Occult Blood Tests

Occult Blood Tests: Overview

Alternative names: Fecal Occult Blood Test, FOBT.

Amounts of blood that come from the digestive system but are too small to be seen can be detected chemically.  The detection of such small amounts of blood ('hidden', or 'occult' blood) may provide early clues to the presence of ulcers, cancers, and other abnormalities.

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Bleeding in the digestive system can be caused by something as insignificant as a little irritation or as serious as cancer.  When bleeding is profuse, a person can vomit blood (hematemesis), pass bright red blood in the stool (hematochezia), or pass black, tarry stool (melena).

Function; Why it is Recommended

There are two types of test:

  1. The traditional guaiac smear test (also called Hemoccult, Seracult, or Coloscreen) is completed and interpreted by a medical professional; these tests are usually only available from a laboratory or a doctor's office.
  2. The newer, flushable reagent pads (EZ Detect, ColoCARE).  These are available without a prescription at many drugstores.

Many patients prefer the flushable reagent pads because there is no stool handling, more privacy, and no laboratory processing.  However, health care providers usually favor the guaiac tests because the large studies that have shown the benefits of colon cancer screening were done with guaiac tests.

Guaiac Smear Test. During a rectal examination, the doctor obtains a small amount of stool on a gloved finger.  This sample is placed on a piece of filter paper impregnated with a chemical (guaiac).  After another chemical is added, the color of the sample will change if blood is present.  Alternatively, the person can take home a kit containing the impregnated filter papers.  The person places samples of stool from about three different bowel movements on the filter papers, which are then mailed in special containers back to the doctor for testing.  If blood is detected, further examinations are needed to determine the source.

These tests are useful in detecting hidden blood in the stool, and are mainly used for colorectal cancer screening.  They may also be performed in the evaluation of anemia.

Positive test results may also indicate the following:

Note that false positives are common.

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Occult Blood Tests:

Occult Blood Tests can help with the following:


Gastrointestinal Bleeding

If gastrointestinal bleeding is suspected due to other symptoms, but no blood is visible in the stool, a doctor may decide to test for occult (hidden) blood.

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