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Occult blood refers to the presence of blood in body fluids/tissues that is not visually apparent and requires laboratory detection.

Medical Explanation

1. Definition
Occult blood = Hemoglobin or its degradation products present in sub-visual concentrations in stool, urine, or other secretions. Grossly visible blood is termed "frank blood"; non-visible blood is "occult".

2. Clinical significance - GI tract
F***l Occult Blood Test (FOBT/iFOBT) is a screening tool for lower GI bleeding.
Guaiac-based FOBT: Detects heme peroxidase activity. Sensitive to dietary peroxidases → dietary restrictions needed.
Immunochemical FOBT/iFIT: Detects human globin via monoclonal antibodies. More specific for human blood, no dietary restrictions, detects bleeding from lower GI tract only as globin degrades in upper GI.

3. Differential diagnosis for positive occult blood
Lower GI sources: Colorectal adenoma/carcinoma, inflammatory bowel disease, infectious colitis, diverticular disease, hemorrhoids, a**l fissure.
Upper GI sources: Peptic ulcer disease, gastritis, esophagitis, Mallory-Weiss tear - detectable only by guaiac FOBT.
False positives: Dietary peroxidases, vitamin C >250mg/day, iron supplements, NSAIDs.
False negatives: Ascorbic acid, rapid GI transit, bleeding above ligament of Treitz for iFOBT.

4. Diagnostic workflow
Positive iFOBT/FOBT → requires further evaluation with colonoscopy ± upper endoscopy to localize bleeding source and rule out colorectal neoplasia. It is not diagnostic for cancer, only indicates mucosal disruption.

5. Other contexts
Urine: Occult blood on dipstick = hematuria not visible on microscopy. Detects RBCs, free Hb, myoglobin. Needs urine microscopy + renal workup.
Sputum: Occult hemoptysis in TB/bronchiectasis.