First, obtaining a patient's medical history is important because it directs the evaluation.

• If the bloating/distention is continuous rather than intermittent, then enlargement of abdominal organs, abdominal fluid, tumors, or obesity are probable causes.
• If the bloating/distention is associated with increased flatulence, then bacteria and excessive gas production are likely factors.
• If a diet history reveals the consumption of large amounts of milk or dairy products (lactose), sorbitol or fructose, then the maldigestion and malabsorption of these sugars may be the cause of the distention.
• When individuals complain of flatulence, it may be useful for them to count the number of times they pass gas for several days. This count can confirm the presence of excessive flatulence since the number of times gas is passed correlates well with the total amount (volume) of passed gas. It is normal to pass gas up to 20 times a day.
• If an individual complains of excessive gas but passes gas fewer than 20 times per day, the problem is likely to be something other than too much gas. For example, the problem may be the foul odor of the gas (often due to ingestion of sulfur-containing foods), the lack of ability to control the passing of gas, or the soiling of underwear with small amounts of stool when passing gas. 

Simple abdominal X-rays

Simple X-rays of the abdomen, particularly if they are taken during an episode of bloating or distention, can often confirm air as the cause of the distention since large amounts of air can be seen easily within the stomach and intestine. Moreover, the cause of the problem may be suggested by noting where the gas has accumulated.

Small intestinal X-rays

X-rays of the small intestine, in which barium is used to fill and outline the small intestine, are particularly useful for determining if there is an obstruction of the small intestine.

Gastric emptying studies

These studies measure the ability of the stomach to empty its contents. For gastric emptying studies, a test meal that is labeled with a radioactive substance is eaten and a Geiger counter-like device is placed over the abdomen to measure how rapidly the test meal empties from the stomach. A delay in emptying of the radioactivity from the stomach can be caused by any condition that reduces emptying of the stomach (for example, pyloric stenosis, gastroparesis).

Ultrasound, CT scan, and MRI

Imaging studies, including ultrasound examination, computerized tomography (CT), and magnetic resonance imaging (MRI), are particularly useful in defining the cause of distention that is due to enlargement of the abdominal organs, abdominal fluid, and tumor.

Maldigestion and malabsorption tests

Two types of tests are used to diagnose maldigestion and malabsorption; general tests and specific tests.

The best general test is a 72-hour collection of stool that measures fat in the stool; if maldigestion and/or malabsorption exist because of pancreatic insufficiency or diseases of the lining of the small intestine for example, celiac disease), the amount of fat in the stool will increase.

Specific tests can be done for maldigestion of individual sugars that are commonly maldigested, including lactose and sorbitol (a sweetener in low calorie foods). 

Hydrogen/methane breath tests

The most convenient way to test for bacterial overgrowth of the small intestine is hydrogen/methane breath testing. Normally, the gas produced by the bacteria of the colon is composed of hydrogen and/or methane. For hydrogen/methane breath testing, a non-digestible sugar, lactulose, is consumed. At regular intervals following ingestion, samples of breath are taken for analysis. When the lactulose reaches the colon, the bacteria form hydrogen and/or methane. Some of the hydrogen or methane is absorbed into the blood and eliminated in the breath where it can be measured in the samples of breath.

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