What the risks are:

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).

Special 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.