A Positive Faecal Occult Blood Test: What Does It Mean?

A positive faecal occult blood test shows that blood, or components of blood, have been found in the stool in an amount too small to see.

On its own this result does not mean you have bowel cancer. Haemorrhoids, polyps and other conditions of the bowel can also produce a positive result.

But a positive result should not be waved away as unimportant either. When a FIT (faecal immunochemical test) done for bowel cancer screening is positive, a colonoscopy is usually needed to find where the bleeding is coming from.

What is a faecal occult blood test?

Small amounts of bleeding in the digestive tract do not always change the colour of the stool. So someone looking at their own stool may see no blood at all.

Faecal occult blood tests are used to detect exactly this — bleeding too small to be seen.

Bowel cancer screening today often uses FIT (Faecal Immunochemical Test). FIT looks for haemoglobin in the stool using antibodies raised against human haemoglobin.

Why does the test come back positive?

A positive test can have many causes. Among them:

• Polyps of the colon or rectum
• Colorectal cancer
• Haemorrhoids
• Diverticular disease
• Inflammatory bowel disease
• Other sources of bleeding in the bowel

What matters is not simply knowing that blood is present, but working out where the bleeding is coming from.

Does a positive result mean I have cancer?

No. This is what worries patients most.

A positive faecal occult blood test is not a diagnosis of cancer. In a large proportion of people with a positive result, no bowel cancer is found.

That said, colon polyps and colorectal cancer can also cause occult bleeding, so a positive result should be investigated properly.

What happens if the test is positive?

If a FIT done for bowel cancer screening is positive, the next step is usually a colonoscopy.

During a colonoscopy the large bowel and rectum can be examined directly. Any of the following may be seen:

• Polyps
• Points of bleeding
• Inflammation
• Diverticula
• Tumours

Biopsies can be taken where needed. If polyps are found, a good proportion of suitable ones can be removed during the same procedure.

Could I not just repeat the test?

This is an important question.

Repeating the test is not the right way to make sense of a positive screening FIT. Bleeding can be intermittent, so a second test may well come back negative.

For this reason a positive FIT should not be dismissed on the grounds that “the second test was negative, so there is nothing wrong.”

Which investigation is needed is decided by weighing your age, your symptoms, your medical history and the kind of test that was used.

I have haemorrhoids — could that be why it is positive?

It could. But it is not right to think “I have haemorrhoids, so that is certainly where the blood is from.”

Someone with haemorrhoids may also have a colon polyp or another bowel condition at the same time. So a positive screening FIT should not be put down to haemorrhoids alone.

I take a blood thinner — could that explain it?

Gastrointestinal bleeding can occur in people taking aspirin, antiplatelet or anticoagulant medicines.

But it is not right to put a positive FIT down to the medicine and leave it uninvestigated.

Does eating meat before the test affect the result?

That depends on which test was used.

With the older guaiac-based faecal occult blood tests (gFOBT), some foods and medicines can affect the result. Modern FIT tests look for human haemoglobin, so they usually need no special dietary restriction.

This is why it matters whether the result in your hand is a FIT or another kind of occult blood test.

My stool looks normal — how can blood be positive?

“Occult blood” means, by definition, bleeding that cannot be seen. Stool that is completely normal in colour and appearance does not rule out a positive test.

So the thought “I see no blood, the result must be wrong” is not correct.

Can occult blood be positive when my blood counts are normal?

Yes.

Small or intermittent bleeding in the bowel may not yet have caused anaemia. Normal haemoglobin and iron levels do not cancel out a positive faecal occult blood test.

Which symptoms matter particularly?

Assessment matters especially when a positive faecal occult blood test is accompanied by one of the following:

• Unexplained weight loss
• Iron deficiency anaemia
• A new and lasting change in bowel habit
• Recurring abdominal pain
• A marked change in the appearance of the stool
• Visible blood in the stool
• Black, tarry stools
• Loss of appetite or marked tiredness
• A family history of colorectal cancer or advanced adenomatous polyps

The absence of these symptoms does not mean a positive screening test needs no investigation.

When is urgent assessment needed?

A positive faecal occult blood test is not usually in itself a reason to go to an emergency department. But urgent assessment may be needed if there is:

Why does colonoscopy matter?

A faecal occult blood test does not show where the bleeding is coming from. A colonoscopy allows the large bowel to be seen directly.

More importantly, colonoscopy is not only a diagnostic procedure. Some polyps that carry the potential to become cancer can be found and removed before that happens.

This is why, in the right person, a colonoscopy after a positive screening test matters not only for finding disease that is already there but for preventing colorectal cancer.

Remember

• A positive faecal occult blood test does not mean cancer.
• But a positive result does need its cause looked into.
• When a screening FIT is positive, assessment by colonoscopy is usually needed.
• Heavy bleeding, black stools, fainting or severe pain may need urgent assessment.