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Diagnosing Irritable Bowel Syndrome

Irritable bowel syndrome, or IBS, is a chronic condition that affects the large intestine, or colon. NYU Langone gastroenterologists are experts in diagnosing the condition. 

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The large intestine is the section of the gastrointestinal tract that helps the body absorb water and convert indigestible food into stool before it exits the body. People with IBS may be more aware of the normal contractions that move food through the intestine, a process called peristalsis. This heightened sensitivity can result in symptoms, such as abdominal pain, cramping, and diarrhea. 

Other symptoms of IBS can include constipation, gas, and bloating. Many people experience these symptoms occasionally, but if they occur at least three days per month for three months or longer, IBS may be the cause. 

IBS symptoms can vary from person to person. Physicians categorize IBS into three main types: type D, in which a person’s predominant symptom is diarrhea; type C, in which the main symptom is constipation; and type M, in which bouts of diarrhea and constipation alternate. 

IBS is not contagious or life threatening. However, once symptoms begin, they typically persist for years and can have a significant impact on your quality of life. Our doctors can provide treatment to help minimize these symptoms.

The causes of IBS are unclear, although many people develop symptoms after eating certain foods or experiencing emotional distress or anxiety. IBS flare-ups can also be triggered by recent viral or bacterial infections in the gastrointestinal tract, which can lead to an imbalance of  “good” and “bad” bacteria in the intestine. 

Though the two conditions are commonly mistaken for one another, IBS is not a form of inflammatory bowel disease, a group of disorders that includes Crohn’s disease and ulcerative colitis. Inflammatory bowel disease, or IBD, is an autoimmune disease that causes inflammation in the intestines. This is not the case in people with IBS. However, some of the symptoms of irritable bowel syndrome and inflammatory bowel disease are similar, including abdominal pain and irregular bowel habits. If you see blood in your stool or if diagnostic tests reveal inflammation in the digestive tract, you probably have a different condition from IBS. 

There is no single test to confirm a diagnosis of IBS. A gastroenterologist at NYU Langone may diagnose the condition by recognizing a pattern of symptoms based on a detailed medical history and physical exam. 

Medical History and Physical Exam

When taking a medical history, your NYU Langone physician may ask detailed questions about your bowel habits and whether you have a family history of digestive disorders. He or she may ask whether you have traveled recently, whether you have taken antibiotics recently, and what foods you eat regularly. These details may help your physician determine whether an external trigger is causing symptoms. 

You should tell your physician if you have any other conditions and what medications, vitamins, and supplements you take. It is also very important to give your physician a copy of the results of all tests and procedures done under the care of another health care provider.

A physician also performs a thorough physical exam, which includes a rectal exam. He or she gently inserts a gloved finger into the anus. This can help your physician determine whether the muscles of the rectum—the lower part of the large intestine, where stool is stored before it passes through the anus—are functioning normally. Your physician also can assess any tenderness in the area and identify any unusual masses or sores. If there is stool in the rectum, your physician can assess its consistency and whether there is any blood in it. 

Your physician may advise making dietary changes or taking medication before recommending further testing. If these approaches fail to relieve your symptoms, your doctor may perform one or more tests to rule out other conditions that cause similar symptoms. 

Blood Tests

Your physician may recommend one or more blood tests to look for signs of infection, inflammation, or anemia, in which there is a decrease in the number of red blood cells in the bloodstream. Although a blood test can’t confirm that IBS is causing your symptoms, it can help the physician determine whether another condition is responsible for your digestive complaints. 

Based on your symptoms, diet, and medical and travel history, your physician may suggest a blood test for celiac disease. This is an autoimmune condition in which the body’s immune system has an abnormal response to gluten, a protein found in wheat, barley, and rye. 

Your doctor may also order a blood test to check for evidence of iron or vitamin deficiencies, which may indicate that certain nutrients aren’t being absorbed during digestion. These deficiencies can occur when the lining of the intestines is inflamed, as with inflammatory bowel disease. Nutritional deficiencies are not usually seen in IBS. 

Your doctor may also test your blood for evidence of thyroid dysfunction. These conditions can cause symptoms that resemble those of IBS. 

Stool Test 

Your physician may ask you to submit a sample of your stool for laboratory testing, which can reveal the presence of bacteria, parasites, or blood. The results of a stool test may also indicate elevated levels of proteins, such as calprotectin or lactoferrin, which may point to an infection or inflammation in the digestive tract. If a stool test detects inflammation or blood, a condition other than IBS may be causing your symptoms.  

