A physician may be able to diagnose irritable bowel syndrome (IBS) based on a medical history and a physical examination. A medical history can include questions about the duration and severity of symptoms, changes in bowel functioning and the relation of symptoms to bowel functioning. Diet, current medications, activity and stress levels may also be discussed. A physical exam can identify signs of other illness that may be causing the symptoms.

Patients may be diagnosed with IBS when they have experienced abdominal pain or discomfort for at least 12 weeks (not necessarily consecutive) in the past year. In addition, patients must meet at least two of the following criteria for a diagnosis of IBS:

• Pain is relieved with the passage of stool
  
• Change in frequency of bowel movements when pain begins
  
• Change in appearance of stool when pain begins

There is no specific diagnostic test used to identify IBS. Instead, tests may be used to rule out other conditions as the cause of a patient’s symptoms. These may include lactose intolerance, infection, diverticulosis, thyroid disorders, inflammatory bowel diseases (e.g., Crohn's disease or ulcerative colitis) or cancer. Tests may be selected based on a patient’s symptoms, the likelihood of other causes of symptoms, as well as the cost and safety of each test.

Tests used to rule out other causes of symptoms in patients with IBS include:

• Blood tests. Drawing of blood from a vein for laboratory analysis. These tests may rule out other conditions that may cause symptoms similar to IBS (such as celiac disease or thyroid disorders).
  
  
• Fecal tests. Laboratory analysis of a stool sample. This may reveal infection or hidden (occult) bleeding, which may indicate various other causes for symptoms.
  
  
• Lactose intolerance tests. Tests that determine whether patients are unable to process lactose (commonly found in dairy products), which may be causing symptoms such as abdominal pain, gas and diarrhea. These tests include a breath test or the systematic elimination of dairy products from the diet under physician supervision.
  
  
• Barium enema x-ray. X-rays taken after a patient has received an enema of a chalky, white substance (barium) that allows internal organs to show up on x-rays. These imaging tests are used to look for any abnormalities that may be causing the symptoms.
  
  
• Endoscopy. An imaging test in which a flexible tube with a camera is inserted into the digestive tract, either through the mouth or the anus. This allows images to be taken of the intestines. Types of endoscopy include colonoscopy (to view the entire large intestine) and sigmoidoscopy (to view the lower portion of the large intestine). These tests may cause spasms and abdominal pain in patients with IBS. They are often recommended as screening tests for colon cancer in patients over the age of 50, but may also be used to rule out conditions that causes similar symptoms to IBS (e.g., inflammatory bowel disease).
  
  
• Anorectal manometry. Balloon catheters are inserted into the anus and rectum and used to test the strength and function of the anal sphincter muscles and exclude anorectal incontinence.
  
  
• Other radiological exams. A computed axial tomography (CAT) scan or an abdominal ultrasound may be used to produce images of internal organs in the abdomen and pelvic areas. These tests allow a physician to look for any abnormalities in these areas.

If no other medical disorder is found to be the cause of a patient’s symptoms, a physician may diagnose IBS based on the patient’s symptoms.