Fecal Occult Blood Test Health Article

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.