Clinical utility of amylase and lipase.

Q In cases of suspected pancreatitis, sometimes the doctors order amylase, sometimes they order lipase, and sometimes they order both. Do these assays duplicate each other? When should a physician order amylase, and when should he order lipase? Are they STAT tests?

A The clinical use of amylase, lipase, and trypsin assays to diagnose pancreatitis is based on the presence of high concentrations of these digestive enzymes, which are released and absorbed into the blood when pancreatic tissue is damaged. Factors that cause levels to rise above normal include the extent and severity of the damage and the capacity of the body to clear the blood of the circulating tissue products. A small amount of damage coupled with high renal clearance of amylase, for example, might never result in an elevated amylase level. Consequently, establishing a "one test fits all" criterion for the diagnosis of pancreatitis is impossible.

Amylase assays are more useful early in the disease because amylase typically rises during the two hours after symptoms begin. Amylase levels frequently return to normal range within 36 hours, however, so an amylase value is not conclusive in a patient who says the stomach pain is four days old. A lipase assay is of value in this type of patient. Although lipase levels may peak about 24 hours after symptoms start, levels remain elevated for up to 14 days. In terms of multiples of the upper reference limit (URL), peak values of lipase tend to be higher than amylase values.

The specificity of the amylase assay has been introduced in some test systems by pretreatment of the sample with immunologic reagents to block salivary amylase. In mild cases of pancreatitis, elevated release of amylase into the blood and subsequent renal clearance are potentially measured by the amylase creatinine clearance ratio. Serum trypsin assays are notoriously crude or problematic, but a urine dipstick assay called Actim Pancreatitis is a promising test for abnormal release of the pancreatic proteolytic enzyme trypsinogen.

The University of Missouri Health System has published an algorithm for the diagnosis and management of acute pancreatitis in hospitalized patients that might be useful in your setting. The algorithm uses amylase or lipase, depending on the length of time symptoms have persisted, and recommends a criterion of three times the URL. (1) Supplemental studies may be required to establish the diagnosis and the severity of the disease. These include endoscopic retrograde cholangiopancreatography or ERCP, computed tomography, and measurement of C-reactive protein (to assess infection in the damaged pancreas).

Your question about appropriate turnaround time for the assay is a complicated medical, political, and practical question that should not be arbitrarily answered. In the hospitalized patient who is being managed expectantly, turnaround time is less critical. Measures will be in place to minimize the severity of pancreatic damage should pancreatitis be present.

The emergency department (ED), however, presents a more demanding set of circumstances. But here, too, the goal of patient management is to avoid progression of the disease while the diagnostic studies are in progress. Ideally, test results should be available to ED doctors in a time frame that allows them to make the most effective and least costly decisions regarding admission, treatment, and discharge. Sometimes, a single test cannot provide the answer--rather, testing over time is necessary. At the risk of being arbitrary, I recommend handling ED testing as a high priority; the optimal turnaround time for these assays in this setting is within an hour of receipt of the sample.

--Louis Buettner, MD

Consulting Pathologist

Southern Diagnostic Laboratories Inc.

Tuscaloosa, AL

Reference

1. University of Missouri Health Care. Acute Pancreatitis Algorithm. Available at http://www2.muhealth.org/[approximately]outcomes/pancreatitis/algorithms.html. Accessed August 1, 2005.