FIT / Hemoccult – the easy-to-perform test for blood in the stool

Tests for blood in the stool are convenient and quick, but have some disadvantages. They are recommended for anyone over 50 who does not want a colonoscopy.

How does the test work?

To ensure their supply with nutrients, malignant tumors contain many delicate blood vessels. Intestinal polyps are also often well supplied with blood. Bleeding from these blood vessels is more frequent than from the normal intestinal mucosa, although the amount of blood is (usually) only small and cannot be seen with the naked eye. Doctors then speak of “occult” blood. These traces of blood are detected by the tests.

Depending on the test, one or more small stool samples (from three consecutive defecations) are required. A special paper is placed in the toilet to collect the stool. Then a little stool is spread on a piece of paper or poured into a tube. This is sent by post to the laboratory or handed in to the pharmacy or doctor, where the sample is analyzed. Some tests can also be evaluated by the person having the test.

If the test is negative, this means that no blood was found. A “positive” test result only means that blood has been detected. The test does not answer where it comes from.

What tests are available for blood in the stool?

The best known is the “Guaiak test”, which is sold under the brand name “Hämoccult”, for example. He is looking for a molecule called “heme”. It is part of the red blood pigment (hemoglobin) and is found in humans and animals.

The detection method is a chemical reaction using resin from the guaiac tree. This is why this test is also called the “guaiac-based fecal occult blood test” (abbreviated to gFOBT). It provides information on whether blood is present in the stool sample or not. The amount of blood it contains cannot be determined with this test. As a rule, the guaiac test becomes “positive” from a blood loss of around 0.5 milliliters. It should be made with samples from three consecutive defecations.

One sample is sufficient for the “FIT”. This abbreviation means “fecal immunochemical test” (also abbreviated to iFOBT). Such tests are also available from various providers. What they all have in common is that they are more precise than the guaiac tests and use special antibodies to specifically search only for human blood pigment (hemoglobin) in the stool. In addition to the mere detection of blood, some of these tests also allow an assessment of how much blood is contained in the stool. As a rule, they detect a blood loss of around 0.25 milliliters or more.

How good is the test?

This also depends on how well the person being tested follows the test instructions and whether the test is carried out every one or two years or only once.

  • Guaiak test: Out of 100 people with colorectal cancer, this test detects an average of around 50. Out of 100 larger adenomas, it detects 11 to 31. It therefore misses most cases of the disease and also most of the preliminary stages of colorectal cancer. Nevertheless, if carried out regularly, it can reduce the mortality rate from bowel cancer by around a quarter.

FIT: Out of 100 people with colorectal cancer, a first-time FIT detects the disease in around 80 on average. Out of 100 people with advanced adenomas, who are therefore at a higher risk of colorectal cancer, the test finds around 50.
How good this test is depends, among other things, on how the detection limit is set: An FIT that detects ten micrograms of haemoglobin per gram of stool will find more cases of bowel cancer than one that only indicates at 20 micrograms. The catch is that the test with the ten microgram limit also triggers more “false alarms”.
The usual detection limit is 20 micrograms.

Advantages and disadvantages

What are the advantages of the test?

The stool sample can be taken at home at any time; it is not necessary to take a stool sample beforehand. Another plus point is the low cost.

What are the disadvantages of the test?

