How Ulcerative Colitis Is Diagnosed

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Ulcerative colitis has symptoms similar to many other digestive conditions, which can make diagnosis challenging. Because treatment is needed to induce remission and to prevent the disease from worsening, getting an accurate and timely diagnosis is important.

Gastroenterologists may use a variety of tests in order to understand what’s going on with a patient who is suspected of having ulcerative colitis, but it is typically a colonoscopy with biopsies that is used to make the diagnosis.

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A colonoscopy is a way for a physician to see inside the large intestine. In ulcerative colitis, the large intestine will have certain characteristics that point to inflammatory bowel disease (IBD). Ulcerative colitis is a condition that is part of inflammatory bowl disease.

There may be inflammation that begins in either the rectum or the last part of the large intestine (the sigmoid colon) and spreads upwards through the rest of the colon. That inflammation occurs in the wall of the large intestine and will look red and swollen. There may also be ulcers (sores) on the intestinal lining.

During the course of the test, biopsies (small pieces of tissue) will be taken from various parts of the colon and sent to a lab for testing. The results of these tests can help in making the diagnosis of ulcerative colitis.

A colonoscopy is done by passing a long, thin, flexible tube (called a colonoscope) with a light on the end through the anus and up through the colon.

Patients must prepare for this test by emptying the bowel of stool. How this is done will vary based on patient and physician preference, but in general, strong laxatives are used to purge the colon of any fecal matter. In most cases, the preparation is done the day or the afternoon before the test. Patients will follow the instructions provided by the physician on how to prep and will fast until the time of the test the next day.

The colonoscopy itself is done under sedation, so patients won’t feel any discomfort or remember it. The sedation is given just prior to the colonoscopy via an IV. After the physician has completed the test and taken the necessary biopsies, patients are monitored while the sedation wears off and are then able to be driven home and have something to eat (as per physician’s instructions).

In some cases, the physician or another member of the healthcare team may give some feedback right after the test, which is why it’s a good idea to have a friend or relative available to help remember the conversation. In the case of a diagnosis, there may also be a follow-up scheduled later to discuss the results of the biopsies or to make a plan for next steps.

Other Imaging Studies

Other imaging tests such as x-rays, barium enema, upper gastrointestinal series, sigmoidoscopy, upper endoscopy, CT enterography, MR enterography, or capsule tests to visualize the small bowel may also be used during the process of diagnosing ulcerative colitis.

Your doctor may use other imaging tests, but these generally don’t provide as much information about ulcerative colitis as a colonoscopy.

The changes that ulcerative colitis causes in the colon may be visible via these other tests, but it will not be possible to see the entire colon and to get biopsies. They may be used to rule out other conditions as a cause for signs and symptoms but are not going to have as much use in the diagnostic process for ulcerative colitis.

Labs and Tests

Blood Tests

Blood tests will provide information about how the signs and symptoms are affecting the body, but they aren’t used solely to diagnose ulcerative colitis.

Red blood cell and white blood cell counts, in particular, are useful in getting a fuller picture of the body and if the ulcerative colitis is causing another condition, such as anemia. Erythrocyte sedimentation rate and C-Reactive Protein tests may also be helpful.

Other blood tests may be used to monitor the progress of the disease, especially during a flare-up, but may not be of much use in making the initial diagnosis.

Stool Tests

A stool test, called Calprotectin, might be used as part of the complete workup. It won’t be diagnostic for ulcerative colitis, but rather is used to rule out other potential causes of diarrhea or bloody diarrhea.

You may be asked to provide a stool sample or given a container to collect stool in at home. The sample is returned to the lab where it can be tested for blood, parasites, and bacteria.

People with IBD can also have bacterial infections, and indeed are more prone to them, so a stool test or a stool culture may be used to either confirm that or to rule it out.

Differential Diagnoses

Some of the common symptoms of ulcerative colitis, such as left-sided abdominal pain and diarrhea, can be caused by other conditions, so it will be important to rule those out in making the diagnosis.

