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The fecal occult blood test (FOBT), also known as an occult blood test or the benzidine test (Gregersen reaction), assesses stool for trace amounts of blood not visible on inspection. The assay supports evaluation of gastrointestinal mucosal injury due to peptic ulcer disease, primary or metastatic tumors of the esophagus, stomach, small or large intestine, inflammatory bowel disease, intestinal tuberculosis, and helminthiasis. It is used to estimate the extent of mucosal damage, to monitor response to therapy in conditions such as peptic ulcer disease, ulcerative colitis, Crohn disease, and intestinal tuberculosis, and to help gauge overall disease severity and prognosis.

Occult gastrointestinal bleeding may accompany a wide spectrum of disorders and can be difficult or impossible to identify visually or by routine microscopy. When bleeding is brisk and arises from the lower gastrointestinal tract, bright red blood or clots may be evident; bleeding from the upper tract often produces black, tarry stools due to digestion of hemoglobin. In milder or intermittent bleeding, stool appearance remains normal, and intact erythrocytes may be sparse or absent microscopically. FOBT detects chemically altered hemoglobin rather than intact red cells, enabling identification of small-volume blood loss. A positive FOBT indicates mucosal disruption with blood loss into the gastrointestinal lumen and is observed in conditions such as peptic ulcer disease, ulcerative colitis, Crohn disease, intestinal polyps, helminthic invasion, and primary or metastatic gastrointestinal tumors. The test contributes to early detection of colorectal neoplasia, for which occult bleeding frequently begins at an early stage. Extraintestinal or proximal sources of blood—epistaxis, gingival or pharyngeal bleeding, esophageal varices, erosive esophagitis, and hemorrhoids—can also yield positive results and should be considered during interpretation. Analytical yield improves with repeat testing. A negative result does not exclude erosive, ulcerative, or neoplastic disease and must be interpreted alongside clinical assessment and other laboratory and imaging studies. The Gregersen (benzidine) reaction is a commonly used method for detecting occult blood in stool, but it is one of several methodologies available for fecal occult blood testing.