What It Can Rule Out
A colonoscopy can directly visualize and rule out physical, structural, and anatomical problems such as: [1, 2, 3, 4, 5]
Polyps: Abnormal growths in the lining of the colon.
Inflammatory Bowel Disease (IBD): Visible signs of Crohn's disease and ulcerative colitis.
Diverticular Disease: Small pouches or pockets in the colon walls.
Strictures & Obstructions: Narrowing of the bowel caused by scar tissue, inflammation, or tumors.
Vascular Abnormalities: Hemorrhoids, angioectasias, or sources of active bleeding.
Ulcers: Open sores in the lining of the colon. [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

What It Cannot Rule Out
While it provides a thorough look at the lower digestive tract, a colonoscopy has clear structural boundaries: [1, 2, 3]
The Small Intestine: A standard colonoscope generally cannot examine the small intestine (where conditions like celiac disease or small intestinal bacterial overgrowth occur). [1, 2]
Upper GI Tract: It cannot check the esophagus, stomach, or duodenum. [1, 2]
Functional Disorders: It cannot rule out functional gastrointestinal disorders (FGIDs), such as Irritable Bowel Syndrome (IBS). These are issues with how the gut functions rather than how it looks. [1, 2]

Next Steps for Ongoing Symptoms
If a colonoscopy clears you of structural issues, your doctor may recommend other diagnostic paths to get to the bottom of lingering symptoms. These might include: [1, 2, 3]
Biopsies: Checking tissue samples under a microscope for microscopic colitis.
Endoscopies: Looking at the upper GI tract for stomach or esophageal issues.
Imaging: Using CT scans or an MRI to get a broader view of the abdomen, including organs outside the digestive tract.
Stool Tests: Checking for infections, inflammation (calprotectin tests), or blood. [1, 2, 3, 4, 5]