Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1089 page)

   Trauma, bleeding diatheses

   Vasculitis (polyarteriosis nodosa, Henoch purpura, Schönlein purpura)

   Amyloidosis

   Hiatal hernia

   Neurofibromatosis

   Kaposi sarcoma

   Hematobilia

   Limitations

   If using guaiac-based test, individuals should be instructed to avoid aspirin and other NSAIDs, vitamin C, red meat, poultry, fish, and some raw vegetables because of diet–test interactions that can increase the risk of both false-positive and false-negative (specifically, vitamin C) results.

   The sensitivity and specificity of a gFOBT has been shown to be highly variable and varies based on the brand or variant of the test; specimen collection technique; number of samples collected per test; whether or not the stool specimen is rehydrated; and variations in interpretation, screening interval, and other factors.

   A gFOBT test must be performed properly with three stool samples obtained at home. A single-stool sample FOBT collected after digital rectal examination in the office is not an acceptable screening test, and it is not recommended.

   FIT has several technologic advantages when compared with gFOBT. FIT detects Hb; therefore, it is more specific for human blood than guaiac-based tests are. In addition, because globin is degraded by digestive enzymes in the upper GI tract, FIT also are more specific for lower GI bleeding, thus improving their specificity for colorectal cancer. At this time, the optimal number of FIT stool samples is not established, but two samples may be superior to one.

   Drugs causing intestinal bleeding (e.g., aspirin, corticosteroids, and NSAIDs) and drugs causing colitis (e.g., methyldopa and a variety of antibiotics) can cause positive test results.