Stool occult blood test

Alternative names 
Fecal occult blood test (FOBT)


A fecal occult blood test (FOBT) is a noninvasive 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 that a person has colorectal disease, 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 without a prescription 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 a doctor’s office.

Many consumers prefer the flushable reagent pads because there is no stool handling and no laboratory processing. However, health care providers usually favor the guaiac tests because the large studies that have shown the benefits of colon cancer screening were done with guaiac tests.

How the test is performed
See description under specific type of FOBT.

How to prepare for the test
See description under specific type of FOBT.

How the test will feel
See description under specific type of FOBT.

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


  • Noninvasive  
  • Low cost


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

Normal Values
A “negative” test result is usual and is normal.

What abnormal results mean
Positive test results may indicate the following:

  • Bleeding esophageal varices  
  • Colon polyp or colon cancer  
  • Esophagitis  
  • Gastritis  
  • GI (gastrointestinal) trauma  
  • GI tumor  
  • Hemorrhoids  
  • Fissures  
  • Inflammatory bowel disease  
  • Peptic ulcer  
  • Complications of recent GI surgery  
  • Angiodysplasia of the colon

Note that false positives are not uncommon.

Additional conditions under which the test may be performed include the following:

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

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 the following:

  • Bleeding gums following a dental procedure  
  • Ingestion of red meat within 3 days  
  • Ingestion of fish, turnips, or horseradish

Drugs that can cause GI bleeding include anticoagulants, aspirin, colchicine, iron supplements in large doses, NSAIDs (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.

Johns Hopkins patient information

Last revised: December 6, 2012
by Dave R. Roger, M.D.

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