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Fecal Occult Blood Test (FOBT)

Last updated July 23, 2019

Approved by: Maulik P. Purohit MD, MPH

Fecal Occult Blood Test (FOBT) is performed to detect the presence of minute quantities of blood in stool, not visible to the naked eye (hence the term ‘occult’, or hidden blood).

What are the other Names for this Test? (Equivalent Terms)

  • Guaiac Smear Test
  • Immunoassay FOBT
  • Stool Occult Blood Test

What is Fecal Occult Blood Test? (Background Information)

  • Fecal Occult Blood Test (FOBT) is performed to detect the presence of minute quantities of blood in stool, not visible to the naked eye (hence the term ‘occult’, or hidden blood)
  • The presence of such small quantities of blood in stool indicates bleeding into the gastrointestinal tract, often a sign of polyps (finger-like projections from the walls of intestines, or commonly from the colorectal area) that may be benign or cancerous. Thus, FOBT mainly aids in the early detection of colon cancer
  • It is recommended that FOBT be done annually, in adults aged over 50 years. The test may also be ordered, as per the discretion of the physician, based on family history, or when a bleeding into the gastrointestinal tract is suspected

Fecal occult blood maybe detected in the following ways:

  • Guaiac Smear method/gFOBT: In this method, a chemical reagent that changes color in the presence of blood, is used
  • Immunochemical FOBT/iFOBT/FIT: In this method, antibodies (proteins that react with substances called antigens) against hemoglobin (protein present in red blood cells) is used
  • Over-the-counter (OTC) flushable reagent pad/tissue: Makes use of chemicals that change color in the presence of blood. The individuals may themselves perform the test at home, and record any color change

Even when an individual has a polyp or early cancer, bleeding from the growth may occur, only intermittently. Hence, it is recommended to perform FOBT on 3 separate stool samples, taken on 3 separate days, to increase the chances of finding any hidden bleeding.

  • While the immunochemical test is specific for blood from the lower gastrointestinal tract (colon), the Guaiac-based test and the OTC test are non-specific and detect any blood in the stools - even blood that is swallowed, due to nosebleeds or gum bleeds, or bleeding from the upper portions of the gastrointestinal tract. Hence, some preparation is required for the Guaiac Smear and OTC tests
  • An individual, who has occult blood in their stools, may be subjected to further investigation, to find out the source of bleeding. This may include procedures, such as sigmoidoscopy or colonoscopy. Tubes fitted with cameras, are passed into the colon and lower end of the large intestine, to check for any growth or polyps and tissue samples taken for examination

What are the Clinical Indications for performing the Fecal Occult Blood Test?

Clinical indications for performing Fecal Occult Blood Test include:

  • Screening for colon cancer, every year after the age of 50 years, or as determined by the physician, based on the family medical history
  • Unexplained anemia (low levels of hemoglobin in blood) and when the physician suspects bleeding into the gastrointestinal tract, to be the probable cause
  • Presence of any black tarry stools
  • Symptoms, such as loss of weight, appetite, abdominal pain, and altered bowel movements

How is the Specimen Collected for Fecal Occult Blood Test?

Sample required: Stool - the American Cancer Society recommends testing on 3 different stool samples, collected on 3 separate days.


  • For Guaiac Smear Test: The stool sample, free from urine or water, is collected in a clean container and a thin smear is transferred onto a card with a stick and allowed to dry. When 3 such samples have been collected, the cards are sent to the lab for testing
  • For Immunochemical FOBT: A long-handled brush is used to obtain a sample from the surface of the stool and placed on a card and allowed to dry. Once, 3 such samples have been taken, the cards are sent to the lab for testing
  • OTC flushable reagent pad/tissue: The pad is dropped into the toilet bowl after a bowel movement. It changes color in the presence of any blood, which can be noted by the individuals themselves. However, blood in urine, or from menses, as well as the use of toilet cleaners, may influence the results

Preparation required:

The Guaiac Smear Test and the OTC reagent pad testing require preparation. These 2 tests will detect blood from any part of the gastrointestinal tract, unlike the Immunochemical Test that detects blood only from lower GI portions (colon).

It is advised to:

  • Avoid having any dental procedures, up to 3 days before collection of stool samples
  • Avoid taking NSAIDs, such as aspirin and ibuprofen, if possible for 7 days, prior to testing
  • Avoid foods, such as red meat, turnips, radish, broccoli, mushrooms, and drugs, such as colchicines for 3 days prior to testing, as they may give a positive test result, even in the absence of any blood (false positive result)
  • Avoid vitamin C supplements and fruits/vegetables containing vitamin C, such as guava, peppers, oranges, kiwi fruit, etc. for 3 days prior to testing, as they may give false negative results (i.e. no color change observed, even in the presence of blood)

What is the Significance of the Fecal Occult Blood Test Result?

A positive Fecal Occult Blood Test result indicates:

  • For the Guaiac Smear Test:
    • Bleeding could be from any part of the gastrointestinal tract - due to ulcers, inflammatory bowel disease, polyps, hemorrhoids, or diverticulosis
    • The blood could have been swallowed and the original source of bleeding, may be the gums, or caused due to nosebleeds
    • False positive results (a positive test result in the absence of any bleeding) maybe caused by food items such as red meat, turnips, radish, broccoli, and mushrooms, or due to drugs such as colchicine
  • For the Immunochemical Test: Bleeding could be from the lower part of the gastrointestinal tract (colon)
  • Any positive test result requires further workup, in order to find the source of bleeding

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.

Additional and Relevant Useful Information:

Colonoscopy is a screening method used to check for any abnormal growth in the colon, in individuals more than 50 years of age or earlier; if there is a family history or colon cancer. The following article link will help you understand colonoscopy surgical procedure.


Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

References and Information Sources used for the Article:

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: Dec. 31, 2013
Last updated: July 23, 2019