malabsorption test

malabsorption test, any of a group of noninvasive medical procedures used to diagnose abnormalities associated with poor absorption of nutrients. Malabsorption of nutrients can result from surgical alterations or physiological disturbances of the gastrointestinal tract. For example, the removal of a significant portion of the bowel can cause a malabsorption condition known as short-bowel syndrome. In addition, diffuse mucosal disease, such as tropical sprue, can interfere with absorption, and diseases of the liver or pancreas may prevent digestive enzymes from reaching the intestines. Bacterial overgrowth in the intestines can interfere with glucose absorption, and the stomach’s failure to produce a substance called intrinsic factor will prevent the absorption of vitamin B₁₂ (cobalamin), which leads to pernicious anemia.

Persons who have a low serum vitamin B₁₂ level and who are suspected of having pernicious anemia usually are required to undergo the Schilling test. Radioactive vitamin B₁₂ is administered orally, and the amount excreted in the urine over the next 24 hours is measured. Malabsorption is confirmed if less than 8 percent of the vitamin B₁₂ is excreted in the urine.

Steatorrhea is the excretion of an excessive amount of fat in the stool, which is diagnostic of fat malabsorption when the amount of fat in the diet is normal. Stool specimens are collected for three days following two days of a diet containing 100 grams of fat per day. The excretion of more than six grams of fat daily indicates fat malabsorption, which may occur in persons with pancreatic disease, in those with diffuse mucosal disease, and in those who have undergone massive small-bowel resection.

A five-carbon sugar, d-xylose, is absorbed in the duodenum and proximal jejunum. It is not metabolized and is excreted unchanged in the urine. The d-xylose absorption test measures the absorption ability of the jejunum. Lowered excretion indicates diminished intestinal absorption usually caused by a decreased absorptive surface, infiltrative intestinal disease, or bacterial overgrowth.