Fat malabsorption studies
Fat malabsorption is usually the first test obtained because many disease processes result in fat malabsorption.Instruct patients to consume a normal amount (80-100 g/d) of fat before and during the collection. Based on this intake, fecal fat excretion in healthy individuals should be less than 7 g/d.
For a quantitative measurement of fat absorption, a 72-hour fecal fat collection is often performed and is considered the criterion standard.Raman et al have suggested that a novel clinical test that uses levels of serum retinyl palmitate to identify severe cases of fat malabsorption may be useful relative to the 72-hour fecal fat test. [11]
Qualitative tests include the acid steatocrit test and Sudan III staining of stool, but these tests are less reliable.
D-xylose test
If the 72-hour fecal fat collection results demonstrate fat malabsorption, the D-xylose test is used to document the integrity of the intestinal mucosa.
Facilitated diffusion in the proximal intestine primarily absorbs D-xylose. Approximately half of the absorbed D-xylose is excreted in urine, unmetabolized. If the absorption of D-xylose is impaired due to either a luminal factor (eg, bacterial overgrowth) or a reduced or damaged mucosal surface area (eg, surgical resection, celiac disease), urinary excretion is lower than normal.
Cases of pancreatic insufficiency usually result in normal urinary excretion because the absorption of D-xylose remains intact.
Carbohydrate absorption studies
A simple sensitive test for carbohydrate malabsorption is the hydrogen breath test, in which patients are given an oral solution of lactose. [12, 13] In cases of lactase deficiency, colonic flora digest the unabsorbed lactose, resulting in an elevated hydrogen content in the expired air. Bacterial overgrowth or rapid transit also can cause an early rise in breath hydrogen, necessitating the use of glucose instead of lactose to make a diagnosis. However, 18% of patients are hydrogen nonexcretors, causing a false-negative test result.
Bile salt absorption studies
The bile salt breath test can determine the integrity of bile salt metabolism. The patient is given oral conjugated bile salt, such as glycine cholic acid with the glycine radiolabeled in the carbon position. The bile salt is deconjugated and subsequently metabolized by bacteria, leading to a radioactively labeled elevated breath carbon dioxide level in the presence of interrupted enterohepatic circulation, such as bacterial overgrowth, ileal resection, or disease.
Schilling test
Malabsorption of vitamin B12 (cobalamin) may occur as a consequence of a deficiency of intrinsic factor (eg, pernicious anemia, gastric resection), pancreatic insufficiency, bacterial overgrowth, ileal resection, or disease. The three-stage Schilling test results often can help differentiate these conditions.
13C-D-xylose breath test
Hope et al suggested that small intestinal malabsorption in chronic alcoholism may be determined by a13C-D-xylose breath test. [14] The investigators evaluated the13C-D-xylose breath test in 14 alcoholics, compared the breath test results with those of untreated celiac patients and healthy controls, and correlated the breath test findings to morphologic findings of the duodenal mucosa. [14] Their findings showed significantly reduced absorption of13C-D-xylose in the alcoholic individuals relative to the healthy controls, whereas the time curve of13C-D-xylose absorption in the alcoholics was similar in appearance to that of the untreated celiac patients. In addition, despite few light microscopic changes in the alcoholic individuals, morphologic pathology, primarily a reduced surface area of microvilli, was observed under electron microscopy in the majority of the patients. [14]
