How is irritable bowel syndrome (IBS) diagnosed?

Your doctor should be able to diagnose irritable bowel syndrome (IBS). Doctors usually start by noting your symptoms and medical and family history. While there is no definitive test for IBS, there are tests that are helpful for confirming some indicators and eliminating other possible conditions that could be causing symptoms such as abdominal pain and bloating. A blood test and a stool test, for example, will help rule out anemia, other digestive issues, and infections. Other tests may include a breath test to check for bacterial or digestive issues, a colonoscopy, and an upper gastrointestinal tract endoscopy, which may be accompanied by a biopsy. Other tests may include an X-ray done after drinking barium to make the bowel more visible. If no other conditions are indicated, your doctor may decide to recommend medications and diet changes to help treat IBS; if the treatment works, that's a good indicator that you have the condition.

Some blood tests are required to help doctors assess how active your inflammatory bowel disease (Crohn's disease or ulcerative colitis) is. First is a complete blood count, which will indicate if you are anemic or if your blood counts are low. Further, the elevation of platelets and other markers of inflammation can allow your doctor to gauge how severe your disease is.

Your metabolic panel will be assessed to see if you're dehydrated. Are your electrolytes normal? Are there any liver abnormalities? Particularly, these are things that doctors will monitor while you're on certain types of therapies for Crohn's disease.

If you've had certain bowel resections or if you're very malnourished, your doctor will want to make sure that you're not running low on certain vitamins and nutrients, such as B12, iron and vitamin D.

Additionally, a common infection that is associated with inflammatory bowel disease (IBD) is called Clostridium difficile. This infection often mimics an inflammatory bowel disease flare. Doctors will test your stool to determine which is actually causing your symptoms at the time—Clostridium difficile or an IBD flare. Another stool study for IBD, fecal calprotectin, can give us clues regarding your degree of disease activity.

Healthcare professionals use a symptom checklist called the Rome Criteria to diagnose irritable bowel syndrome (IBS). It requires that people have at least three months of recurrent abdominal pain or discomfort along with two or more of the following:

Abdominal pain or discomfort that is:

  • relieved with defecation
  • associated with a change in frequency of stool
  • associated with a change in form (appearance) of stool

When diagnosing IBS, doctors will also ask if you have these other symptoms:

  • changes in stool frequency (often defined as more than three bowel movements per day or fewer than three bowel movements per week)
  • lumpy/hard or loose/watery stools
  • changes in the passage of stools (straining, urgency or feeling of incomplete evacuation)
  • passage of mucus
  • bloating or feeling of abdominal distension

In addition to taking a complete medical history that includes a careful description of symptoms, your doctor may do one or more of the following:

  • Order lab tests
  • Order a flexible sigmoidoscopy or, for older patients, a colonoscopy
  • Conduct a pelvic exam to rule out ovarian tumors and cysts or endometriosis, which may cause symptoms similar to IBS
  • Test for lactose intolerance
  • Test a stool sample for signs of bleeding
  • Order an imaging test of the bowel and abdomen, such as a computerized tomography (CT) scan or small bowel series
  • Test for gluten allergy, called celiac disease

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Irritable bowel syndrome, or IBS, can be diagnosed by criteria that were established by a panel of experts which are called the Rome III Diagnostic Criteria. IBS is defined as repeat stomach pain or discomfort for at least 3 days in a month for the last 3 months. And there should be two or more associated symptoms, which include feeling better with a bowel movement, changes in the number of times you have a bowel movement and a change in the appearance of the bowel movement.

Dr. Philip E. Tanner, MD

Irritable bowel syndrome (IBS) can be diagnosed based on symptoms. If diarrhea is the predominant symptom, a celiac panel will be ordered to rule out celiac sprue, an autoimmune disease. A complete blood count will be used to rule out acute infection, and a sedimentation rate will be used to check for inflammation in the body. Stool studies to check for blood in the stool, parasites, ova, and infection will also be ordered. Sometimes, thyroid function tests and colonoscopy will also be ordered. IBS is largely a diagnosis of exclusion, and treatment is aimed at relieving symptoms and reducing flares.

Dr. Dawn Marcus

Most people with symptoms of irritable bowel syndrome (IBS), which causes chronic stomach pain, bloating, and problems with diarrhea and constipation, need an evaluation that includes an analysis of stool and blood samples, and a sigmoidoscopy—which is a test in which your doctor inserts a small flexible tube (about the width of a finger) with a small video camera into your rectum to look into the end of your colon. People 50 years and older will probably need to have a more complete examination of the colon, called a colonoscopy. This also uses a narrow, flexible tube with a camera to see the entire length of the colon.

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Because there are no specific tests for irritable bowel syndrome (IBS), the illness must be diagnosed based on symptoms and by process of elimination, sometimes with the use of tests for other conditions.

The doctor takes a complete medical history, including a careful description of your symptoms. A physical exam and some routine laboratory tests are likely to be part of the exam, and a stool sample is useful for evidence of bleeding. The doctor will also ask whether your symptoms started after an episode of gastroenteritis, or if they seem to be triggered by specific foods or medications, particularly milk products (to rule out lactose intolerance) and foods and beverages that contain fructose or sorbitol. You may need to keep a food diary for a few weeks to help identify foods that provoke symptoms.

