Testing for hidden blood in stool

Testing stool samples in a laboratory (Jupiterimages / Photos.com)

Fecal occult blood (FOB) tests can detect hidden traces of blood that may be caused by tumors or polyps in the bowel. If such traces are found, an endoscopy is performed to determine whether they're caused by a tumor or pre-cancerous tissue changes. It has been proven that, when combined with an endoscopy, the FOB test can lower the risk of dying of bowel cancer.

Bowel cancer and advanced-stage bowel polyps can cause slight bleeding in the bowel. FOB tests can find traces of blood that can't be seen with the naked eye (occult blood). But not every tumor or polyp can be detected this way because not all of them bleed. Blood in the stool can also be caused by many benign (non-cancerous) conditions, such as hemorrhoids. This is why stool blood tests are meant to be done as preliminary tests. If blood is found, a colonoscopy is then performed to find the cause. So doing a FOB test makes the most sense if you're also prepared to have an endoscopy of the bowel if the test results are abnormal.

There are two different ways to test for blood in the stool. Stool guaiac tests detect parts of the blood pigment hemoglobin using a chemical reaction. Immunochemical tests look for antibodies to hemoglobin, a component of blood, in order to find traces of blood.

For screening purposes, German statutory health insurers only have to cover the costs of the stool guaiac tests “Hämoccult,” “HemoFec” and “HemoCare.” For people who have statutory health insurance in Germany, bowel cancer screening using these tests is free from the age of 50. Some insurers will also cover the costs of an immunochemical test.

Stool guaiac tests

To do this type of FOB test, two pea-sized samples of stool are put on a special test card using an applicator. These test cards can be closed with a flap, like an envelope. A test set contains three test cards for three successive bowel movements, ideally on three different days. You can prepare the FOB test at home and take it to your doctor’s practice in person or send it by mail. The samples will then be checked for blood in a laboratory.

Some people are put off by the idea of FOB tests because they think they're disgusting or unhygienic, or because they're embarrassed to send the test card by mail. But there's no need to worry about this: the test cards can be sealed so that they're clean and do not smell.

Certain foods and medications can distort the results of stool guaiac tests. These mainly include red meat like beef, lamb and liver. It's therefore recommended that you avoid these foods before doing the test. Anti-clotting medications like acetylsalicylic acid (ASA, the drug found in Aspirin) and larger amounts of vitamin C can also influence FOB test results.

If you take medication and you aren't sure whether this may affect your test results, ask your doctor before doing the test. You'll find more information on how to do the FOB test correctly in the test instructions.

Advantages and disadvantages of stool guaiac tests

FOB tests have the advantage of being rather easy to do and having no direct adverse effects. Stool guaiac tests have also been tested in large studies. These studies showed that the test reduces the risk of dying of bowel cancer, provided an endoscopy is also done if your test results are abnormal.

The disadvantage of these tests is that they're not very reliable. They often produce abnormal results even though cancer isn't the cause. This is called a "false positive" test result. This can happen because things like hemorrhoids, stomach ulcers or inflammations in the stomach or bowel may cause bleeding too. Women shouldn't do a FOB test while they are on their period. The test may also need to be postponed if you have diarrhea or bleeding gums.

About 50 to 80% of abnormal test results later prove to be a false alarm – nothing abnormal is found during an endoscopy following the test.

In a second possible type of error, the results of the test may be normal despite the presence of advanced-stage polyps or bowel cancer. This is called a “false-negative” test result. About 40 to 70% of cancerous bowel tumors are not detected by the test. So it's important to take any symptoms that worry you seriously – even if your last test results were normal.

Research results on stool guaiac tests

Studies show that fewer people over the age of 45 die of bowel cancer if they do a stool guaiac test every two years, and then have an endoscopy if their test results are abnormal. Expressed in numbers, studies over a period of more than 10 years found that:

  • Without FOB tests: Out of 1,000 people who did not do the test, about 10 died of bowel cancer.
  • With FOB tests: Out of 1,000 people who did the test every two years, about 8 to 9 died of bowel cancer.

This means that stool guaiac tests prevented 1 to 2 out of 1,000 people from dying of bowel cancer.

Fecal immunochemical test (FIT)

Compared to guaiac tests, immunochemical tests aren't as easily affected by diet, and aren't as complicated to analyze in a laboratory. It's important to know that painkillers or antirheumatic drugs can affect the results of immunochemical stool tests. If you use these medications, you should ask your doctor whether you can stop taking them for a short while.

Immunochemical stool tests can detect traces of blood slightly better than stool guaiac tests. But they also don't tell us anything about the cause of bleeding, so an endoscopy is done if blood is detected.

Not all German statutory health insurers cover the costs of immunochemical tests. It's best to contact your health insurance fund beforehand to ask whether they will cover the costs.

Other types of stool tests

Other tests include the M2-PK test and a stool DNA test. These don't currently play a large role in bowel cancer screening. The M2-PK test looks for an enzyme that is thought to be a sign of bowel cancer when found in stool. The DNA test looks for fragments of genetic material from cancer cells found in stool. It's not clear whether these tests are able to decrease the risk of dying of bowel cancer. The costs are not covered by statutory insurance funds in Germany.

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Please note that we do not provide individual advice on matters of health. You can read about where to find help and support in Germany in our information “How can I find self-help groups and information centers?”

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