Clinical assessment of patients with chronic or severe gastrointestinal (GI) disease may include evaluation of fecal samples, CBC, biochemical and thyroid profiles, urinalysis, imaging studies for masses and foreign bodies, endoscopy, and biopsy. Fecal analysis includes direct smear, fecal flotation, zinc sulfate centrifugation, Clostridium perfringens enterotoxin assay, fecal cytology, giardia testing, cryptosporidium testing, fecal occult blood test, and sometimes fecal culture.

These testing procedures can be complemented by several more direct GI absorption and function tests to assist in the diagnosis and treatment of GI disease.

The TLI test detects trypsinogen that leaks into the circulation from the pancreas and can be used in the diagnosis of both exocrine pancreatic insufficiency (EPI) and pancreatitis in dogs and cats. TLI is of test of choice (sensitive and specific) for diagnosing EPI; affected animals have low TLI concentrations. An 8 to 12-hour fast is required prior to sampling, and exocrine pancreatic supplements should be discontinued for 24 hours prior to testing. Evaluation of fecal fat, starch, and muscle content is neither sensitive nor specific for making a diagnosis of EPI.

Pancreatitis can be a difficult diagnosis to confirm. Amylase and lipase are not reliable tests for ruling pancreatitis in or out. In dogs with severe pancreatitis, amylase and lipase are elevated in only 50 to 60% of cases. Furthermore, amylase and lipase can be increased in diseases other than pancreatitis, including gastritis, intestinal obstruction, liver disease, and renal failure, and subsequent to corticosteroid administration. Amylase and lipase are considered even less useful in diagnosing pancreatitis in cats. While an elevated TLI concentration is specific for pancreatitis in dogs, it is not a highly sensitive indicator. Although marked azotemia may also cause an increase in TLI concentrations, this test is normal in most patients with renal failure. In addition to laboratory tests, the history, physical examination findings, imaging studies, and treatment response are important components in establishing a diagnosis of pancreatitis. Cats with inflammatory bowel disease (IBD) frequently have elevated TLI concentrations, which likely reflects concurrent pancreatitis. In contrast, elevated TLI concentrations are unusual in dogs with inflammatory bowel disease. Some cats with an elevated TLI have been found to have focal pancreatic hypertrophy and not pancreatitis. Rarely, an elevated TLI can be caused by pancreatic neoplasia.

Laboratory tests available for evaluation of small intestinal function are limited. Tests such as fat balance studies, xylose absorption, and breath hydrogen testing are impractical and often insensitive. Serum concentrations of folate and cobalamin are simple tests of small intestinal function that can be helpful in the diagnosis and treatment of patients with GI disease. Folate and cobalamin are absorbed in different parts of the small intestine so that an abnormal concentration of either one may help to determine the precise location of intestinal disease. Serum folate and cobalamin concentrations must be measured after an 8 to 12-hour fast to reduce dietary influence. As it is important to have excluded EPI to correctly interpret folate and cobalamin test results, TLI should be measured simultaneously. In general, folate and cobalamin concentrations are reported to have good specificity but low sensitivity in detecting small intestinal disease.

Disease of the proximal small intestine may cause a decreased folate concentration, whereas disease of the ileum may cause cobalamin to be decreased. With diffuse small intestinal mucosal disease, both cobalamin and folate concentrations may be decreased. Small intestinal bacterial overgrowth (SIBO) in dogs may cause increased serum folate concentrations with or without decreased serum cobalamin concentrations. Note that sample hemolysis will cause folate concentrations to be falsely increased because red blood cells contain high concentrations of folate. EPI can influence serum folate and cobalamin concentrations by causing cobalamin malabsorption and SIBO.

Changes in serum folate and cobalamin concentrations also may have treatment implications. Many cats with severe cobalamin deficiency as a result of GI disease will not respond to treatment unless supplemented with cobalamin. Cobalamin needs to be administered parenterally at a dose of 250-1000 ug SQ or IM weekly for 8-12 weeks. Folate deficiency in cats can cause megaloblastic anemia (macrocytic, non-regenerative), anorexia, lethargy and poor growth. Folate can be administered orally in large doses to overcome the decreased absorption caused by small intestinal disease. The dose administered in one reported case of folate deficiency was 0.5 mg/day for 30 days. In dogs, supplementation with folate or cobalamin is rarely necessary.