ADAM Health Encyclopedia

Fecal occult blood test (FOBT)

Definition

A fecal occult blood test (FOBT) is a noninvasive test (nothing enters the body). This test detects hidden (occult) blood in the stool. Such blood may come from anywhere along the digestive tract. Hidden blood in stool is often the first, and in many cases the only, warning sign that a person has colorectal disease, including colon cancer.

Alternative Names

Why the Test is Performed

This test is mainly performed yearly for colorectal cancer screening. It may also be performed in the evaluation of anemia.

Advantages:

• Noninvasive
• Low cost

Disadvantages:

• Detects blood in stool, but not its cause.
• False-positive results are common with some testing methods. This may cause unneeded anxiety about cancer and lead to unnecessary further tests.
• False-negative results are also common and may miss disease in its early stages.

How the Test is Performed

There are two types of FOBTs: 1) the traditional guaiac smear test (Hemoccult, Seracult, Coloscreen), and 2) the newer, flushable reagent pads (EZ DetectT, ColoCARE). They are both useful in detecting hidden blood in the stool, and are mainly used for colorectal cancer screening.

The tests differ in the way they are performed. The flushable reagent pads are available without a prescription at many drugstores. In contrast, the traditional guaiac smear test is completed and interpreted by a medical professional, and these tests are usually available from a laboratory or a doctor's office.

Many consumers prefer the flushable reagent pads because there is no stool handling 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.

See description under specific type of FOBT for how these tests are performed.

How to Prepare for the Test

See description under specific type of FOBT.

How the Test Will Feel

See description under specific type of FOBT.

Risks

A negative test does not necessarily mean there are no colorectal diseases present. Not all polyps bleed, and not all polyps bleed all the time. That is why a FOBT must be used with one of the other more invasive screening measures (sigmoidoscopy, colonoscopy, double barium contrast enema).

Considerations

Colonoscopy is generally recommended as the preferred follow-up test to a positive FOBT.

Factors that can cause this test to be less accurate include the following:

• Bleeding gums following a dental procedure
• Eating red meat within 3 days of the test
• Eating turnips or horseradish

Drugs that can cause GI bleeding include anticoagulants, aspirin, colchicine, iron supplements in large doses, NSAIDs (anti-inflammatory analgesics), and corticosteroids.

Drugs that can cause false positive measurements include colchicine, iron, oxidizing drugs (for example, iodine, bromides, and boric acid), and reserpine.

Large amounts of vitamin C can cause false-negative results on most FOBTs.

In general, avoiding food is not recommended, with the exception of red meat as described above.

Normal Results

A "negative" test result is normal. It means no blood was found in the stool.

What Abnormal Results Mean

Positive test results may indicate the following:

• Angiodysplasia of the colon
• Bleeding esophageal varices
• Colon polyp or colon cancer
• Complications of recent GI surgery
• Esophagitis
• Fissures (cracks around the anus)
• Gastritis
• GI (gastrointestinal) trauma
• GI tumor
• Hemorrhoids
• Inflammatory bowel disease
• Peptic ulcer

Note that false positives are common.

Additional conditions under which the test may be performed include the following:

• Colon cancer screening
• Evaluation of anemia