APPROACH TO THE PATIENT WJTH SUSPECTED MALDIGESTION AND/OR MALABSORPTION



In view of the large number of tests that are available to study patients suspected of having malabsorption, a logical sequence in their use is indicated in order to arrive at a diagnosis most expeditiously (Fig. 36-2]. In general the diagnosis of malabsorption of fat is best established by a quantitative 72-hour fecal fat analysis. If this is normal, the patient may still have a selective ab­normality for the absorption of carbohydrates, es­pecially if symptoms are largely those of watery diarrhea, cramps, and excessive flatus. The most frequent causes of abnormal carbohydrate absorp­tion are shown in Table 36-7. Specific tests for carbohydrate malabsorption include oral toler­ance tests with the suspected carbohydrate asso­ciated with measurement of H2 in expired air or the level of the sugar in the blood, checking also to see if the symptoms are reproduced. Typically the feces have a low pH (<6) and there is a sig­nificant fecal osmotic gap due to the high con­centration of fecal short chain fatty acids pro­duced by colonic bacteria from unabsorbed carbohydrates.

If malabsorption for fat is demonstrated (>6 grams/24 hours in the stool), a next logical step would be an upper gastrointestinal radiographic study, including the small bowel, and a xylose absorption-excretion test. A normal xylose test makes a diffuse mucosal abnormality highly un­likely and suggests an abnormality in digestion, such as pancreatic deficiency or deficiency of bile salts. A radiographic study may demonstrate either diffuse or segmental disease, including fis­tulas, blind loops, or other areas of stasis. A truly abnormal xylose test (see description of the test for its limitations) may be caused by bacterial ov­ergrowth with direct metabolism of the sugar, but more frequently it is caused by some form of diffuse mucosal abnormality. If the latter is sus­pected, a peroral jejunal biopsy should be per­formed. The approach to the patient suspected of having bacterial overgrowth is described later in this section.

This sequence of tests is usually sufficient to establish the cause of malabsorption, although other tests, such as the response to a gluten-free diet for celiac disease or the study of pancreatic exocrine function, may also be indicated.