  • Parasitic infection. Infection with some parasites may cause pain and bloody stools. This cause might be suspected if there has been recent travel to an area where these infections are more common. 
  • Bacterial colitis. Colitis is the condition of having inflammation in the colon, regardless of the cause. Bacterial infections (such as from E​. coli) can cause colitis.
  • Clostridioides difficile infection. This bacterial infection causes many symptoms similar to ulcerative colitis and requires a different treatment to clear it.
  • Crohn's disease. Crohn’s disease and ulcerative colitis are both forms of IBD, but they are treated differently in some cases and therefore making the distinction is important.
  • Ischemic colitis. This condition is caused by a lack of blood flow to part of the colon and requires immediate treatment.
  • Microscopic colitis. This type of colitis, while it does cause diarrhea, does not cause bloody diarrhea.
  • Viral infection. Gastroenteritis (“stomach flu”) or other viral infections will also cause pain, vomiting, and diarrhea, but most people usually recover in a few days.

Frequently Asked Questions

  • What are the earliest symptoms of ulcerative colitis?

    Most people with the condition initially have symptoms caused by inflammation of the rectum:

    • A sensation of pressure in the rectum (tenesmus)
    • Rectal bleeding or bloody stools
    • An urgent need to have a bowel movement

    That said, up to 15% of people have symptoms of severe ulcerative colitis from the start, which include incontinence (even during sleep), mucus in the stool, abdominal cramps, fatigue, and fever.

  • What are ulcerative colitis stools like?

    If you have ulcerative colitis, your bowel movements will be loose or watery, and contain blood, mucus, and/or pus. Your stool also likely to have a foul, sulphur-like odor.

  • How did I get ulcerative colitis?

    It's likely to have started with a bacterial or viral infection of your colon. Even after the initial infection was resolved, your immune system continued to send white blood cells to your colon, causing persistent inflammation and leading to ulcers (sores) in the lining of you intestines. Why this immune system overreaction happens to some people isn't well understood, but genetics, gut microbiome (the bacteria that inhabit the digestive system), and environmental factors likely are involved.

  • How long does it take to cure ulcerative colitis once it's diagnosed?

    Ulcerative colitis is a chronic disease, meaning there's no cure. However, there are ways to manage the disease, including medications, special diets—specifically one that eliminates grains, sugar, and starchy and processed foods—and, in severe cases, surgery to remove parts fo the colon that are affected.

9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  4. Inflammatory Bowel Disease Group, Chinese Society of Gastroenterology, Chinese Medical Association. Evidence-based consensus on opportunistic infections in inflammatory bowel disease (republication)Intest Res. 2018;16(2):178-193. doi:10.5217/ir.2018.16.2.178

  5. Adams SM, Bornemann PH. Ulcerative Colitis. Am Fam Physician. 2013;87(10):699-705.

  6. Rubin DT, Ananthakrishnan AN, Siegel CA. ACG clinical guideline: ulcerative colitis in adultsAm J Gastroenterol. 2019;114(3):384-413. doi:10.14309/ajg.0000000000000152

  7. Ungaro R, Mehandru S, Allen PB, et al. Ulcerative colitisLancet. 2017;389(10080):1756-1770. doi:10.1016/S0140-6736(16)32126-2

  8. Crohn's & Colitis Foundation. Overview of ulcerative colitis.

  9. Stanford Health Care. Expert treatments for ulcerative colitis.

Additional Reading
  • Dans LF, Martínez EG. "Amoebic dysentery." BMJ Clinical Evidence. 2007;2007:0918. 
  • Kornbluth A, Sachar DB. "Practice Parameters Committee of the American College of Gastroenterology. Ulcerative colitis practice guidelines in adults." Am J Gastroenterol. 2010;105:501–523.
  • Pardi DS, Kelly CP. Microscopic colitis. Gastroenterology. 2011;140:1155–1165. doi: 10.1053/j.gastro.2011.02.003.​

By Amber J. Tresca
Amber J. Tresca is a freelance writer and speaker who covers digestive conditions, including IBD. She was diagnosed with ulcerative colitis at age 16.