Occult Blood Test (FOBT – Fecal Occult Blood Test)

 



1. Objective

The objective of the Fecal Occult Blood Test (FOBT) was to detect hidden (occult) blood in the stool, which was not visible to the naked eye, and served as an early indicator of gastrointestinal bleeding, ulcers, colorectal cancer, or polyps.

2. Principle

The test was based on the principle that the presence of blood in the stool could catalyze a chemical reaction. Most commonly, guaiac-based FOBT used the pseudoperoxidase activity of hemoglobin to oxidize guaiac resin in the presence of hydrogen peroxide, producing a blue color change if blood was present. Alternatively, immunochemical tests (iFOBT or FIT) used antibodies to detect human hemoglobin.

3. Materials

  • Stool collection container

  • Guaiac test cards or immunochemical test kits

  • Applicator sticks

  • Hydrogen peroxide reagent (for guaiac method)

  • Gloves

  • Instruction leaflet (for at-home tests)

  • Control solutions (positive/negative, if available)

4. Procedure (Guaiac Method)

  • The patient collected stool samples from different parts of a bowel movement on designated test card areas using an applicator stick.

  • After the sample dried, the test card was opened and developer (hydrogen peroxide) was applied.

  • The test was observed for a blue color change within a specific time (usually 30–60 seconds).

  • A blue color indicated a positive result for occult blood.

(For immunochemical method, the test device was dipped in stool extract and results were interpreted based on colored lines forming on the test strip.)

5. Result

  • Positive: A visible blue coloration (in guaiac method) or appearance of the test line (in FIT) indicated the presence of occult blood.

  • Negative: No color change (in guaiac) or only control line visible (in FIT) indicated absence of detectable blood.

6. Uses

The test was commonly used to:

  • Screen for colorectal cancer in asymptomatic individuals

  • Detect bleeding from ulcers, hemorrhoids, diverticulosis, or inflammatory bowel disease

  • Monitor for gastrointestinal bleeding in anemia workups

7. Consultation

Following a positive result, a consultation was provided by a physician, often recommending further diagnostic investigations such as colonoscopy or sigmoidoscopy to determine the source of bleeding. Dietary and medication history was also reviewed to rule out false positives or interferences.

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