Diagnosing malabsorption involves confirming that it is occurring as well as identifying the cause and type. Some patients may not have symptoms or may have symptoms that indicate other conditions. Therefore, tests are necessary for diagnosis.

A physical examination can be used to identify signs of malabsorption or any other condition that may be causing the symptoms. A medical history may include discussion of symptoms, eating habits, alcohol use, current medications, family history of disease and any recent surgeries or foreign travel. In general, patients exhibiting signs of unexplained weight loss, steatorrhea (fat buildup in stool) and anemia may be tested for malabsorption.  

Blood tests will also be performed to determine whether a patient is anemic or lacking in certain nutrients (e.g., low vitamin D may point toward fat malabsorption). These preliminary tests include:

• Complete blood count (CBC). Includes measurement of the number of white and red blood cells. This test is used to identify signs of infection, anemia and various nutrient deficiencies (e.g., vitamin B-12, which is important for digestion).
  
  
• Chem20. Measures the levels of 20 different chemical substances (e.g., protein, glucose, calcium) released from various tissues in the body. Abnormal levels may indicate problems with the tissues that secrete the substance.

Blood tests are typically followed by fecal tests in which stool samples are analyzed in a laboratory. A 72-hour fecal fat measurement requires the collection of a patient’s entire stool output over 72 hours. Then the amount of fat in stool is measured. This is the most reliable test for diagnosing fat malabsorption.

The next test typically performed is the D-xylose absorption test, which assesses whether malabsorption is a problem of absorption (involving the intestinal lining) or digestion (e.g., involving pancreatic insufficiency) within the small intestine. During this test, a patient is given an oral dose of D-xylose (a type of sugar). D-xylose can be absorbed without pancreatic enzymes, but is not used by the body, so it typically exits the body in urine. Urine samples are analyzed for traces of D-xylose.

If D-xylose levels are abnormal, the physician will typically perform an endoscopy. This test allows the physician to view the patient’s digestive tract via a small, flexible tube (with a light and camera) that is inserted through either the mouth or anus. Photos and tissue samples (small bowel biopsy) can be taken during this procedure. Biopsy may identify many different disorders, such as celiac disease, tropical sprue, bacterial overgrowth and inflammatory bowel disease (e.g., Crohn's disease).

If the biopsy is normal, additional tests, such as breath tests, which measure the amount of a particular substance in the breath, may be performed. Common breath tests may include:

• Hydrogen breath test. A test for lactose intolerance. This test measures breath samples for hydrogen after ingestion of a drink that contains high levels of lactose. The body makes excess hydrogen when lactose is not broken down properly in the small intestine. Thus, too much hydrogen in the breath may indicate a failure to absorb lactose.
  
  
• Bile salt breath test. Measures the amount of carbon dioxide in breath after oral administration of bile salt. This measures how well digestion is occurring, and may be able to identify whether malabsorption is the result of liver disease or bacterial overgrowth in the small intestine. This test indicates malabsorption of fats and fat soluble vitamins.
  
  
• Triolein breath test. A test of fat malabsorption. This test measures the level of carbon dioxide in breath after ingestion of a drink containing 14-C triglycerides (a fat-like compound). The test measures a specific type of carbon dioxide that the body normally produces when it digests fats. However, lung disease, thyroid disorders, obesity and other conditions may interfere with test results. The effectiveness of this test in diagnosing malabsorption is not clear.

The Schilling test for B12 malabsorption may also be performed. However, this three-stage urine test that involves the administration of oral and intravenous (into a vein) doses of vitamin B-12 followed by a urine analysis, is becoming increasingly rare.

Other tests that may be used to identify the cause of malabsorption include:

• Barium x-ray. X-rays that are taken after a patient has received a chalky substance (barium) that allows easy viewing of internal organs on x-ray. These x-rays can help identify abnormalities within the digestive tract, such as obstruction, tumors and thickening of the intestinal lining due to inflammation. For an upper gastrointestinal (GI) series, patients drink the barium solution before testing. For a lower GI series, barium is introduced rectally (by enema) prior to testing.
  
  
• Computed axial tomography (CAT). Simultaneous x-rays taken from different angles. A CAT scan allows a physician to analyze the condition of the intestines as well as the pancreas and lymph nodes.
  
  
• Pancreatic function tests. Used to identify abnormalities in the functioning of the pancreas, which can lead to malabsorption. These tests include:
  
  
• • Secretin stimulation test (SST). Test in which a tube is inserted through a patient’s nose into the digestive tract until it reaches the upper portion of the small intestine. Then a hormone called secretin (which normally indicates the presence of food in the small intestine) is released into the area through the tube. Intestinal secretions produced in response to the hormone are removed for examination. This test measures the ability of the pancreas to respond when food particles move into the small intestine from the stomach.
    
    
  • Lundh test. Similar to the SST, except that intestinal secretions are stimulated by a liquid meal containing carbohydrates, proteins and fat. This test is not considered as sensitive as the SST.