It's especially important to consider emotional and psychological triggers. The doctor will want to know what prompted the visit and will ask about your lifestyle and stress level. It's not unusual for a traumatic life event such as divorce or the loss of a job to wreak havoc on the bowels and the psyche.

Other symptoms that accompany the pain may offer clues. If there is pain in the lower abdomen and a change in bowel movements, an abnormality in the large intestine may be present. A combination of abdominal pain and fever can signal inflammation (for example, diverticulitis), which requires immediate medical attention.

Another major diagnostic clue is bleeding from the digestive tract. People with IBS can have rectal bleeding, but IBS does not cause bleeding. Instead, bleeding reflects another cause, such as hemorrhoids. Bright red blood generally comes from the lower digestive tract, while black, tarry blood generally comes from the upper gastrointestinal (GI) tract. If there is bleeding, more tests must be performed to determine the cause.

During the physical exam, the physician will look for tenderness in the abdomen. If the tenderness is located in the lower right part, it may signal ileitis or appendicitis, and in the upper right part, gallstones and inflammation of the gallbladder. The doctor will also check for a mass, which might be a tumor, a large cyst, or impacted stool. If the patient has IBS, the physical exam will usually not reveal anything other than perhaps a mildly tender abdomen. And lab tests are generally normal in IBS patients.

IBS can be diagnosed clinically based on comprehensive history and complete physical examination. It is the clinical diagnosis made when abdominal pain and altered bowel habits are reported and there are no “alarm” features. The alarm features are history of weight loss, blood in stool, anemia and significant family history for inflammatory bowel disease, celiac disease or colorectal cancer. IBS is not a diagnosis of exclusion. The Rome III criteria are used to diagnose IBS—Recurrent abdominal pain or discomfort at least 3 days per month in the last 3 months associated with 2 or more of the following: a) improvement with defecation b) onset associated with a change in stool frequency c) onset associated with a change in stool form/consistency. Criteria should be fulfilled for the last 3 months and symptom onset should have been at least 6 months prior to diagnosis. Based on the prominent stool type, IBS is sub classified as IBS-D (with diarrhea); IBS-C (with constipation); IBS-M (with occurrence of both loose and hard stools); IBS-U (undefined, neither loose nor hard stools). 

Irritable bowel syndrome (IBS) is diagnosed clinically, which means that it is diagnosed by symptoms and not a specific test. Doctors usually use the "Rome III criteria": Recurrent abdominal pain or discomfort for at least three days per month for the last three months AND at least two of the following:

  • Improvement of the discomfort with a bowel movement
  • Bowel movements become either more or less frequent when the discomfort starts
  • The stool is changed in appearance or form (harder or softer) when the discomfort starts

It is important to always see your doctor if you suspect you have IBS, so they can make sure it is nothing more serious first.

The diagnosis is usually made based on historical symptoms and ruling out other gastrointestinal disorders. Symptoms of alternating constipation/diarrhea are usually primarily constipation with eventual passage of substantial amounts of stool from the colon, and cramping and bloating.

Dr. William B. Salt, MD

The diagnosis of irritable bowel syndrome (IBS) is based upon:

  1. The identification of certain symptoms (positive symptom diagnosis)
  2. Determination of the predominant symptom pattern (alternating diarrhea and constipation, mostly diarrhea or mostly constipation)
  3. The absence of "red flags" that signal the possibility of another disease masquerading as IBS
  4. Identification of certain key historical features (history of abuse, persistent symptoms following the acute onset of gastroenteritis and associated anxiety or depression)
  5. Recognition of associated functional symptoms and syndromes
  6. Clues in the family history
  7. Consideration of differential diagnosis
  8. The judicious application of appropriate medical testing
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The road to a diagnosis of irritable bowel syndrome (IBS) can be an arduous journey. Doctors will want to rule out other GI disorders such as parasites, gluten sensitivity, small intestine diseases, and lactose intolerance. A diagnosis of IBS is made when people experience:

  • At least 3 months of abdominal pain or cramping that is present at least 25 percent of the time
  • A noticeable difference in consistency and frequency of stool that can be hard, loose, watery, or poorly formed
  • A feeling of incomplete evacuation or the need to strain
  • A sense of urgency
  • Mucous in stool
  • Bloating or a feeling of abdominal distention.

To diagnose irritable bowel syndrome, your doctor will conduct a number of medical tests, including a physical examination, to help rule out other conditions such as polyps, inflammation, food intolerance or allergies and celiac disease. Specific symptom guidelines, called the Rome III criteria, help physicians assess and treat patients.

If you have some of the characteristic symptoms of irritable bowel syndrome, such as mucus in your stool and bloating, your doctor will first run certain diagnostic tests on you to rule out other disorders. If your results are negative, your doctor may diagnose you with irritable bowel syndrome if your symptoms have the following history and features:

1. You've had abdominal discomfort for at least 12 consecutive or nonconsecutive weeks out of the last year.

2. Your abdominal discomfort has two of these three characteristics:

  • When the discomfort starts, your bowel movement frequency changes.
  • When the discomfort starts, the appearance or form of your bowel movement changes.
  • Your discomfort is relieved when you have a bowel movement.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.