How does Fecal Occult Blood Tests (FOBT) Work?

The long-term incidence rate of cancer in the US is consistent among men and women over the past decades. Among these cases, colorectal cancer is one of the top three leading causes of cancer-related mortalities with 49,920 reported deaths in 2009. Since then, the US government encourages every individual to voluntarily undergo fecal occult blood tests especially those who have higher risk of getting colorectal cancer. Furthermore, the US continues to accelerate its efforts in controlling cancer, which eventually showed significant reduction in the mortality and incidence rates for the last 10 years. Since cancer is less treatable at it progresses to the final stage, early detection is their primary concern, if not total prevention.

What is Stool Occult Blood Test?

Fecal Occult Blood Test

Fecal occult blood test or simply occult test is a stool blood screening test for the detection of colorectal cancer. It primarily checks for presence of blood in the stool that is invisible to the naked eye. Since the process of bleeding inside the intestine is slow and mostly irregular, blood does not change the stool color neither result to a visible bright red blood in the stool. Only detection through the occult test is possible.

Types of Fecal Occult Blood Tests

There are actually three kinds of fecal occult blood tests available either through laboratory processing in the hospitals/clinic or in drugstores. The occult blood testing process and materials used are also different. They may also vary in the collection method. However, in general, they still require three stool samples for proper occult blood testing.

Traditional Chemical Test or Guaiac Test

The Guaiac test is the most common occult blood test performed in the hospital setting. It is cheaper when compared to other occult blood tests. In Guaiac test, the medical professional uses a solution containing guaiac and an oxidizing chemical agent to test stool for hidden blood. If the color of the smear changes to blue after mixing, it indicates a positive result. This is mainly due to the interaction of heme portion of hemoglobin in the blood with Guaic.

Immunologic Stool Blood Tests

This type of occult blood test uses a solution containing a globin antibody combined with a small amount of gold. Globin is the protein part of the hemoglobin found in the blood. As the antibody-Gold complex binds with globin, it creates a visible line in the strip indicating a positive result.

Flushable Reagent Stool Blood Tests

Flushable reagent pads are available in the drug stores. It is a home stool blood testing kit, which does not need stool handling or laboratory processing commonly required in stool tests. The process is even simpler. Just place the disposable pads on the toilet after defecation and record color changes, which usually appears after 2 minutes. After noting the changes in the card, send it to the health care provider.

Fecal Occult Blood Test Preparations

For an immunologic occult test and flushable reagent pads, preparations are minimal or no restrictions at all. However, the Guaiac test has a number of restrictions in order to lessen if not totally eradicate errors during testing. Here are a few points to consider before taking the occult test.

  1. Avoid eating red meat, turnips or horseradish three days prior to the occult test. These foods also contain the heme part of the blood, which may show a false positive result.
  2. Do not take medicines such as corticosteroids, colchines, aspirins, NSAIDs and anticoagulants prior testing. These drugs can cause minimal gastrointestinal bleeding thus resulting to a false positive measurement.
  3. Avoid iron and other oxidizing drugs like boric acid. They too may give a false positive result.
  4. Avoid taking Vitamin C supplements prior occult blood testing. It causes unnecessary interactions with the reagents and may give a false negative result.

In general, every laboratory test has its own limitations and discrepancies and this includes fecal occult blood tests. They are neither absolute nor 100% accurate all the time. Hence, a negative result does not mean you are cancer-free neither a positive result does not necessarily mean you have colorectal cancer. That is why healthcare providers still require further tests and follow-up examinations for confirmation. Either way, early detection or prevention is better than late treatment.

Lena Butler

My name is Lena Butler. I live in San Diego, California. I work as a customer service representative for I attended the University of San Diego and majored in marketing. I enjoy spending time at home, working on my painting and playing with my two pet rabbits, Carl and Lenny, when I am not here sharing interesting posts :)

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