  • The test can miss the diagnosis, especially in early-stage tumors that are easily curable. Out of 100 people with colorectal cancer, the Guaiak test misses the disease in 62 to 93, while the FIT misses it in around 20 to 30.
    There are several reasons for this: Firstly, bacteria in the intestine break down any blood so that the test can no longer find it. Secondly, intestinal tumors and polyps do not bleed constantly. The probability of bleeding increases with the size of the tumor. The “serrated adenomas”, which are considered precancerous lesions, also bleed rather rarely compared to other types of adenomas. And thirdly, it is possible that vitamin C ingested with food, other substances or very hot weather may cause the test to fail, which is why the blood in stool test is more likely to find advanced cancers than early-stage cancers. Of the intestinal tumors found with the blood-in-stool test, five to six out of ten had a good chance of recovery. In comparison: without colorectal cancer screening, around four to five out of ten tumors were at this stage when diagnosed.
  • The test can trigger a “false alarm”.
    In around six out of 100 healthy people, the guaiac test detects blood in the stool where there is none. One reason for this is that various substances can influence the chemical reaction in the guaiac test, including some medications. Taking anti-inflammatory drugs such as acetylsalicylic acid (known as Aspirin®, for example), diclofenac or ibuprofen can lead to minor intestinal bleeding. The test may then be “positive” even though there is no bowel cancer. As the guaiac test does not differentiate between human and animal blood, the consumption of meat (steak, tartare) can also lead to a “false positive” test result.
  • The test for blood in the stool must be repeated regularly.
    Experience shows that the motivation to have it done annually or at least every two years decreases over time – but the incidence of bowel cancer increases with age.
  • Because the test, unlike colonoscopy, cannot remove adenomas, it does not reduce the risk of developing bowel cancer in the future.

What does it mean if the result is "positive"?

Blood was then found in the stool, but the diagnosis of “bowel cancer” is by no means certain. In a large study, statistically around eight out of 100 women with a “positive” guaiac test had bowel cancer. In around 30 women, other, comparatively harmless intestinal problems such as hemorrhoids or intestinal diverticula were the reason for the test result.

Stomach ulcers, nosebleeds, blood clotting disorders and other illnesses, even bleeding gums, can also lead to “positive” test results. And even a healthy person without bowel cancer loses around 0.5 to 1.5 milliliters of blood every day through the mucous membrane in the gastrointestinal tract. To find out the diagnosis and make sure that there is no colon cancer behind it, a colonoscopy makes sense.

For every 100 adults between the ages of 40 and 64 who have a “positive” FIT, statistically around 24 have a tumor at least one centimeter in size or an advanced adenoma that is very likely to become cancerous.

Repeating the test for blood in the stool if it was “positive” is useless. Because even if it is negative the second time, this does not reduce the probability of colorectal cancer.

How much does the test for blood in the stool cost?

Basic insurance covers the costs for people aged between 50 and 69 every two years (less deductible and excess) if the test has been prescribed by a doctor. Depending on the insurance conditions, some supplementary insurances pay for early detection examinations in full or in part.

If a colonoscopy is performed on the basis of a positive test result, it is no longer a screening colonoscopy, but a diagnostic one. The basic insurance then covers the costs, minus the deductible and excess.

At what intervals should the test for blood in the stool be repeated?

It makes medical sense to repeat the Guaiak test every year or the FIT every two years. This applies to people with an average risk of colorectal cancer. The results of a single test are significantly worse.

When is the test not useful?

If there are already symptoms that could indicate bowel cancer, the blood in stool test is the wrong examination method.

For people with a family history of increased risk, it can at best be a supplement to colonoscopy, but not a substitute for it.

What can I do myself?

  • Anyone who opts for the blood in stool test as an early detection measure should be prepared to take the test at least every two years.
  • For the test to be meaningful, it is important to adhere to the specifications. With the guaiac test, for example, this means avoiding certain vegetables and fruit for three days beforehand and not eating any meat, poultry or fish.
  • The stool samples should not be taken completely superficially.
  • Once the stool samples have been taken, the FIT should be carried out as soon as possible and the guaiac test within six days.
  • If the test result is “positive”, in the vast majority of cases only a colonoscopy will provide the answer as to whether bowel cancer is present. The test for blood in the stool also requires a basic willingness to have a colonoscopy in this case.

For patients

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University Hospital Zurich
Department of Gastroenterology and Hepatology
Raemistrasse 100
8091 Zurich

Tel. +41 44 255 85 48
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