Guaiac
Formal Definition
Fecal occult blood test (FOBT) using guaiac-impregnated paper and hydrogen peroxide to detect peroxidase activity from heme in stool; a positive result indicates the presence of blood in stool, either from upper or lower GI sources, and requires further workup.
How It's Used on the Ward
"Guaiac positive" or "guaiac the stool" — ordered in any patient with anemia, GI symptoms, or concerning exam; results as "guaiac pos/neg".
Example
""Patient admitted with hemoglobin of 7.2, down from 11 last month — no obvious source. Rectal exam: guaiac positive, no masses palpable. Starting GI workup: colonoscopy scheduled for tomorrow.""
Clinical Context
Guaiac test detects heme, not intact red cells — upper GI bleed (melena source) and lower GI bleed both positive. False positives: red meat, turnips, horseradish, certain medications (iron, NSAIDs). False negatives: vitamin C (antioxidant interferes with reaction), old sample. A positive guaiac is a finding, not a diagnosis — it mandates investigation. A patient with guaiac-positive stool and a dropping hemoglobin needs urgent GI evaluation regardless of symptom severity.
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