Testing for IBS: How is it Diagnosed?


There is currently no one test or biological marker to diagnose irritable bowel syndrome (IBS). What that means is that there is no way to easily test for IBS using a blood, stool, or tissue sample, although research is underway. While this can make things more complicated for those exhibiting symptoms to quickly arrive at a diagnosis, your doctor will help you get there.

While there is no definitive test for diagnosis, the starting point is typically a physical exam. Your doctor will check for abdominal pain, bloating, and may listen to your abdomen with a stethoscope for any sounds, as well taking your full medical history, including family digestive disorders (such as celiac disease, inflammatory bowel disease, or colon cancer), as well as any recent infections, stressful events, medications you are on, and your diet. There could be other testing performed to rule out other conditions, as many other disorders share similar symptoms with IBS (1, 2, 3).


The Rome Criteria

Although there is often cause to rule out other conditions, IBS is not merely a disorder of exclusion (diagnosed by what it is not). There are also a set of criteria to measure the likelihood of IBS in a patient, known as the Rome criteria. To be diagnosed with IBS under these guidelines a patient must have:

  • The onset of symptoms at least 6 months ago
  • Abdominal pain lasting an average of at least one day a week in the last three months, plus at least two of the following:
    • Pain-related to passing a bowel movement (either before, during, or after)
    • Altered stool consistency
    • Altered frequency of bowel movements 

If you are diagnosed with IBS based on those criteria, your doctor may also categorize your IBS into one of 3 main subtypes depending on your symptoms: constipation-predominant IBS (IBS-C), diarrhea-predominant IBS (IBS-D), or IBS with mixed symptoms of diarrhea and constipation (IBS-M) (1,2,3). 


More Severe Symptoms

While IBS can be diagnosed using just these criteria above and in the absence of any other testing, certain symptoms and conditions will likely cause your doctor to run more tests to rule out a more serious illness. For example:

  • Weight loss
  • Anemia
  • Bloody or black stool
  • Bleeding from the rectum
  • Fever
  • Nausea
  • Stomach pain or diarrhea at night
  • A patient over the age of 50 (3,2)


Additional testing

If there are no red flag symptoms that cause immediate worry, your doctor may still test to see if your IBS symptoms are related to other conditions. Testing can include stool tests, blood tests, breath tests, or further diagnostics (1,2).

  • Blood Tests: Your doctor may run bloodwork to check for Celiac disease if you have diarrhea-predominant IBS (IBS-D) or mixed diarrhea and constipation (IBS-M), as well as to rule out anemia or infection (2). 
  • Stool tests: Your doctor may have you collect a stool sample to send or take somewhere for testing. Testing could include lab work to check your stool for blood, infection, bacteria, parasites,  or malabsorption issues. If you have severe diarrhea they may check for bile acid malabsorption, which is an excess of a digestive enzyme produced in your liver (1,3). 
  • Breath tests:  Hydrogen breath tests can be used to check for certain issues as well, for example:
  • Lactose intolerance: Some people don’t produce enough lactase, an enzyme you need to digest the sugar found in dairy products. If this is an issue you have, you may have problems very similar to IBS, like abdominal pain, gas, and diarrhea. You could also be put on a temporary elimination diet omitting dairy for a few weeks to see if it could potentially be the culprit of your symptoms. 
  • SIBO:  A breath test can help determine whether you have bacterial overgrowth in your small intestine, or SIBO. People who have had bowel surgery, altered motility (like IBS or Diabetes), immunodeficiency, low gastric acid, or cirrhosis/pancreatitis are more likely to have this condition (3).


Diagnostic procedures 

  • Colonoscopy. Your doctor will use a thin, flexible tube to examine the entire length of the colon, usually under light sedation, to look for issues such as colon cancer or inflammatory bowel disease. 
  • X-ray or CT scan. These are tests that generate images of your abdomen and pelvis for your doctor to rule out other symptom causes, especially if abdominal pain is an issue. Your doctor might fill your large intestine with barium to coat them and make issues more visible to an X-ray. This can also be called a lower GI series.
  • Upper endoscopy. Your doctor will inspect your upper digestive tract using a long tube with a camera on the end of it, which is inserted down your throat and into the esophagus. He or she may perform a biopsy by taking a small tissue sample from your small intestine to check for any bacterial overgrowth.  If celiac disease is suspected, he or she may also recommend an upper endoscopy. (1,3)

If the results of further testing come back negative, and your symptoms align with the Rome criteria, your doctor may diagnose you with IBS. If one or more tests come back positive, you may have another issue entirely, or you could have a secondary condition along with IBS. Because there is not a concrete marker to indicate that you do have IBS one way or the other, and so many other issues mimic the same characteristics of the disorder, arriving at a diagnosis can be complicated. Your doctor will make that call based on the symptoms you exhibit and in the absence of any more serious issues.  By learning all you can about your IBS and working with your clinicians to come up with an individualized treatment plan, you can find a  solution that works for you (1,2).



1) “Diagnosis of Irritable Bowel Syndrome,” niddk.nih.gov, National Institute of Diabetes and Digestive and Kidney Diseases, accessed September 1, 2021,  https://www.niddk.nih.gov/health-information/digestive-diseases/irritable-bowel-syndrome/diagnosis.

3) “Irritable Bowel Syndrome,” mayoclinic.org, Mayo Clinic, accessed September 1, 2021, https://www.mayoclinic.org/diseases-conditions/irritable-bowel-syndrome/diagnosis-treatment/drc-20360064.

2) “Testing in IBS,” aboutibs.org, International Foundation for Gastrointestinal Disorders, accessed September 1, 2021, https://aboutibs.org/signs-and-symptoms/testing-in-ibs/.