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Subject: Occult Blood, Stool
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Occult bleeding refers to the initial presentation of a positive fecal occult blood test (FOBT) result and/or iron deficiency anemia, when there is no evidence of visible blood loss to the patient or physician. The differential diagnosis for occult GI bleeding is broad. Some of the more common causes include colon cancer, esophagitis, peptic ulcers, gastritis, inflammatory bowel disease, vascular ectasias, portal hypertensive gastropathy, and gastric antral vascular ectasias. However, less common causes such as gastroesophageal cancers, hemosuccus pancreaticus, hemobilia, and infections also need to be considered. Non-GI sources of blood loss such as hemoptysis and epistaxis can also cause a positive FOBT. FOBT falls into two primary categories based on the detected analyte: guaiac based (gFOBT) and immunoassay based (FIT). gFOBT are the most common stool blood tests in use for colorectal cancer screening, and they detect blood in the stool through the pseudoperoxidase activity of heme or hemoglobin, whereas immunochemical-based tests react to human globin.
Normal range: negative.
Screens for carcinomas (particularly colon) and polyps of the GI tract
Identifies GI bleeding related to upper GI bleeding (gastric ulcer)
Screens for diverticulitis and colitis
GI malignancies (colon)
Ischemic bowel disease
Inflammatory lesions (ulcerative colitis, Crohn disease, shigellosis, amebiasis)
Trauma, bleeding diatheses
Vasculitis (polyarteriosis nodosa, Henoch purpura, Schönlein purpura)
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.