sometimes two or three FOB tests are done on two or three separate samples of faeces, obtained on different days. This is because a bleeding disorder of the gut may only bleed 'now and then'. So, not every sample may contain blood. A series of two or three samples done on several days may be more accurate in detecting a bleeding gut disorder.

Screening for bowel cancer

In general, the earlier bowel cancer is diagnosed, the better the chance of a cure. Bowel cancer often causes no symptoms until it is fairly advanced. However, early small cancers often bleed slightly into the gut without you realising. An FOB test is often positive if you have early bowel cancer, before it causes symptoms.

Therefore, in April 2006 a national bowel cancer screening programme is to be begin. All people in the UK aged 60-69 will be invited to do an FOB test every two years. The aim is to detect people with the early stages of bowel cancer. (For details, see www.cancerscreening.nhs.uk/bowel/).

© EMIS and PIP 2006   Updated: February 2006