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Amylases are a group of hydrolases that degrade complex carbohydrates into fragments. Amylase is produced primarily by the exocrine pancreas where the enzyme is synthesized by the acinar cells and then secreted into the intestinal tract by way of the pancreatic duct system. Amylases also are produced by the salivary glands, small intestine mucosa, ovaries, placenta, liver, and fallopian tubes. Pancreatic and salivary isoenzymes are found in serum.
Diagnosis and management of pancreatitis
Evaluation of pancreatic function
In acute pancreatitis, a transient rise in serum amylase activity occurs within 2 to 12 hours of onset; levels return to normal by the third or fourth day. A 4- to 6-fold elevation of amylase activity above the reference limit is usual with the maximal levels obtained in 12 to 72 hours. However, a significant number of subjects show lesser elevations and sometimes none. The magnitude of the elevation of serum enzyme activity is not related to the severity of pancreatic involvement. Normalization is not necessarily a sign of resolution.
In acute pancreatitis associated with hyperlipidemia, serum amylase activity may be spuriously normal; the amylasemia may be unmasked either by serial dilution of the serum or ultracentrifugation.
A significant amount of serum amylase is excreted in the urine, and therefore, elevation of serum activity is reflected in the rise of urinary amylase activity. Urine amylase, as compared to serum amylase, appears to be more frequently elevated, reaches higher levels and persists for longer periods. However, the receiver operator curves (ROC) of various serum and urine amylase assays demonstrated that all urine assays had poorer diagnostic utility than all serum assays. In quiescent chronic pancreatitis, both serum and urine activities are usually subnormal.
Because it is produced by several organs, amylase is not a specific indicator of pancreatic function. Elevated levels also may be seen in a number of nonpancreatic disease processes including mumps, salivary duct obstruction, ectopic pregnancy, and intestinal obstruction/infarction.
Amylase results may be elevated in patients with macroamylase. Macroamylase refers to a high molecular weight form of amylase that is present in a patient's serum. Different causes of macroamylase have been suggested, the most common being amylase complexed with an immunoglobulin. The large size of the macroamylase complex prevents its excretion in the urine. As a result, the serum amylase is usually elevated. This elevated amylase is not diagnostic for pancreatitis. By utilizing serum lipase and urinary amylase, the presence or absence of macroamylase may be determined.
0-30 days: 0-6 U/L
31-182 days: 1-17 U/L
183-365 days: 6-44 U/L
1-3 years: 8-79 U/L
4-17 years: 21-110 U/L
> or =18 years: 26-102 U/L
1. Soldin SJ: Pediatric Reference Ranges, AACC Press, Washington DC, Second edition. 1997
2. Tietz Textbook of Clinical Chemistry. Edited by CA Burtis, ER Ashwood. WB Saunders Company, Philadelphia, 1999
3. Swaroop VS, Chari ST, Clain JE: Acute pancreatitis, JAMA 2004;291:2865-2868