fecal occult blood test

fecal occult blood test, method used to analyze feces for the purpose of diagnosing a disease or disorder in humans or animals.

In humans the fecal occult blood test is a low-cost method for detecting gastrointestinal bleeding, which may be the first sign of carcinoma of the colon or rectum. Although the false-positive rate for this test is low, the false-negative rate is high. The test also is more likely to detect lesions in the right (ascending) colon because these lesions bleed more than those in the left (descending) colon. Routine surveillance for colorectal cancer depends on periodic fecal occult blood testing combined with direct visualization of the lower colon with a sigmoidoscope (see sigmoidoscopy). Individuals who are at increased risk for colorectal cancer and should be screened regularly are identified by any of the following criteria: age greater than 50 years; previous colorectal cancer or adenoma; family history of colorectal cancer or polyps in a first-degree relative or another genetic predisposition (e.g., cancer family syndrome); history of ulcerative colitis or Crohn disease; or personal or family history of genital or breast cancer.

Stool cultures are obtained when diarrhea is severe and particular bacteria such as Salmonella, Shigella, or Giardia are suspected. If a parasitic infection is suspected, the stool is examined under a microscope for the eggs or cysts of parasites such as pinworms (Enterobius vermicularis) or roundworms (Ascaris lumbricoides).