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Fecal occult blood test (FOBT)

A non-invasive test that detects the presence of hidden (occult) blood in the stool. Such blood may arise from anywhere along the digestive tract. Hidden blood in stool is often the first, and in many cases the only, warning sign a person has of colorectal diseases including colon cancer.

There are two types of FOBTs:

  1. the traditional guaiac smear test (Hemoccult, Seracult, Coloscreen), and
  2. the newer, flushable reagent pads (EZ DetectT, ColoCARE). They are both useful in detecting hidden blood in the stool, and are mainly used for colorectal cancer screening. The tests differ in the way they are performed. The flushable reagent pads are available OTC at many drugstores. In contrast, the traditional guaiac smear test is completed and interpreted by a medical professional and these tests are usually available from a laboratory or doctor's office. Many consumers prefer the flushable reagent pads because there is no stool handling and no laboratory processing. However, doctors still primarily favor the guaiac tests.

Alternative Names: Stool occult blood test

Why the test is performed: This test is mainly performed for colorectal cancer screening. It may also be performed in the evaluation of anemia.

Advantages: Non-invasive and low cost


  • Detects blood in stool, but not its cause.
  • False-positives are common with some testing methods and may cause unwarranted anxiety about cancer and lead to unnecessary further tests.
  • False-negative results are common and may miss disease in its early stages.

Normal Values: A "negative" test result is usual and is normal.

What abnormal (positive) results may indicate:

  • Bleeding esophageal varices
  • Colon polyp or colon cancer
  • Esophagitis
  • Gastritis
  • GI (gastrointestinal) trauma
  • GI tumor
  • Hemorrhoids
  • Fissures
  • Inflammatory bowel disease
  • Peptic ulcer
  • Recent GI surgery
  • Angiodysplasia of the colon
Additional conditions under which the test may be performed:
  • Colon cancer screening, evaluation of anemia

What the risks are: A negative test does not necessarily mean there are no colorectal diseases present. Not all polyps bleed, and not all polyps bleed all the time. That is why a FOBT must be used with one of the other more invasive screening measures (sigmoidoscopy, colonoscopy, double barium contrast enema).

Special considerations: False positives can occur.

Colonoscopy is generally recommended as the preferred follow-up test to a positive FOBT.

Factors that can cause this test to be less accurate include:

  • Bleeding gums following a dental procedure.
  • Ingestion of red meat within 3 days (most FOBTs).
  • Ingestion of fish, turnips, or horseradish.
  • Drugs that can cause GI bleeding include anticoagulants, aspirin, colchicine, iron supplements in large doses, NSAID drugs (anti-inflammatory analgesics), and corticosteroids.
  • Drugs that can cause false positive measurements include colchicine, iron, oxidizing drugs (for example, iodine, bromides, and boric acid), and reserpine.
  • Large amounts of Vitamin C can cause false-negative results on most FOBTs.

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