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 are 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 cannot be seen with the naked eye (occult blood). But not all tumors and polyps bleed, so these tests cannot detect every tumor or polyp. Blood in the stool can also be caused by many harmless conditions. 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 it only makes sense to do a FOB test if you are also prepared to have an endoscopy of the bowel if the test results are abnormal.

Stool guaiac tests are currently the most commonly used type of FOB test. These tests detect parts of the blood pigment hemoglobin using a chemical reaction. In immunochemical tests, the hemoglobin is detected using antibodies instead. 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 higher costs of an immunochemical test.

The test

To do a 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. You can prepare the FOB test at home and take it to your doctor’s practice in person or send it by mail. A laboratory will then check the stool samples for blood.

Some people are put off by the idea of FOB tests because they think they are disgusting or unhygienic, or because they are embarrassed to send the test card by mail. But there is no need to worry about this: the test cards can be sealed so that they are 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 is 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 are not sure whether this may affect your test results, ask your doctor before doing the test. You will 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 are not very reliable. They often produce abnormal results even though cancer is not 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.

About 5 to 8 out of 10 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 4 to 7 out of 10 cancerous bowel tumors are not detected by the test. So it is 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 tests

The main alternative to stool guaiac tests are fecal immunochemical tests. Like guaiac tests, these tests detect occult blood in stool, but using antibodies against hemoglobin in blood rather than a chemical reaction. Compared to guaiac tests, immunochemical tests are not as easily affected by medication or diet, and are not as complicated to analyze in a laboratory.

Like stool guaiac tests, immunochemical tests cannot tell us anything about the cause of bleeding, so an endoscopy is done if blood is detected. So far, only guaiac tests have been proven to reduce the number of deaths from bowel cancer in conclusive studies. Most German statutory health insurers do not currently cover the costs of immunochemical tests. It is best to contact your health insurance fund beforehand to ask whether it will cover the costs.

Other types of stool tests

Other tests include the M2-PK test and a stool DNA test. These do not 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 is 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 counseling services?”

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