Fecal occult blood testing

Fecal occult blood testing provides a low-risk alternative to invasive colonoscopy when screening for colorectal cancer. Two methods are currently used.

Guaiac-based tests, used for over 30 years, identify the presence of blood in the stool by detecting the peroxidase activity of the heme molecule in hemoglobin. However, these tests are not specific for human blood and can produce false-positive results when red meat (which contains heme) or fruits and vegetables containing peroxidase are eaten in the days preceding a guaiac-based test. Moreover, bleeding in the upper gastrointestinal tract from use of nonsteroidal anti-inflammatory (NSAI) drugs, or high doses of vitamin C within days of testing, may cause false negatives.

Fecal immunochemical tests (FITs) are newer fecal occult blood tests that have a higher sensitivity and specificity for the detection of adenomas and cancer than guaiac-based tests because they use antibodies specific to human globin to detect globin in the stool.Therefore, FITs can be performed without the dietary or medication restrictions of guaiac-based tests. In addition, FITs are highly specific for detecting lower gastrointestinal bleeding but not upper gastrointestinal bleeding, because globin is largely degraded by upper gastrointestinal enzymes.(1)

Variations in stool sample collection (dry slide or liquid-based) and analysis (manual or automatic), as well as sample stability are important factors to consider in maximizing accurate results.