Fecal Occult Blood Test

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Fecal Occult Blood Test

Definition

The fecal occult blood test (FOBT) is a rapid test for detecting the presence of blood hidden in the stool and caused by gastrointestinal bleeding.

Purpose

This test is performed to detect the presence of blood in the feces. It is also routinely used (in conjunction with a rectal examination performed by a physician) to screen for colorectal cancer, particularly after age 50. However, a positive result does not necessarily indicate cancer, as positive results can be associated with a wide variety of conditions, such as:

peptic, gastric, or stress ulcers
esophageal varices
colon or pancreatic cancer
tumors
gastritis
gastrointestinal bleeding
hemorrhage
ulcerative colitis
diverticulitis
alcohol abuse
Crohn's disease
intussusception

Precautions

Certain foods and medicines can influence the test results. Diets rich in red meat, poultry and fish could cause false-positive FOBT results, as can excessive amounts of green leafy vegetables. Certain drugs can also cause false-positive results; they include aspirin, cortisone, iron, potassium preparations, nonsteroidal antiinflammatory (NSAID) drugs, thiazide diuretics, colchicine, and reserpine. False-negative findings may result from the ingestion of large amounts of ascorbic acid (vitamin C ) from dietary supplements or foods such as citrus fruits. It is also important that the specimen not be contaminated by menstrual discharge.

Description

In most cases, the collection of stool samples can be done at home, using a kit supplied by the physician. The kit contains a specially prepared card onto which a small sample of stool is spread, using a stick provided in the kit. The sample is placed in a special envelope and either mailed or brought to the physician's office or test site for analysis.

Most tests for occult blood test are based upon the fact that hemoglobin, the oxygen carrying protein in blood, has peroxidase activity. In the presence of hydrogen peroxide, hemoglobin will catalyze the oxidation of a dye such as gum guaiac, benzidine, or o-toluidine, creating a blue color. The dye is present on the card. When hydrogen peroxide is applied to the card and mixed with the stool, the blue color will form if an abnormal amount of blood is present. Rapid immunochemical tests are also available.

Preparation

The patient may be requested to avoid meat, fish and poultry for two or three days prior to the test.

Aftercare

There is no aftercare required for this test.

Complications

There are no complications associated with this test.

Results

As described above, many factors can result in false-positive and false-negative findings. Therefore, it is important to note that a true-positive finding only signifies the presence of blood; it is not necessarily an indication of cancer. The National Cancer Institute states that, in their experience, less than 10% of all positive results are caused by cancer. Alternatively, a negative result (meaning no blood was detected) does not guarantee the absence of colon cancer, which may bleed only occasionally or not at all. The physician will want to follow up on a positive result with further tests, depending upon other factors in the patient's history or physical examination. Endoscopic exam is often recommended when the cause of GI bleeding is unknown.

Health care team roles

Nurses should determine if the patient has been experiencing signs of a peptic ulcer (epigastric pain between meals), and should note any recent bleeding episodes related to dental problems. The physician, laboratory technician, or scientist should ensure that the sample is analyzed within 48 hours of collection.