Fecal occult blood test

The fecal occult blood test detects bleeding in the gastrointestinal tract in a simple, fast, and low-cost manner. Although the treatment is not definitive for colorectal cancer or any other disease, it is commonly used as a colorectal cancer screening test, and is a leading cause of cancer death.

When malignant or cancerous cells develop in the intestine, they can cause superficial bleeding that is not visible to the naked eye. To obtain supernatural fecal blood, a small portion of a stool specimen must be extracted using chemical or immunochemical processes. The early identification of gastrointestinal bleeding aids in the diagnosis of gastrointestinal conditions.

The importance of using fecal blood as a colorectal cancer early detection tool is huge. Hemorrhoids, anal fissures, intestinal polyps, ulcerative colitis, and Crohn’s disease are some of the other reasons for blood in the stool.


Occult gastrointestinal haemorrhage is suspected (including potential screening test for colorectal carcinoma).


The most common method is to use impregnated guaiac paper to perform an easy and low-cost qualitative chemical search. Haem (in feces) has pseudoperoxidase activity, which catalyzes the release of free radical oxygen from hydrogen peroxide (in the forming solution), resulting in the oxidation of phenolic chromogens (in guaiac) to quinones and the formation of blue pigment. Several (e.g., three consecutive) bowel movements or a rectal examination are used to collect feces samples.

Guaic Test

The guaiac test, also known as a fecal occult blood test, has been around for a number of years. A slide with squares of paper covered with guaiac, a solvent derived from the resin of the Guaiacum genus fruit, is used to do this. A tiny portion of the fecal (stool) specimen is placed on the slide’s paper squares. A developer solution of hydrogen peroxide is added to the paper ( H2O2). If there is blood in the sample, the hemoglobin iron (Fe) catalyzes the association of the paper guaiac with the H2O2, resulting in a blue colour. For a reaction, here’s a simplified formula:

Alpha guaiaconic acid + H2O2 —(Hemoglobin)—–> Blue Quinone compound

The disadvantage of the guaiac procedure is that the guaiac response is not exclusive to blood, and the treatment does not detect 100% of colorectal cancers. Another disadvantage is that the examination can interfere with some foods and medications, resulting in a false result. Horseradish and turnips possess the peroxidase enzyme, which can trigger a false-positive result when present in high amounts in the diet. Furthermore, due to its high iron content, red meat can provoke a (false) positive reaction. An ulcer drug called cimetidine contains a blue dye that can cause test results to be misinterpreted. Excess dietary vitamin C suppresses the guaiac response, resulting in a false-negative test.

Occult Blood Test Kit
Commercial Occult Blood Test Kit

Test Interpretation

The fecal occult blood test is nothing more than a blood involvement screening test that isn’t specific to any disease. While the test is said to be very accurate in identifying colon cancer-related bleeding, it does not guarantee 100% accuracy.

Positive findings should be followed up by more conclusive tests like colonoscopies or radiographic trials. Any occult blood tests are more vulnerable than others, which should be taken into account when assessing test results. Because certain small bowel lesions bleed only sometimes, a test can come out negative even though the patient is sick. For this cause, several specimens are usually examined. Patients who continue to have digestive problems after unfavorable occult blood samples should be checked using more precise procedures.


False positives

  • Animal haemoglobin or vegetable peroxidase in the diet (tests may recommend exclusion of red meat and certain uncooked fruits and vegetables for 3 days before and during collection period).
  • Aspirin and nonsteroidal anti-inflammatory medications (NSAIDs); use of rectal drugs (Note: iron therapy should not affect guaiac-based tests).

False negatives

  • GI lesions, like cancer, can bleed sometimes or not at all.
  • If there is a significant amount of blood leakage from the upper GI tract, it is insensitive (haem is metabolised in the small bowel).
  • Vitamin C intake at a high dosage (reducing agent)

Other methods of detecting fecal occult blood

Immunochemical Test

By using an antibody to the globin chain of hemoglobin, the immunochemical assay (ICT) detects fecal occult blood. The ICT is exempt from CLIA requirements and has some benefits over the guaiac test:

  • Hemoglobin is soluble in dried feces, so testing can be postponed for several days without losing reactivity.
  • Since the test is selective for hemoglobin, no dietary limitations are needed.


  1. Estridge, Barbara H., and Anna P. Reynolds. Basic Clinical Laboratory Techniques. Clifton Park, NY: Delmar Thomson Learning, 2012
  2. Hubbard J D. A Concise Review of Clinical Laboratory Science. Lippincott Williams and Wilkins.
  3. Provan D and Krentz A. Oxford Handbook of Clinical and Laboratory Investigation. Oxford University Press

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