Overview

The Stool for Occult Blood Test detects hidden (occult) blood in the stool that is not visible to the naked eye. It is primarily used to screen for colorectal cancer, polyps, or gastrointestinal (GI) bleeding.

There are two main types:

• Guaiac-based FOBT (gFOBT) – uses a chemical reaction to detect heme.

• Immunochemical FOBT (iFOBT or FIT) – uses antibodies to detect human hemoglobin (more specific).

Risk Assessment / Indications

The test is typically used in the following contexts:

Screening

• Adults over 50 years (colorectal cancer screening)

• Earlier if there’s a family history of colorectal cancer

Diagnostic

• Iron-deficiency anemia of unclear origin

• Unexplained weight loss or abdominal pain

• Suspected GI bleeding

• Inflammatory bowel disease (as a supportive test)

Negative for occult blood
→ No hidden blood detected in stool.

Interpretation

Important Notes:

• False positives: Can occur due to diet (red meat, some vegetables), NSAIDs, or bleeding gums.

• False negatives: May occur if bleeding is intermittent or low volume.

Pre-test Instructions (for gFOBT):

• Avoid red meat, NSAIDs, and vitamin C for 2–3 days prior (not needed for FIT).

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