Fecal occult blood test

Some foods, medications and dietary supplements may affect the results of the fecal occult blood test, including red meat, vitamin C supplements, blood thinners, broccoli and mushrooms.

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The fecal occult blood test (FOBT) chemically checks your stool for hidden (occult) blood. Occult blood in the stool may indicate cancer or polyps in the colon or rectum because cancers and sometimes large polyps commonly bleed. Conversely, it's important to remember that not all cancers bleed and most small polyps don't bleed. The blood often is passed in such small amounts that it's undetectable by the human eye and can be detected only through chemical testing.

A positive FOBT — meaning blood is found — doesn't reveal the source of the bleeding. Additional tests are needed to further evaluate your colon and rectum and sometimes your upper gastrointestinal tract, which includes your esophagus, stomach and small bowel. Unfortunately, the fecal occult blood test is a relatively insensitive method for colon cancer screening and should be combined with another method that looks directly at the colon at regular intervals.

How do you prepare?

Various foods, medications and dietary supplements can adversely affect the FOBT results. For instance, they can indicate that blood is present when it isn't (false-positive) or miss the presence of blood when it actually is present (false-negative). Because of this, you may be advised to modify your diet for about three days before stool testing.

This is one example of a fecal occult blood test. Using an applicator stick, you apply a stool sample to two test windows on a special card. You then mail the card to your doctor's office or lab.

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Not everyone can stop taking certain medications. Discuss your dietary questions and medications with your doctor ahead of time. He or she can provide you with specific instructions, if necessary.

Substances you may be asked to avoid include:

• Aspirin
• Nonsteroidal anti-inflammatory drugs, such as ibuprofen (Advil, Motrin, others)
• Blood thinners, such as warfarin (Coumadin)
• Fresh broccoli, cabbage, cauliflower, cucumbers, horseradish, mushrooms, potatoes, radishes, turnips, cantaloupe, grapefruit, carrots, Jerusalem artichokes and other high-fiber foods
• Red meat
• Vitamin C supplements

In addition, iron supplements, which darken stool, may sometimes make it difficult to interpret the test.

The preparations for each type of FOBT are slightly different. Discuss with your doctor the type of test you'll undergo to determine what preparations are necessary.

How is it done?

Typically, your doctor will give you a kit that explains how to take the FOBT stool sample at home. Because cancers may bleed intermittently, you'll generally be asked to take samples on three consecutive days.

Various types of kits are available, but usually you obtain a stool sample for testing and store it in a supplied container or use an applicator stick to smear it on a chemically treated card. You then return the container or card in person or by mail to a lab or your doctor's office for analysis. It's important that samples be returned promptly, as instructed, to ensure accuracy.

Once the sample reaches the lab, a chemical is applied that reacts with the fecal sample and produces a different color when it comes in contact with blood.

Results

Your doctor will recommend a follow-up schedule based on your test results.

Once the card reaches a doctor's office or a lab, a chemical is applied to the back of the card over the test windows. If blood is present, the chemical reacts and appears as a different color.

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• Negative test results. If your test results are negative, it means that no blood was found. If you're at average risk of colon cancer, you can probably wait a year to repeat the test.
• Positive test results. If your test results are positive, it means that blood was detected. However, it's possible that the blood may have come from sources other than colon cancer or a polyp, such as from a stomach ulcer, a hemorrhoid, or even from your mouth or nose. Or the results could be incorrect — a false-positive. More testing — such as a colonoscopy or flexible sigmoidoscopy exam and a barium enema — is needed to locate the source of the bleeding. Sometimes you may need additional tests. These might include an upper endoscopy, an upper gastrointestinal X-ray or a small bowel X-ray.

An FOBT often is performed in conjunction with a flexible sigmoidoscopy exam to allow additional screening of the colon above the rectum and sigmoid colon.