The main use for the FOB test is as a screen for early bowel cancer. 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; other follow-up procedures would need to be done to find the source of the bleeding because the blood may also indicate other problems.

Another use of the FOB test is to look for a cause of anaemia, such as might be caused by a bleeding ulcer. If you have symptoms of anaemia, such as tiredness or a low haemoglobin and haematocrit, and/or bloody or dark stools, your doctor may look at the FOB test.

Faecal occult blood tests are usually done as part of a routine examination, possibly annually after age 50, or as directed by your doctor. Most people who have them done do not have any symptoms. They are used mainly as a screening tool for early detection of bowel cancer.

Your doctor may also order a FOB test if they suspect that you have an unexplained anaemia or symptoms, such as bloody or dark, tarry stools.

The FOB test is normally negative. A positive result indicates either blood in the stool or an interfering substance. More than one sample is done to:

1. catch bleeding that happens now and again; and
2. help rule out false positives.

False positives for the FOB test fall into three classes:

1. substances or conditions that cause bleeding, such as bleeding gums following a dental procedure or drugs that may cause gastrointestinal bleeding, such as anticoagulants, aspirin, steroids, and large doses of iron preparations;
2. other sources of haemoglobin, such as eating red meat within three days before testing; and
3. other substances that will react with the FOB test, such as eating fish, turnips, horseradish, or drugs such as colchicine and oxidising drugs (for example, iodine and boric acid). Your doctor may ask you to stay off your medications and follow dietary restrictions before collecting the FOB test samples.

False negatives may be caused by large doses of vitamin C and by not collecting multiple samples (because many conditions only produce blood now and again).