Diagnostic markers for exocrine pancreatic disease

Exocrine pancreatic disorders are common in dogs and cats. Based on clinical findings, pancreatitis is the most-common exocrine pancreatic disorder in small animals, followed by exocrine pancreatic insufficiency, exocrine pancreatic neoplasia, and some rare conditions.

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The following discussion will focus on diagnostic markers for pancreatitis and exocrine pancreatic insufficiency in dogs and cats.

•Diagnostic markers for pancreatitis

The true incidence of pancreatitis in dogs and cats is unknown, but a recent study suggests that, similarly to human beings, pancreatitis in dogs remains undiagnosed in the vast majority of cases. This might be due to subclinical or mild disease in some patients that are not presented to a veterinarian. The low rate of diagnosis also might be due to non-specific clinical signs and diagnostic findings in dogs and cats with pancreatitis.

Thus, in spite of the many advances in the diagnosis of canine and feline pancreatitis, the diagnosis remains challenging in many cases. Many minimally invasive diagnostic tests for canine and feline pancreatitis have been described. However, few have been found to be clinically useful.

•Trypsinogen activation peptide (TAP)

When trypsinogen is activated to trypsin, a small peptide, trypsinogen activation peptide (TAP) is cleaved from the trypsinogen molecule. Under normal conditions activation of trypsinogen is limited to the small intestine. Thus, normal dogs and cats have no or only minimal concentrations of TAP circulating in the blood stream. During pancreatitis, trypsinogen is activated prematurely in pancreatic acinar cells, and TAP is released into the vascular space.

Unfortunately, clinical trials have shown that the diagnostic indices for TAP are suboptimal in both dogs and cats. This is most likely due to the short serum half-life of TAP in combination with the fact that veterinary patients with pancreatitis rarely are presented during the first 24-48 hours after disease initiation.

•Serum lipase activity

Serum lipase activity has been used to diagnose canine pancreatitis for several decades. However, it has been well recognized that serum lipase activity is neither very sensitive, nor very specific for pancreatitis. This is due to the fact that many different cells in the body synthesize and secrete lipases that all can contribute to the serum lipase activity measured. Serum lipase activity decreases in dogs after pancreatectomy indicating that some of the lipase activity present in the serum does originate from the exocrine pancreas. However, considerable serum lipase activity remains in dogs after pancreatectomy, indicating that lipase activity in serum also originates from other tissues.

Also, many non-pancreatic conditions, such as gastrointestinal, renal, or hepatic disorders or even heat stress or glucocorticoid administration, have been associated with an increase in serum lipase activity. While some dogs with pancreatitis have elevated serum lipase activity, others display no or only mild elevations. Thus, in dogs, serum lipase activity only should be used as a screening test until the diagnosis can be confirmed by other, more-specific, diagnostic modalities.

Also, if serum lipase activity is analyzed, it should be interpreted cautiously, and only elevations of three to five times the upper limit of the reference range should be considered suggestive of pancreatitis.

Cats with experimentally-induced pancreatitis showed significantly increased serum lipase activities, but cats with spontaneous pancreatitis did not. In one study, not a single cat with pancreatitis had a serum lipase activity outside the reference range. Thus, it appears that, in cats, serum lipase activity is of no clinical usefulness for the diagnosis of pancreatitis.

•Serum amylase activity

Like serum lipase activity, serum amylase activity is neither sensitive nor specific for pancreatitis. While some dogs with spontaneous pancreatitis have an elevated serum amylase activity, others have serum amylase activities in the normal range. Furthermore, many dogs with non-pancreatic conditions have elevations of serum amylase activity. As for serum lipase activity, serum amylase activity only should be used as a screening tool for canine pancreatitis until the diagnosis can be confirmed by more-specific diagnostic modalities.

Serum amylase activity has been shown to be decreased in cats with experimentally-induced pancreatitis, but in cats with spontaneous disease serum amylase, activity was of no clinical value.

•Serum trypsin-like immunoreactivity (TLI)

Serum trypsin-like immunoreactivity (TLI) measures trypsinogen, but also trypsin and some trypsin molecules bound to proteinase inhibitors. During health, a small amount of trypsinogen is secreted into the vascular space, but very little or no trypsin is present in serum during health. Serum TLI concentration has been shown to be specific for exocrine pancreatic function in both dogs and cats.

Dogs with experimentally-induced pancreatitis have increased serum canine TLI concentrations. Also, some dogs with spontaneous pancreatitis have elevations of serum cTLI; however, the sensitivity of serum TLI concentration for spontaneous pancreatitis is less than 40 percent. Taking into consideration the much slower turn-around time for serum cTLI determination compared to those of serum amylase and lipase activities, serum cTLI concentration has little diagnostic advantage over serum amylase and lipase activities in dogs with pancreatitis.