Hydrogen Breath Test

A hydrogen breath test detects the presence of hydrogen and methane in exhaled air. These gases are often elevated in people who have trouble digesting certain foods. A breath test is useful if your physician suspects that you have lactose or fructose intolerance, or a condition called small intestinal bacterial overgrowth. 

Lactose intolerance is caused by low levels of the enzyme lactase, which is needed to digest lactose, a sugar found in milk and other dairy products. In fructose intolerance, drinks like fruit juices and foods like watermelon, honey, and apples may be difficult to tolerate. 

Small intestinal bacterial overgrowth is the result of too much bacteria in the small intestine. This can lead to abdominal bloating or fullness, frequent or loose bowel movements, and sometimes signs of malabsorption, such as nutritional deficiencies and weight loss. 

A diagnosis of lactose intolerance or small intestinal bacterial overgrowth may help your physician understand the source of your symptoms—which can be similar to those of IBS—and recommend treatment. It is important to note that people with IBS can also have lactose and fructose intolerance, as well as small intestinal bacterial overgrowth. Symptoms such as bloating, irregular bowel movements, and abdominal pain often overlap. 

The hydrogen breath test is noninvasive and painless. However, it may take up to four hours to complete. Your doctor asks you to avoid eating or drinking anything other than water for 12 hours before the test and to follow a specific diet for 24 hours beforehand. You cannot smoke or exercise within one hour of the test. You may also be asked to refrain from taking certain medications or supplements, like probiotics, on the day prior to testing.

During the test, you exhale into a balloon-like bag. Then you drink a solution containing small amounts of different sugars, including lactose and fructose. Over the next few hours, you breathe into the bag every 15 minutes. A gastroenterologist analyzes the breath samples to determine whether your body is having trouble digesting these sugars. 


If the results of blood or stool tests suggest inflammation in the large intestine, your physician may recommend a colonoscopy, a procedure used to visualize the large intestine. 

Depending on your symptoms, your doctor may also recommend a colonoscopy to rule out a large polyp—a fleshy sac that can form on the colon wall—or colorectal cancer. These conditions may cause abdominal pain, a change in bowel habits, a partial blockage in the intestine, anemia, or other symptoms. 

To perform a colonoscopy, a gastroenterologist inserts a thin, flexible instrument with a camera and a light on one end into the anus and maneuvers it through the large intestine. The camera broadcasts images of the intestine to a computer monitor, enabling the physician to identify any signs of damage and to check for any unusual growths. A physician may also perform a biopsy during a colonoscopy, in which he or she removes a tiny piece of intestinal tissue and sends it to a lab for further testing. 

If you are 50 or older, even if you have no symptoms at all, NYU Langone doctors recommend a colonoscopy to screen for colon cancer. Those who have certain risk factors—like inflammatory bowel disease or a family history of colorectal polyps or cancer—should start screening at a younger age.

Imaging Tests

If the results of blood and stool tests point to a diagnosis other than IBS, your doctor may recommend imaging tests. Although they cannot confirm a diagnosis of IBS, they enable your doctor to view the gastrointestinal tract, including the abdominal organs. The results of imaging tests may indicate that a condition other than IBS is causing your symptoms.

At NYU Langone, radiologists who specialize in gastroenterological imaging interpret these tests. They have the experience to identify any gastrointestinal problems that may appear on imaging tests. 


X-rays use electromagnetic radiation to take pictures of structures inside the body. If you experience frequent constipation, your physician may recommend an X-ray to find out whether any stool is impacted in your intestines and to check for any blockages. 

CT Enterography 

CT enterography is an enhanced CT scan that creates three-dimensional, cross-sectional images of the small intestine and other structures within the abdomen. One hour before the test, you drink a contrast agent, which highlights the intestinal lining. 

A doctor may recommend CT enterography if the results of blood tests or a colonoscopy indicate intestinal inflammation and he or she suspect you may have inflammatory bowel disease. 

MR Enterography 

An MRI scan uses a magnetic field and radio waves to create computerized, three-dimensional images of structures in your body. Like CT enterography, MR enterography uses a contrast agent to enhance images of the intestines. The technician gives you the contrast agent to drink before the scan. 

Your physician may recommend this test if a colonoscopy or endoscopy suggests that there may be inflammation or thickness in the lining of the intestines. MR enterography is also useful in establishing the size of the affected area of the intestine.

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