The serum
amylase concentration reflects the balance between the rates of
amylase entry into and removal from the blood.
Hyperamylasemia can result either from an increased rate of entry of
amylase into the circulation and/or a decreased metabolic clearance of this
enzyme. The pancreas and salivary glands have
amylase concentrations that are several orders of magnitude greater than that of any other normal tissue, and these two organs probably account for almost all of the serum
amylase activity in normal persons. A variety of techniques are now available to distinguish pancreatic from salivary-type
isoamylase. Pancreatic
hyperamylasemia results from an insult to the pancreas, ranging from trivial (cannulation of the pancreatic duct) to severe (
pancreatitis). In addition, loss of bowel integrity (
infarction or perforation) causes pancreatic
hyperamylasemia due to absorption of
amylase from the intestinal lumen.
Hyperamylasemia due to salivary-type
isoamylase is observed in conditions involving the salivary glands. In addition, this type of
hyperamylasemia occurs in conditions in which there is no clinical evidence of
salivary gland disease, such as chronic
alcoholism, postoperative states (particularly postcoronary bypass),
lactic acidosis,
anorexia nervosa or
bulimia, and
malignant neoplasms that secrete
amylase.
Hyperamylasemia can also result from decreased metabolic clearance of
amylase due to
renal failure or
macroamylasemia (a condition in which an abnormally high-molecular-weight
amylase is present in the serum). Patients with
abdominal pain and a markedly elevated serum
amylase (more than three times the upper limit of normal) usually have
acute pancreatitis, and additional serum
enzyme testing is not helpful. Patients with smaller elevations of serum
amylase often have conditions other than
pancreatitis, and measurement of a serum
enzyme more specific for the pancreas (pancreatitic
isoamylase,
lipase or
trypsin) is frequently of diagnostic value in such patients.