Fecal Occult Blood Test (FOBT)
Reviewed
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.
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.
Understanding this test
Use the reference interval and units printed by the laboratory that performed the test. A result outside that interval is not a diagnosis by itself, and a result inside it does not rule out every condition. Your symptoms, history, medicines, timing, and related tests can change what the result means.
- 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).
Evidence source
U.S. National Library of Medicine: Fecal Occult Blood Test (FOBT)