Amylase and Lipase Tests Health Article

Definition

Amylase and lipase tests are performed to aid in the differential diagnosis of acute abdominal pain. Amylase and lipase are digestive enzymes made by the pancreas. An enzyme is a protein that accelerates a biochemical

reaction. Both enzymes are members of the hydrolase class, which means that they split a substrate by the addition of water. Amylase catalyzes the hydrolysis of starch forming maltose. The maltose can be converted to glucose by other enzymes. Lipase splits triglycerides, forming glycerol and fatty acids as the final product.

Purpose

Epigastric pain and abdominal tenderness associated with acute appendicitis is difficult to distinguish from acute pancreatitis. Serial measurements of amylase and lipase are used to exclude a diagnosis of acute pancreatitis when results are within normal limits. One or both enzymes may be increased in acute pancreatitis, but neither enzyme is specific. In addition to acute pancreatitis, amylase is increased mumps, some malignancies, ectopic pregnancy, alcoholic liver disease, peptic ulcers, intestinal obstruction, and renal failure. Lipase is increased in renal failure, intestinal obstruction and liver disease. Use of both enzymes increases diagnostic sensitivity and specificity to around 90%. Acute pancreatitis is highly likely when the plasma amylase is increased to more than twofold normal and lipase is increased more than fivefold normal. When plasma amylase is increased, but the lipase is normal, a nonpancreatic condition is almost always the cause. When amylase is increased more than twofold and lipase is increased but less than fivefold, renal failure, pancreatitis, intestinal obstruction, peptic ulcer disease, and acute pancreatitis are possible causes.

In acute pancreatitis, plasma amylase becomes elevated two to 12 hours after an episode of acute abdominal pain. Levels peak in 12-72 hours, usually reaching two to six times the upper limit of normal, then return to normal by four days. Amylase is a small protein and is excreted in urine in significant quantities. Urinary amy-lase rises in parallel with plasma amylase, but reaches higher levels and remains elevated for seven to 14 days. Renal excretion of amylase is increased in pancreatitis, and the ratio of amylase to creatinine clearance is a more specific test for acute pancreatitis than is plasma amylase. Lipase in plasma becomes abnormal four to eight hours following an episode of acute pancreatitis, reaches a peak level two to 50 times normal in approximately one day, and remains elevated for seven to 10 days. Recently, serum and urine levels of trypsinogen-2 have been shown to be very sensitive and specific indicators of pancreatits, but assays are not yet widely available.

Precautions

Blood for measurement of amylase and lipase is collected by venipuncture. The nurse or phlebotomist should follow standard precautions for the prevention of transmission of bloodborne pathogens. Amylase and lipase should not be used as screening tests for future pancreatic disease. Neither enzyme is likely to be elevated in chronic pancreatitis because enzyme production is decreased by chronic disease. Up to 1% of persons have increased plasma amylase owing to formation of a complex between amylase and immunoglobulins. This condition is termed macroamylasemia and it occurs more frequently in the older population. Plasma amylase is elevated above normal, but urinary amylase is low in this condition. The presence of natural amylase inhibitors in the diet may decrease plasma amylase activity. Drugs that may increase amylase include morphine, phenformin, ethanol, and contraceptives. Lipase may be increased by ethanol, codeine, and narcotics. Results are dependent upon the method used and normal values may vary significantly between laboratories.

Description

Amylase and lipase tests are usually performed on a blood sample, but amylase testing can also be performed on urine. Enzymes are usually measured by determining the rate of product formation under controlled pH and temperature. Measurements are reported as units of activity rather than in mass units. Reference methods for amy-lase and lipase are labor intensive and difficult to automate. There are several methods for measuring amylase and lipase each with its own advantages and disadvantages. A common method of measuring amylase is based upon the hydrolysis of a synthetic glucose polymer that is labeled at one end with p-nitrophenol. The amylase splits the substrate into various subunits of glucose. Subunits consisting of three glucose molecules are hydrolyzed by an enzyme in the reagent, alpha-glucosidase, forming glucose and p-nitrophenol. The activity of amylase is proportional to the rate of p-nitrophenol formation. This is determined by measuring the amount of light that the reaction mixture absorbs at 405 nm over a fixed time interval. Lipase is often measured using a cascade of coupling enzyme reactions that yield a colored product. Lipase is incubated with a synthetic diglyceride substrate and splits it forming a monoglyceride and a fatty acid. The monoglyceride is split by an enzyme in the reagent, monoglyceride esterase, to yield glycerol. The glycerol is converted to glycerol-3-phosphate by the enzyme glycerol kinase which attaches a phosphate from adenosine triphosphate (ATP). The glycerol-3-phosphate is oxidized by another enzyme, glycerol phosphate oxidase (GPO), forming dihydroxyacetone phosphate and hydrogen peroxide. In the final step, peroxidase catalyzes the oxidation of a dye by the hydrogen peroxide forming a pink product. The rate of absorbance (color) increase at 500 nm is measured, and is proportional to lipase activity.