Tests For Pancreatic Disease - The Merck Veterinary Manual

Tests For Pancreatic Disease

Pancreatitis:

Serum amylase and lipase activities have been used for several decades to diagnose pancreatitis in both humans and dogs. Unfortunately, neither of these diagnostic tests is both sensitive and specific for pancreatitis. After total pancreatectomy, significant serum amylase and lipase activities remain, indicating that there are sources other than the exocrine pancreas. Clinical data also suggest a specificity for pancreatitis of only ~50% for both of these markers. Many nonpancreatic diseases, such as renal, hepatic, intestinal, and neoplastic diseases, can lead to increases in serum amylase and lipase activities. Steroid administration can also increase serum lipase activity and cause variable responses in serum amylase activity. Thus, in dogs, measurement of serum amylase and lipase activities are of limited clinical usefulness for the diagnosis of pancreatitis and should only be used until a more definitive diagnostic test, such as abdominal ultrasound, serum canine pancreatic lipase immunoreactivity concentration, or exploratory laparotomy, can be performed. Serum amylase and/or lipase activities that are 3-5 times the upper limit of the reference range, in patients with clinical signs that are consistent with pancreatitis, are suggestive of such a diagnosis. However, it is important to note that ~50% of dogs in this group do not have pancreatitis. In cats, serum amylase and lipase activities are of no clinical value for the diagnosis of pancreatitis. While cats with experimental pancreatitis show an increase in serum lipase activity and a decrease in serum amylase activity, these changes are not seen in cats with spontaneous disease.

Serum trypsin-like immunoreactivity (TLI) concentration measures mainly trypsinogen, the only form of trypsin that should be circulating in healthy individuals. However, trypsin, if present in the serum, is also detected by the assay. In healthy animals, serum TLI is low; during pancreatitis, however, an increased amount of trypsinogen leaks into the vascular space, which leads to an increase in serum TLI. Trypsin that has been prematurely activated may also contribute to this increase. However, both trypsinogen and trypsin are quickly cleared by the kidney. In addition, any prematurely activated trypsin is quickly removed by proteinase inhibitors, such as α₁-proteinase inhibitor and α₂ -macroglobulin. In turn, α₂-macroglobulin-trypsin complexes are removed by the reticuloendothelial system. Thus, serum half-life for TLI is short, and a significant degree of active inflammation is required to increase serum TLI. In dogs, serum TLI is of limited usefulness for the diagnosis of pancreatitis. While it is more specific than serum amylase and lipase activity, its sensitivity is lower. For most veterinarians, the longer turnaround time for this test makes the measurement of serum TLI less desirable as a diagnostic test. Assays for the measurement of pancreatic lipase immunoreactivity (PLI) in canine and feline serum have recently been developed and validated. Serum PLI is highly specific for exocrine pancreatic function. Also, serum PLI is far more sensitive for the diagnosis of pancreatitis than any other diagnostic test currently available.

Pancreatic lipase immunoreactivity concentration specifically measures the mass of classical pancreatic lipase in the serum, rather than its kinetic (enzymatic) activity. At the moment, measurement of serum PLI concentration is only available through the Gastrointestinal Laboratory at Texas A&M University. Serum PLI concentration can be used to diagnose pancreatitis in dogs with renal failure, underscoring the high specificity of this new diagnostic test.

Other tests for the diagnosis of pancreatitis in dogs and cats have been evaluated. However, plasma trypsinogen activation peptide (TAP) concentration, urine TAP concentration, urine TAP:creatinine ratio, serum a₁-proteinase inhibitor trypsin complex concentration, and serum α₂-macroglobulin concentration have all been shown to be of little clinical usefulness for the diagnosis of spontaneous pancreatitis in dogs or cats.

Exocrine Pancreatic Insufficiency:

In the past, several fecal tests were used to diagnose exocrine pancreatic insufficiency (EPI). Microscopic fecal examination for fat and/or undigested starch or muscle fibers are at best useful to suggest maldigestion. However, in light of wide availability of tests for diagnosing EPI, microscopic fecal examination can no longer be clinically justified. Fecal proteolytic activity had been used for several decades to diagnose EPI in small animals. Most of these methods, particularly the radiographic film clearance test, are unreliable. One method, which uses pre-made tablets to pour a gelatin agar, is the most useful. However, false positive as well as false negative results have been reported; the clinical use of fecal proteolytic activity is limited to species for which more specific assays to estimate pancreatic function are not available.

Serum TLI concentration is the diagnostic test of choice for EPI in both dogs and cats. Assays for TLI measure trypsinogen circulating in the vascular space. In healthy individuals, only a small amount of trypsinogen is present in serum. However, in dogs and cats with EPI, serum TLI decreases significantly and may even be undetectable. The reference range for canine TLI is 5-35 mg/L with a cut-off value of £2.5 µg/L considered diagnostic for EPI. Similarly, the reference range for feline serum TLI is 12-82 µg/L, with a cut-off value of ≤8 µg/L diagnostic for feline EPI. Rarely, animals with serum TLI concentrations below the cut-off value do not have clinical signs of EPI. This is probably due to the functional redundancy of the GI tract. At the same time, many dogs and cats with chronic diarrhea and weight loss have mild decreases in serum TLI (2.5-5.0 µg/L). Most of these animals have chronic small-intestinal disease and should be investigated accordingly. However, a small number of these dogs and cats may have EPI. If chronic small-intestinal disease is not diagnosed, serum TLI should be reevaluated in 2-3 mo.

PLI is also highly specific for exocrine pancreatic function and could be used to diagnose EPI. However, there is a small degree of overlap in serum PLI concentrations between normal dogs and dogs with EPI, making the measurement of PLI slightly inferior to TLI for accurate diagnosis.

Recently, an assay for measurement of canine fecal elastase has been developed and validated. This assay is inferior to serum TLI measurement and leads to many false-positive results. It is also more cumbersome and more expensive. It might be useful for the diagnosis of EPI due to obstruction of the pancreatic duct; however, this condition has never been described in dogs or cats.