Fecal Occult Blood Test (FOBT)

Reviewed

Also called: FOBT, stool occult blood test, Hemoccult test, guaiac smear test, gFOBT, immunochemical FOBT (iFOBT or FIT)

A fecal occult blood test (FOBT) checks for hidden blood in your stool. It is a screening test used primarily to detect colorectal cancer early, when treatment is most effective. The test can also help find other causes of digestive tract bleeding. A positive result does not mean you have cancer; it requires follow-up testing (usually a colonoscopy). The test is typically done every year starting at age 45 for average-risk individuals. There are two main types: guaiac FOBT (gFOBT) and immunochemical FOBT (FIT). FIT is considered more accurate. Follow your kit's instructions carefully for accurate results.

Specimen: Stool (feces)

Why this test may be ordered

  • Screening for colorectal cancer (starting at age 45 for average risk, or earlier for higher risk).
  • To investigate symptoms of digestive tract bleeding (e.g., anemia, change in bowel habits).
  • To help distinguish irritable bowel syndrome from inflammatory bowel disease.

Preparation and collection

For some guaiac tests only, you may need to avoid NSAIDs, vitamin C (over 250 mg/day), red meat, and certain raw fruits/vegetables for a few days before testing. Follow the specific instructions in your test kit.

stool (poop)

According to MedlinePlus, there is no known risk to having a fecal occult blood test.

Understanding results

This test checks for hidden (occult) blood in your stool. The result is reported as screen-positive or screen-negative.

  • Screen-positive: Blood was detected in your stool sample. You likely have bleeding somewhere in your digestive tract. Limitation: A screen-positive result does not diagnose cancer. It only indicates possible bleeding in the digestive tract, which could be from polyps, hemorrhoids, ulcers, or other causes. Next step: Your provider will likely recommend a colonoscopy to find the source of bleeding.
  • Screen-negative: No occult blood was found in your stool sample. Limitation: A screen-negative result does not guarantee that you do not have colorectal cancer. Some cancers or polyps may not bleed or may bleed intermittently. Next step: Continue regular screening as recommended by your provider (usually every year for FOBT).
  • FOBT is a screening test only; it cannot diagnose any condition. A positive result requires follow-up testing (e.g., colonoscopy) to find the cause of bleeding.
  • False-positive results can occur due to certain foods, supplements, or medicines (especially with guaiac-based tests).
  • False-negative results can occur if bleeding is intermittent or if the sample is not collected correctly.

What to do next

If your result is screen-positive, your healthcare provider will likely recommend a follow-up test such as a colonoscopy to find the exact cause of bleeding. If screen-negative, continue regular screening as advised by your provider (typically every year).

  • What does my result mean for my risk of colorectal cancer?
  • Do I need any follow-up tests based on my result?
  • How often should I repeat this screening test?

Evidence source

U.S. National Library of Medicine: Fecal Occult Blood Test (FOBT)

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