Ulcerative Colitis Diagnosis: Colonoscopy, Blood and Stool Tests, and More
Published on June 3, 2025
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When diagnosing ulcerative colitis (UC), a healthcare provider will talk to you about your health history, perform a physical exam, and order several kinds of medical tests. Depending on your provider’s schedule, testing availability, and other factors, an accurate diagnosis can take time.
How Doctors Diagnose Ulcerative Colitis
The only way to diagnose UC with certainty is by testing a small sample of tissue from your intestines during a colonoscopy. This is known as a biopsy. But other tests are used to confirm a UC diagnosis, check its severity, see how much of the large intestine is affected, identify complications, and assess if other intestinal issues like irritable bowel syndrome (IBS) or Crohn's disease are present instead.
Health History
To start, your provider will ask you about your symptoms, how long you have experienced them, and if anything makes them better or worse. They’ll want to know about your health history (any other health conditions you have or had) and what medications you take, if any. Since UC may be inherited genetically, they will also ask about your family’s medical history.
Physical Exam
At your appointment, your provider will do a physical exam. They will check your vital signs, like your blood pressure, heart rate, and temperature. To assess your bowel health, they may listen to your abdomen with a stethoscope and feel the area to check for pain or masses. You may also need a rectal exam to check for inflammation around the anus or blood in your stool.
Blood and Stool Tests
Blood and stool test results can rule out any other potential causes of your symptoms and figure out if you have any infections, inflammation, or bleeding.
- Blood tests: A blood test can check for anemia (a low number of red blood cells), inflammation, and infection. Blood tests can also identify nutrient deficiencies like iron, folate, vitamin B12, and vitamin D, which can mean the intestines aren’t absorbing them well.
- Stool tests: A stool test can rule out infections and may confirm a UC diagnosis based on levels of inflammation.
Endoscopy Procedures and Biopsy
Endoscopy procedures (including colonoscopy) are done under sedation (the specific type of anesthesia may vary) which allows your provider to see the intestines through a small camera mounted on a flexible, thin tube. They insert this tube into your rectum and examine your colon for inflammation and bleeding.
During the procedure, providers typically take biopsies (small samples of the intestinal lining) for testing in a lab. Two types of endoscopy are used to diagnose UC:
- Colonoscopy This procedure allows a doctor to inspect the entire colon.
- Flexible sigmoidoscopy This test allows a doctor to inspect the lower portion of the colon, including the rectum, and is used when the colon is severely inflamed.
Imaging
While imaging tests can’t diagnose UC on their own, they can identify any serious complications you may have and the severity of disease.
- X-ray: An X-ray of the abdomen can identify intestinal obstruction, bowel dilation (widening), or a perforated colon (a hole in the intestinal wall).
- Computed tomography (CT) scan: A CT scan of the pelvis or abdomen can reveal inflammation levels and certain complications like strictures (narrowing of the intestines).
- Magnetic resonance imaging (MRI): An MRI uses a magnetic field to take detailed three-dimensional images of the body, and can reveal small tears, ulcers, irritation, and bleeding.
- Enterography: Enterography scans are an enhanced version of a CT or MRI; contrast fluid is ingested before the test to help the organs stand out better on the scans.
Ruling Out Other Conditions
Healthcare providers may need to rule out other health conditions before they can diagnose someone with UC. These conditions, which may have similar symptoms, include:
- Crohn’s disease
- Infectious colitis including bacterial or viral infections
- Radiation colitis
- Tuberculosis
- Colon cancer
The Takeaway
- A definitive ulcerative colitis diagnosis requires a biopsy of intestinal tissue taken during an endoscopic procedure, such as a colonoscopy or flexible sigmoidoscopy.
- Other tests, like bloodwork, stool testing, and imaging, can rule out other conditions, help confirm UC diagnosis, and identify its severity.
- If you think you may have ulcerative colitis, tell your healthcare provider, who can order diagnostic tests if needed.
Resources We Trust
- Cleveland Clinic: Flexible Sigmoidoscopy
- Mayo Clinic: Living With Crohn's Disease or Colitis
- Crohn's & Colitis Foundation: IBD Mimics: Most Common Conditions Misdiagnosed as IBD
- Stanford Health Care: Ulcerative Colitis Diagnosis
- National Institute of Diabetes and Digestive and Kidney Diseases: Diagnosis of Ulcerative Colitis
EDITORIAL SOURCES
Everyday Health follows strict sourcing guidelines to ensure the accuracy of its content, outlined in our editorial policy. We use only trustworthy sources, including peer-reviewed studies, board-certified medical experts, patients with lived experience, and information from top institutions.
Resources
- Diagnosis of Ulcerative Colitis. National Institute of Diabetes and Digestive and Kidney Diseases. September 2020.
- How Is IBD Diagnosed? Crohn’s & Colitis Foundation.
- Ulcerative Colitis - Diagnosis and Treatment. Mayo Clinic. November 22, 2024.
- Diagnosing Inflammatory Bowel Disease. NYU Langone Health.
- IBD Mimics: Most Common Conditions Misdiagnosed as IBD. Crohn's & Colitis Foundation.
- Lynch WD et al. Ulcerative Colitis. StatPearls. June 5, 2023.
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Yuying Luo, MD
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