ADDITIONAL LABORATORY TESTING

Other specific laboratory tests may provide additional information about the nature and location of intestinal disease. In some cases, a definitive diagnosis may be provided. Some of these tests are referred to as GI function tests. It is important to point out, however, that many of the essential functions of the GI tract, such as secretion, motility, and immunologic surveillance and protection, are not readily evaluated.

Folate (vitamin B₉) is a water-soluble B-group vitamin. Serum folate concentrations are influenced by dietary intake, small intestinal brush border enzyme activity, and the number and function of specific folate carriers in the proximal small intestine. Dietary deficiency of folate is extremely uncommon, and low serum folate concentrations strongly suggest proximal small intestinal mucosal disease.¹⁰ Proper sample handling is important since hemolysis causes the release of folate from red blood cells, resulting in spuriously high concentrations.

The trypsin-like immunoreactivity assay (TLI) determines serum trypsinogen and trypsin concentrations. Small amounts of these enzymes can be found in the bloodstream after synthesis by the pancreatic acinar cells. Subnormal TLI concentrations are a sensitive and specific marker for EPI.¹¹ Administration of oral pancreatic extracts does not affect serum TLI concentrations. However, cats should be fasted for 12 hours before serum collection because patients with borderline exocrine pancreatic function may have results within the normal range.

Additional testing for pancreatitis may be warranted in any cat with chronic signs of GI disease, especially if triaditis complex—concurrent inflammation of the pancreas, hepatobiliary system, and GI tract—is suspected. The feline pancreatic lipase immunoreactivity (fPLI) test has higher sensitivity and specificity when compared with abdominal ultrasonography for identifying pancreatic inflammation, although the two tests may provide complementary information.¹² Measuring serum amylase and lipase activities is no longer considered useful, and TLI concentrations are of limited value for diagnosing feline pancreatitis.

ABDOMINAL IMAGING

Abdominal radiography in cats with chronic diarrhea is usually unrewarding but may indicate a foreign body, an obstruction, an intussusception, a traumatic or congenital hernia, megacolon, or a mass lesion. An upper GI barium series may help to detect motility disorders, bowel thickening, partial obstruction, bowel torsion or displacement, strictures, or masses. A barium enema may be helpful to assess the large intestinal mucosa.

Abdominal ultrasonography is a more sensitive tool than radiography for detecting abdominal masses, intussusceptions, lymphadenopathy, and intestinal wall thickening or loss of normal intestinal layering.¹³ Abnormal organ parenchyma, masses, and lymph nodes can be aspirated or biopsied with ultrasonographic guidance. Cytologic examination of mesenteric lymph nodes cannot distinguish small cell lymphoma from normal lymph nodes. However, fungal disease, mast cell disease, and large cell lymphoma can be reliably identified with cytologic evaluation of intestinal or lymph node aspirates.

ENDOSCOPY

Depending on the results of the initial diagnostic tests, an endoscopic examination of the GI tract may be considered. This test is relatively noninvasive but does have substantial limitations. Most important, not all of the intestine can be evaluated since much of the small bowel is out of reach. An upper endoscopic examination permits visualization of the esophagus, stomach, and proximal duodenum. The distal ileum may be reached via the colon during a lower endoscopic examination, but the jejunum is inaccessible. In addition, only mucosal lesions can be identified, and focal thickening or submucosal changes will not be appreciated. Thus, endoscopy is most appropriate in patients in which diffuse inflammatory or infiltrative mucosal disease is suspected.

This diagnostic procedure requires specialized equipment and expertise. An inexperienced practitioner may have difficulty entering the duodenum or ileum, and biopsy quality may be suboptimal.¹⁴ Multiple (six to 10) mucosal biopsy samples should be collected from the stomach, duodenum, ileum, and colon for histologic examination, even if the tissue is grossly normal. Handle the samples carefully, and fix them quickly to avoid manipulation artifact or drying.