Ulcerative colitis can be relatively easy to diagnose because it normally affects only the colon and rectum and usually causes an obvious change in daily bowel habits, such as frequent stools containing blood or mucus. Your health professional will conduct a medical history and physical exam before doing other tests.

The colon and rectum can be examined with flexible sigmoidoscopy or colonoscopy, tests in which a doctor examines the inside of the large intestine using a small, lighted scope. In general, colonoscopy is the preferred test because it can be used to examine the entire colon. However, flexible sigmoidoscopy may be all that is needed to diagnose ulcerative colitis. Both procedures can be used to take a sample (biopsy) of intestinal tissue. The diagnosis of ulcerative colitis is made by ruling out other causes of diarrhea and assessing the results of these tests.

Other exams and tests that may be used to evaluate ulcerative colitis include:

• Abdominal X-ray, which provides a picture of structures and organs in the abdomen.
• Barium enema, a test that allows the doctor to examine the large intestine (colon). For a barium enema, a whitish liquid (barium) is inserted through the rectum into the colon and large intestine. The barium outlines the inside of the colon so that it can be more clearly seen on an X-ray.
• Computed tomography (CT) scan, which uses X-rays to produce detailed pictures of structures inside the body.
• Magnetic resonance imaging (MRI), which uses a magnetic field and pulses of radio wave energy to provide pictures of organs and structures inside the body.

A stool analysis (including a test for blood in the stool) is often done, depending on symptoms, to look for blood, signs of bacterial infection, parasites, or the presence of white blood cells. This test can be used to distinguish ulcerative colitis from irritable bowel syndrome (IBS), a less serious condition that sometimes has similar symptoms.

The presence of white blood cells in stool indicates inflammation and infection but is not necessarily a sign of ulcerative colitis. However, white blood cells in stool mean that you do not have IBS. Stool analysis may be done during a flare-up of ulcerative colitis if there is concern that new symptoms are caused by another problem. You can collect a stool sample, or the doctor may obtain it during sigmoidoscopy or colonoscopy.

• Standard blood and urine tests may be done to check for anemia, inflammation, or malnutrition. Depending on the symptoms, an erythrocyte sedimentation rate (ESR, or sed rate) or C-reactive protein (CRP) blood test may be done to look for infection or inflammation. C-reactive protein is a substance produced by the liver as a result of inflammation in the body.
• Biopsy of a sample of tissue from the lining of the intestine may be done. Biopsies are collected during sigmoidoscopy or colonoscopy to confirm the diagnosis of ulcerative colitis. A biopsy also may be done to find out whether a tumor is present. Multiple biopsies for cancer screening are often done in people who have had ulcerative colitis for 8 years or more. Bowel biopsies are painless (other than the potential discomfort of the scope procedure) and remove only a tiny piece of tissue.

In about 10% of people who have symptoms, neither Crohn's disease nor ulcerative colitis can be diagnosed. These people have a form of inflammatory bowel disease called indeterminate colitis, which doctors believe is a combination of Crohn's disease and ulcerative colitis.⁶