CLINICAL TESTS OF DIGESTION AND ABSORPTION



A large number of tests of varying degrees of complexity and specificity are potentially useful in the study of patients suspected of having ab­normalities of digestion and/or absorption. Some of the most useful of these tests will be described briefly.

Fecal Fat Analysis. As a screening measure a qualitative test for stool fat using a Sudan III stained smear is useful in the detection of stea­torrhea. The standard, however, is to measure quantitatively the amount of fat in a three-day stool specimen after the patient has ingested a diet containing 80 to 100 grams of fat daily. Normal fat excretion in these circumstances is less than 6 grams per 24 hours (usually 2.5 grams). Values higher than this reliably indicate fat malabsorp­tion (steatorrhea) but do not indicate the patho­genesis of the abnormality.

Triolein Breath Test. This test using 14C-tri-olein reflects the rate of absorption of this radio­active triglyceride rather than the amount not ab­sorbed, but otherwise its use is comparable to that of quantitative fecal fat analysis. Expired 14C02 is measured hourly for six hours, and normally more than 3.5 per cent of the radioactivity administered as the triolein is recovered under these condi­tions.

Xy lose Absorption-excretion Test. The absorp­tion of D-xylose, a poorly metabolized 5-carbon sugar, reflects the integrity of carbohydrate ab­sorption by the proximal small intestine. Since no enzymatic digestion or solubilization is required prior to absorption, the rate of absorption is un­affected by pancreatic or hepatic (biliary) disease and is a more selective test of mucosal integrity. Usually 25 grams of D-xylose are given orally to a well-hydrated subject, and the amount of the sugar excreted in the subsequent five-hour urine is measured. Normal values exceed 4.5 grams per 5 hours, but this figure may be reduced by age, decreased renal function, excessive body fluid (ascites, edema), or intestinal bacterial over­growth. In the latter case the test should return to normal with the use of antibiotics. As a variation the plasma xylose can be measured one hour after xylose is ingested and should exceed 30 mg/dl.

Radiographic Studies. All patients with mal­absorption should have radiographic studies of the stomach and small intestine. Often the find­ings are nonspecific, with thickening of the mu­cosal folds, modest dilatation of the intestinal lumen, and clumping and segmentation of the bar­ium in a so-called moulage pattern. Occasionally the study may be diagnostic with the demonstra­tion of a stricture, a fistula, or a “blind-loop” (e.g., a small bowel diverticulum). The findings may guide the use of other studies such as biopsy.

Smallintestinal (Jejunal) Biopsy. Peroral bi­opsy of the small intestinal mucosa is frequently indicated in the investigation of malabsorption. summarizes the conditions in which the biopsy is likely to be diagnostic and those in which it is often abnormal but in a nonspecific way.

Vitamin Bu Absorption (the Schilling Test). Vitamin BJ2 (cobalamin) is selectively absorbed in association with intrinsic factor in the distal ileum. An abnormality in its absorption usually represents one or more of four abnormalities:
(a)disease of the distal ileum (e.g., Crohn’s disease), (b] deficiency of intrinsic factor, (c) deficient pan­creatic exocrine function (pancreatic trypsin is necessary to release cobalamin from a binding “R-protein” of gastric juice so that it is free to com­bine with intrinsic factor), and (d) increased uti­lization of cobalamin during transit (e.g., bacterial overgrowth). In the usual form of the test, radio­active vitamin B12 is given orally either alone, with intrinsic factor, or after therapy with a broad-spectrum antibiotic in suspected bacterial over­growth, and its urinary excretion is measured over 24 hours. A flushing dose of 1.0 mg vitamin B12 is given simultaneously to prevent its hepatic storage and to enhance excretion. Normally more than 7 per cent of the orally administered radio­active vitamin B12 will be recovered in the urine. The pattern of abnormalities found in the sequential Schilling test may suggest (a) all three stages abnormal—intrinsic intestinal disease or pancreatic exocrine insufficiency; (b) stage 1 ab­normal, stage 2 normal—intrinsic factor defi­ciency (pernicious anemia); (c) stages 1 and 2 ab­normal, stage 3 normal—bacterial overgrowth.
Other Breath Tests. Several breath tests that as­sess abnormal rates of intraluminal bacterial me­tabolism of specific compounds have been de­veloped. In the bile acid breath test, a 14C-glycine conjugate of a bile acid is given orally and the rate of expired 14C02 is measured. Normally most bile salts are recycled intact in an enterohepatic cir­culation involving ileal reabsorption. Rapid me­tabolism of the conjugated glycine to C02 reflects either bacterial overgrowth within the small in­testine or a blind loop from it, while a delayed but increased production of 14C02 indicates ileal dysfunction with increased spillage of the bile salt into the colon. In an analogous fashion, intralum­inal bacterial metabolism of sugars releases free hydrogen, which can be measured in expired air. Most frequently this test is used to assess specific disorders of carbohydrate digestion and absorp­tion, such as lactase deficiency, in which expired H2 is measured after the ingestion of 50 grams of lactose.

Miscellaneous Tests. There are many other im­portant although nonspecific tests that may reflect malabsorption, such as body weight, serum al­bumin, the prothrombin time (absorption of the fatsoluble vitamin K), and serum levels of cho­lesterol, carotene, folic acid, calcium, and mag­nesium. All of these tests measure only the results of malabsorption and are, in general, of little help in its differential diagnosis.