Testing for occult blood in the stool

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

Stool tests can find traces of blood that may be caused by tumors or polyps. A positive test should be followed up by a colonoscopy to find out whether a tumor or an early stage of a tumor really is the cause. It has been proven that the stool test can then lower the risk of dying of bowel cancer.

Bowel cancer and advanced bowel polyps can cause bleeding in the bowel. Stool tests – also called stool blood test, or fecal occult blood tests (FOBT) – can detect these traces of blood. But they do not provide a definite answer because only few tumors and polyps leave traces of blood in the stool. There are also harmless causes of blood in the stool, so stool tests are more of a preliminary test. If the results are abnormal, you will be referred to have a colonoscopy to find out the cause. Both the stool test and the following colonoscopy are covered by statutory health insurance in Germany for people aged 50 and above.

Stool tests only provide an opportunity to find polyps or bowel cancer early if a bowel endoscopy is performed after abnormal findings in the stool test. So it only makes sense to do a stool test if you are also prepared to have an endoscopy of the bowel if necessary.

Chemical stool tests are the most commonly used. These tests use a chemical reaction to detect traces of hemoglobin, a protein in red blood cells. They are the only stool tests covered by statutory health insurance in Germany. That is why we will look at how these tests work in more detail. Three chemical stool tests are used for screening tests covered by statutory health insurance in Germany: Hämoccult, HemoFec and HemoCare.

The examination

Stool tests can find traces of blood that cannot be seen with the naked eye. To do the test, two pea-sized samples of stool are put on a special test card with an applicator. These test cards can be closed with a flap like an envelope. A test set contains three test cards for samples of three successive bowel movements. You can prepare the stool 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.

Certain foods and medications can distort the stool test results. These mainly include red meat like beef, lamb and liver, for example. It is therefore recommended to 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 stool test results.

If you are not sure whether you are taking medications that might distort the test result and whether you can stop taking them temporarily, it is a good idea to talk to your doctor about it in good time before doing the test. You will find more information on how to prepare the stool test correctly in the test instructions.

Advantages and disadvantages of chemical stool tests

This type of stool test has the advantage of being rather easy to do and having no direct side effects. It is important to note that the chemical stool test was evaluated in large studies that showed that men and women can lower their risk of dying from bowel cancer with this test– provided they also have a bowel endoscopy of the bowel after a positive test result.

The disadvantage of the test is that it is not very reliable. The test often finds abnormalities even though there is no cancer present – this is called a false-positive test result. It can happen because hemorrhoids, stomach ulcers or inflammations in the stomach or bowel may also bleed. A false-positive result can cause unnecessary worrying before getting an all-clear after a colonoscopy.

About 5 to 8 out of 10 abnormal test results later prove to be a false alarm because neither a polyp nor cancer was found during the colonoscopy. But until then, the abnormal result might have been causing quite some anxiety.

The second possible error is that the results of the stool test are normal despite the presence of polyps or bowel cancer. This is called a false-negative test result. About 4 to 7 out of 10 malignant bowel tumors are not detected in a stool test. So it is important to take any symptoms that worry you seriously – even though the last test might have been normal.

Research results on chemical stool tests

Studies show that screening with stool tests can reduce the bowel cancer mortality rate among people over 45 years of age if they do the test every two years. Expressed in numbers, studies over a period of more than 10 years had the following results:

  • Without stool test: Out of 1,000 people who did not do the test, about 10 died of bowel cancer.
  • With stool test: Out of 1,000 people who did the test regularly, about 8 to 9 died of bowel cancer.

This means that the screening saved 1 to 2 out of 1,000 people from dying of bowel cancer.

Other stool tests

Other tests are offered as an alternative to chemical stool tests. These include immunological stool tests, a test called the M2-PK stool test and DNA stool tests. Immunological tests also detect blood hidden in the stool. The M2-PK stool test looks for an enzyme in the stool that is thought to be a sign of bowel cancer. The DNA stool test looks for traces of cancer cells in the stool.

These stool tests can also only give indications of bowel cancer. They can only prevent cancer if abnormal results are followed up with an endoscopy of the bowel. So far only chemical stool tests have been proven to reduce the number of deaths from bowel cancer in conclusive studies. It cannot be said with certainty yet whether the other tests are more suitable for screening. They are not covered by statutory health insurance in Germany.

The stool test puts some people off because they think it is 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 to keep them hygienic and neutralize any odors.

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