What are the difficulties with guaiac testing for fecal occult blood?
The sensitivity of three guaiac tests obtained on three consecutive days is moderately high (up to 80%) for detecting colon cancer because colon cancers typically bleed intermittently.
Additional weaknesses of guaiac testing for colon cancer include the following:
• The benign precursor of colon cancer, adenomatous polyps usually do not bleed and are not detected by guaiac test.
• A positive stool guaiac test cannot distinguish bleeding from colonic cancer versus an upper gastrointestinal (UGI) source (e.g., gastritis, ulcer, esophagitis).
• Microscopic bleeding from the UGI tract—ulcer, gastritis, esophagitis, and so on—can cause the guaiac test to be positive.
• Approximately 5% to 10% of all guaiac tests are falsely positive.
False positivity may be due to recent ingestion of fresh fruits and uncooked vegetables, especially cruciferous vegetables (cabbage, cauliflower, and broccoli) that can catalyze the colorimetric reaction because of the presence of pseudoperoxidase in this produce; or recent ingestion of red meat, especially steak, that contains residual blood from cows or other mammals that can catalyze the reaction just like human blood. Recent ingestion of iron- or bismuth-containing medications (Pepto-Bismol) can cause stools to appear dark-blue to black and result in a false-positive stool guaiac test. Recent aspirin or nonsteroidal anti-inflammatory drug use can cause microscopic GI bleeding. A dried stool specimen can lead to a falsely negative test, whereas rehydration of a dried specimen can lead to a falsely positive test.