The faecal occult blood test helps to diagnose bleeding disorders of the gut (intestine).
What is a faecal occult blood test?
The faecal occult blood (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 back passage (anus). Occult means unseen or invisible.)
Why is the faecal occult blood test done?
Any heavy bleeding into your gut would be obvious because your stools (faeces) would be bloody or a very black colour. However, sometimes there is only a trickle of blood. If you only have a small amount of blood in your stools then the stools look normal. However, the FOB test will detect the blood. So, the test may be done if you have symptoms in the tummy (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 be arranged to find the source of the bleeding - usually, endoscopy and/or colonoscopy.
How is the faecal occult blood test done?
A small sample of stool (faeces) is smeared on to a piece of card. You obtain a sample by using a small scraper to scrape some stool 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 laboratory 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.
Usually two or three FOB tests are done on two or three separate stool samples, obtained on different days. This is because a bleeding disorder of the gut (intestine) 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.
There are some foods and medications which may affect the results of FOB tests, making it appear there was blood present when there wasn't. However, this is quite unusual and generally no dietary advice is now given when doing an FOB test.
Screening for bowel cancer
Screening means looking for early signs of a particular disease in otherwise healthy people who do not have any symptoms and when treatment is likely to be curative. Bowel (colorectal) cancer screening aims to detect colorectal cancer at an early stage when there is a good chance that treatment will cure the cancer.
As colorectal cancer is much more common in older people, the decision has been made for people of a certain age to be invited to participate in the colorectal cancer screening programme. This involves testing three samples of your stools (faeces) for blood. The NHS Bowel Cancer Screening Programme has been introduced in the UK as follows:
- In England, Wales and Northern Ireland, people aged 60-74 are routinely offered screening every two years.
- In Scotland, people aged 50-74 are routinely offered screening every two years.
The first test kit should automatically arrive by post within a few weeks after you reach the age at which screening starts. After your first screening test, you will then be sent another invitation and screening kit every two years until you reach the maximum age.
A normal result is reassuring, but it is a test to look for cancer in people who have no symptoms. If you do have any bowel symptoms, such as a change in bowel habit, persistent loose stools (diarrhoea), tummy (abdominal) pain or weight loss, do not wait for a screening test. Make an appointment to talk to your GP about it.
Further reading and references
Macmillan Cancer Support; Faecal occult blood test
NHS Cancer Screening Programmes; Bowel cancer screening: the facts
Bailey JR, Aggarwal A, Imperiale TF; Colorectal Cancer Screening: Stool DNA and Other Noninvasive Modalities. Gut Liver. 2016 Mar10(2):204-11. doi: 10.5009/gnl15420.
Brenner H, Tao S; Superior diagnostic performance of faecal immunochemical tests for haemoglobin in a head-to-head comparison with guaiac based faecal occult blood test among 2235 participants of screening colonoscopy. Eur J Cancer. 2013 Sep49(14):3049-54. doi: 10.1016/j.ejca.2013.04.023. Epub 2013 May 22.