Small Intestine: Special Tests

By | 2011-08-02

In cases of malabsorption, intestinal biopsy is usually necessary to obtain a definitive diagnosis. However, exocrine pancreatic insufficiency should be ruled out before biopsy, because signs of malabsorption are nonspecific. It is also well recognized that biopsies from up to 50% of patients are considered normal by light microscopy. Therefore, usually before biopsy, a number of indirect tests are performed to assess for intestinal damage, altered permeability, and dysfunction.

Diagnosis of Exocrine Pancreatic Insufficiency

Because the signs of exocrine pancreatic insufficiency cannot be distinguished from primary causes of small intestine, serum trypsin-like immunoreactivity (TLI) measurement must be performed in all cases.

Serum Folate and Cobalamin Concentrations

The assay of serum folate and cobalamin concentrations can be performed on the same serum sample taken for the trypsin-like immunoreactivity test. This assay has limited value in the diagnosis of small intestine diseases, but subnormal folate and cobalamin concentrations secondary to gastrointestinal disease may be detected. Cobalamin deficiency is more common, and the response to treatment of the underlying gastrointestinal disease may be suboptimal if this vitamin deficiency is not corrected. In dogs, the severe cobalamin deficiency recognized in several breeds has been linked to an IF-cobalamin receptor deficiency. Cobalamin deficiency is a common sequel to small intestinal disease in cats, and systemic metabolic consequences have been recognized. Determination of the serum folate and cobalamin concentrations is not recommended for the diagnosis of canine SIBO.

Indirect Assessment of Intestinal Absorption

Attempts to assess intestinal function by measuring the mediated absorption of numerous substrates (e. g., lactose, glucose, vitamin A, D-xylose, triglyceride, and starch tolerance tests) are no longer performed because of a similar lack of sensitivity and specificity.

Xylose/3-O-methyl-D-glucose test The differential absorption of these two sugars eliminates the nonmucosal effects that blight the xylose test, and initial results suggest that the test may be of value in dogs and cats.

Intestinal Permeability

Intestinal permeability is an index of mucosal integrity and is assessed by measuring unmediated uptake of nondigestible probe markers. Tests use a nonmetabolizable probe marker that is excreted in the urine. The permeability probe chromium-51-labeled ethylenediamine tetraacetic acid (51Cr-EDTA) was used in original studies, but the need for a gamma-emitter limited its safe use.

Errors related to nonmucosal factors (including the gastric emptying rate, intestinal transit time, and completeness of urine collection) can be eliminated by concurrently measuring the absorption of two probes with different pathways of absorption. Calculation of their excretion ratio eliminates errors from extramucosal factors because both probes should be affected equally. The ratio, which is altered by villus atrophy or epithelial damage or both, offers a simple, sensitive diagnostic test.

A 5-hour urine collection is performed after oral administration of two sugars. A number of candidates can be used for the probe molecules, and a mixture of one large simple sugar (e. g., lactulose, cellobiose, raffinose) and one small one (e. g., rhamnose, arabinose, mannitol) can be chosen. The cellobiose / mannitol excretion ratio and lactulose / mannitol ratio have been used in companion animals, but with advances in the high-performance liquid chromatography (HPLC) assay of these sugars, the Iactulose / rhamnose test has become the standard test of small intestine permeability.

Tests for Protein-Losing Enteropathy

Historically, intestinal protein loss has been detected by measuring the fecal loss of 51Cr-labeled albumin. The test is unpleasant to perform and potentially hazardous and has largely been discarded, although it remains the standard by which other tests, such as the assay of fecal alpha1-PI, are judged.

Breath Tests

Breath tests are used to assess bacterial metabolism in the gastrointestinal tract. Bacteria synthesize a gas, which is absorbed and excreted in breath. Breath hydrogen tests have been used most extensively because mammalian cells cannot produce hydrogen, and therefore any that is measured must be bacterial in origin. Such tests can assess carbohydrate malabsorption, bacterial colonization of the small intestine, and oro-cecal transit time.

A variety of protocols has been used, including xylose to assess malabsorption, lactulose to assess orocecal transit, and a test meal to assess small intestine bacterial fermentation. Also, a number of recent studies have attempted to standardize the techniques for companion animals. However, these techniques are not widely used even in referral centers.

Unconjugated Bile Salts

The principle of the serum conjugated bile acids test is that conjugated bile salts secreted into the intestine in bile are deconjugated by certain bacterial species and absorbed by the small intestine. Therefore, in theory, increases in small intestine bacterial numbers might result in an increase in serum unconjugated bile acids (SUBA). Preliminary work suggested that the test was sensitive and specific for canine SIBO, although a recent study has questioned its utility.

Miscellaneous Tests

A number of tests for intestinal bacterial metabolites have been devised to detect SIBO. These include the nitrosonaphthol test, urinary indican excretion, bacterial release of sulfapyridine from sulfasalazine, and bacterial release of p-amino benzoic acid (PABA) from a bile salt conjugate (PABA-UDCA). However, none of these tests are widely used in companion animals.