Amylase

Amylase, Serum

Amylases are a group of hydrolases that degrade complex carbohydrates into glucose subunits. The exocrine pancreas and salivary glands produce amylase to facilitate starch digestion. Salivary amylase is also produced by the small intestinal mucosa, ovaries, placenta, liver, and fallopian tubes.  Serum contains two amylase isoenzymes, pancreatic or P-type, and salivary, or S-type in a ratio of 40:60.  Serum amylase may be elevated in acute pancreatitis, salivary gland disease, macroamylasemia, and renal failure.  

Serum amylase levels become elevated in approximately 80% of patients with pancreatitis.  Serum amylase activity tends to increase within a few hours after the onset of pancreatitis. It usually peaks between 12 to 72 hours after the onset of pain at 3 or 4 times the upper limit of the normal range but may rise as high as 10 to 15 times. The height of the increase in serum amylase activity does not correlate well with the clinical severity of the pancreatitis. Increased amylase activity may persist for as little as a day, but usually normalizes within a week. The serum half-life is approximately two hours. 

Slight elevations of amylase are not very specific for pancreatitis. The best diagnostic efficiency is obtained with an amylase at least twice the upper limit of normal and a lipase at least five times the upper limit of normal. 

The American College of Gastroenterology practice guidelines suggest that measuring both serum amylase and lipase is not necessary to diagnose pancreatitis. Serum lipase alone is the preferred test because it is more sensitive, just as specific, increases sooner, and remains elevated longer than amylase. The sensitivity of serum lipase is 91 to 96%, and the sensitivity of amylase is 62 to 74%. The specificity of both tests is 92 to 99%. 

Many disease states other than pancreatitis can increase serum amylase levels including; perforated ulcers, intestinal obstruction, mesenteric infarction, ectopic pregnancy, biliary tract disease, acute alcohol intoxication, diabetic ketoacidosis, liver metastases, duodenitis, abdominal trauma, head trauma, thoracic surgery, and genitourinary disorders. Renal failure is also associated with elevated serum amylase because renal filtration normally accounts for about 25% of its clearance.  

Macroamylase is an enzymatically active complex of amylase with IgG or IgA that is too large to be excreted in the urine. Macroamylasemia is a benign condition that accounts for 2.5% of all cases of hyperamylasemia. It is particularly likely in patients with unexplained chronic hyperamylasemia. Macroamylasemia should be considered in patients with elevated serum amylase and normal urine amylase activity. The amylase to creatinine clearance ratio will be less than 0.2.  

Reference range is 28-100 IU/L for adults. 

Specimen requirement is one SST tube of blood. 

Amylase, Urine

Elevated urine amylase level is a sensitive indicator of acute pancreatitis.  Urine amylase is increased in almost 95% of patients with pancreatitis. The rate of amylase excretion into the urine is increased more often, is elevated higher above the normal range and remains elevated longer than the serum amylase activity.  Elevated urine amylase may also be seen in salivary gland disease, bowel perforation and ketoacidosis.  

Amylase is cleared from the plasma by the kidney at a rate of 3 mL/minute and is present in the urine of most healthy individuals.   Reference range is 1 -17 U/Hr.

Specimen requirement is a timed urine collection in a container without preservative.  Timed collections of 2, 12 or 24 hours are acceptable.

References

Banks PA, Freeman ML, Practice Parameters Committee of the American College of Gastroenterology. Practice guidelines in acute pancreatitis. Am J Gastroenterol, 2006;101:2379-2400.

Akhtar A et al. Measuring both serum amylase and lipase for acute pancreatitis lowers quality and raises cost, Cleve Clin J Med, 2017;84:670-672