Serum feline TLI concentration is increased in cats with experimentally-induced and spontaneous pancreatitis, but as in the dog, the sensitivity of serum fTLI concentration for spontaneous pancreatitis is less than 40 percent. However, until recently even with a suboptimal sensitivity, serum fTLI concentration was the most sensitive diagnostic test for feline pancreatitis.

•Serum pancreatic lipase immunoreactivity (PLI)

Immunoassays for the measurement of canine and feline PLI recently have been developed and validated. Serum cPLI was measured in a group of dogs with exocrine pancreatic insufficiency, and the median serum cPLI concentration was found to be significantly decreased compared to healthy dogs. In fact, serum cPLI concentrations were decreased below the lower limit of the reference range in almost all of these dogs, indicating that serum cPLI concentration is specific for exocrine pancreatic function. In another study, the sensitivity of different minimally-invasive diagnostic tests was compared in dogs with biopsy-proven pancreatitis. The sensitivity of serum TLI concentration was below 40 percent and that of serum lipase activity was less than 60 percent. In contrast, the sensitivity for serum cPLI concentration for pancreatitis was more than 80 percent, which was also higher than the 68 percent reported for abdominal ultrasonography. Thus, serum lipase activity is the most sensitive diagnostic test for canine pancreatitis currently available.

Because these initial studies were performed, serum cPLI concentration has been used clinically with great success. Currently, measurement of serum cPLI is only available through the Gastrointestinal Laboratory at Texas A&M University (www.cvm.tamu.edu/gilab).

Clinical studies in cats also have been promising. In a group of cats with experimentally-induced pancreatitis both serum fTLI and fPLI concentrations did increase initially but serum fPLI stayed elevated much longer than did serum fTLI concentration suggesting that, as in the dog, serum PLI concentration is much more sensitive for pancreatitis than serum TLI concentration.

In another study of cats with spontaneous pancreatitis serum fPLI concentration was more sensitive and specific than serum fTLI concentration, abdominal ultrasonography or abdominal computed tomography. Currently, measurement of serum fPLI concentration is only available through the Gastrointestinal Laboratory at Texas A&M University.

•Diagnostic markers for exocrine pancreatic insufficiency (EPI)

In the past, several fecal tests have been used to diagnose EPI. Microscopic fecal examination for undigested fat, starch or muscle fibers is at best useful to suggest maldigestion. In light of wide availability of tests that are useful for the diagnosis of EPI, microscopic fecal examination no longer can be clinically justified.

Fecal proteolytic activity has been used for several decades to diagnose EPI in small animals. Several variations of the test have been described; the most simple involves digestion of the gelatin emulsion on a piece of radiographic film. Most of these methodologies, especially the radiographic film clearance test, are completely unreliable. One method, which uses pre-made tablets that can be used to pour a gelatin agar, has been found to be most useful. However, false-positive as well as false-negative results have been reported, and the clinical use of fecal proteolytic activity is limited to species for which more-specific assays to estimate pancreatic function are not available.

Serum trypsin-like immunoreactivity (TLI) 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 concentration decreases significantly and can even be undetectable. There are many dogs and cats with chronic diarrhea and weight loss that only have a mild decrease in serum TLI concentration. Most of these animals likely have chronic small intestinal disease and should be worked-up accordingly. However, a small number of these dogs and cats might have EPI. If diagnosis of chronic small-intestinal disease is unsuccessful, then a serum TLI concentration should be re-evaluated after a couple of months.

Like serum TLI concentration, serum PLI concentration is highly specific for exocrine pancreatic function, and theoretically could be used for the diagnosis of EPI. However, there is some overlap for serum PLI between normal animals and animals with EPI, making the measurement for PLI inferior to TLI for accurate diagnosis of EPI.

Recently, an assay for measurement of canine fecal elastase has been developed and validated. Unfortunately, this assay has been shown to be inferior to serum TLI measurement and leads to many false-positive results. It is also more cumbersome and more expensive.

Jörg Steiner received his veterinary degree from the Ludwig-Maximilians University in Munich, Germany in 1992. He completed an internship in small animal medicine and surgery at the University of Pennsylvania from 1992 to 1993 and a residency in small animal internal medicine at Purdue University from 1993 to 1996. He received his Dr.med.vet. degree from the Ludwig-Maximilians University in Munich, Germany in 1995 in recognition of research on feline trypsin and feline trypsin-like immunoreactivity. In 1996, he achieved board certification with the American College of Veterinary Internal Medicine and the European College of Veterinary Internal Medicine. In 2000, Dr. Steiner received a PhD from Texas A&M University for his work on canine digestive lipases and their use for the diagnosis of gastrointestinal disorders in the dog. He currently serves as a clinical assistant professor with the Department of Small Animal Medicine and Surgery at Texas A&M University. Dr. Steiner also serves as co-director of the GI Laboratory at Texas A&M University and is involved in a wide variety of research in small animal gastroenterology.