Serum amylase is secreted in specific isoforms by the salivary glands (s-amylase) and pancreas (p-isoamylase). It predominantly acts to digest starch, glycogen and related polysaccharides. Almost all laboratories currently measure total serum amylase so the result includes both isoenzymes. The reference range is typically 20–300 U/L, but does vary with age and gender. It also varies between laboratories despite attempts to adopt standardised reference methods.
In studies using radiological evidence as the ‘gold standard’ for acute pancreatitis, serum amylase has a sensitivity of 81–95%. However, this does depend on the definition of ‘abnormal’ and the cut-off values chosen. Most guidelines now suggest an amylase concentration 2–4 times the upper limit of normal is optimal for diagnostic accuracy, but this may reduce the sensitivity of the test to as low as 60%.2,3,5 The sensitivity is also influenced by other factors, including the timing of the test and the cause of the pancreatitis.
Timing
In acute pancreatitis, amylase can rise rapidly within 3–6 hours of the onset of symptoms, and may remain elevated for up to five days. However, it has a short half-life of 12 hours so the concentration can normalise within 24 hours. This significantly reduces its value as a diagnostic test relatively early in the clinical course.
Cause of pancreatitis
In pancreatitis due to hypertriglyceridaemia, the serum amylase can be normal in up to 50% of cases. This is due to interference with the assay by either a circulating inhibitor or the hyperlipidaemia itself. A number of studies have also suggested that amylase may be less elevated in alcohol-induced pancreatitis compared to other causes.
Many conditions (see Table) can increase serum amylase so it is not specific for pancreatitis. These conditions include various intra- and extra-abdominal illnesses and drugs. Macroamylasaemia is an uncommon condition in which amylase rises because its clearance is reduced.1,2,5
Given that up to 60% of the total serum amylase originates from non-pancreatic sources, measuring the pancreatic isoenzyme may improve the diagnostic accuracy in acute pancreatitis. However, this isoenzyme also rises in many of the other non-pancreatic causes of hyperamylasaemia. There are few studies on whether measuring the isoenzyme significantly improves the diagnostic accuracy of acute pancreatitis.2,5 Consequently, pancreatic amylase is not routinely measured in most laboratories.
Table - Causes of elevated serum amylase and lipase 2,5
Causes
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Amylase
|
Lipase
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Abdominal conditions
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acute pancreatitis, pancreatic trauma, perforated viscus, intestinal infarction and obstruction, peritonitis, acute cholecystitis, appendicitis, hepatitis, abdominal aortic aneurysm, ruptured ectopic pregnancy, fallopian and ovarian cysts
|
acute pancreatitis, pancreatic trauma, perforated viscus, intestinal infarction and obstruction, peritonitis, acute cholecystitis, appendicitis, hepatitis, abdominal aortic aneurysm, malignancy (especially oesophagus, stomach, duodenum, pancreas)
|
Extra-abdominal conditions
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salivary disease, renal failure, ketoacidosis, pneumonia, cerebral trauma, burns, anorexia nervosa and bulimia
|
renal failure, ketoacidosis, fat embolism, bony fractures
|
Drug induced
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azathioprine*, colaspase, sulphonamides, tetracycline*, didanosine, methyldopa*, oestrogens*, frusemide, 5-aminosalicyclic acid*, valproate*, thiazides*, glucocorticoids, nitrofurantoin*, rifampicin*, tacrolimus*, metronidazole*, 6-mercaptopurine*, cyclosporin*, cisplatin*
|
adrenocorticotropic hormone*, tetracycline*, oestrogens, frusemide*, valproate*, thiazides*, rifampicin*, metronidazole*, zalcitabine, opioids, methylprednisolone*, indomethacin*
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Others
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macroamylasaemia, idiopathic hyperamylasaemia
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mumps, hyperlipoproteinaemia
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* These drugs can cause acute pancreatitis, but can also elevate pancreatic enzymes without pancreatitis.