Amylase hydrolyses starch, breaking glycosidic bonds to convert glycogen polymer to simpler carbohydrates. Ptyalin, in human salivary secretions, is an α-amylase. An alpha-amylase is also secreted by the pancreas into the small intestine. Alpha, beta (microbes and plants) and gamma (animals and plants) amylase act at different points of the carbohydrate polymer chain. The optimum pH of alpha-amylase is 6.7–7.0 (i.e. acidic milieu). Its small size (50-55 kDa) allows amylase to easily filter through the glomerulus for a predominantly renal clearance, supplemented also by a reticuloendothelial scavenge.
The traditional clean-catch (or even a 24-hour) urinary amylase, supplanted by serum amylase measures has, in the clinical setting of a putative acute pancreatitis, itself been supplanted by the serum lipase level (higher (95%) specificity) such that an elevated serum amylase with normal serum lipase may suggest a problem outside the pancreas. Serum amylase can rise within 3-6 hours of symptom onset.
Lipase typically stays elevated for up to two weeks, while amylase concentrations remain elevated for only up to five days: amylase is not as clinically useful where there is a delay between symptom onset and patient presentation. Important for fat digestion, pancreatic lipase concentrations are 100-fold higher than concentrations in other tissues, such as the duodenum, stomach, adipose tissue and lung. In children, unlike in adults, the lipase level may relate to disease severity.
Because amylase is also found in non-GIT tissue, albeit in small levels, and although hyperamylasemia is primarily seen in salivary and pancreatic disease, it may also be seen in other gastrointestinal diseases, with malignancy, and in gynaecological pathology. Conversely, reduced amylase levels, though rare, can be seen in preeclampsia, cystic fibrosis, and liver disease. The serum amylase is tightly regulated in the body. There is a balance between the rate of production and the rate of clearance. Elevated amylase may be due to an increase in pancreatic or extra-pancreatic production or a decreased rate of clearance. Amylase therefore has a short half-life of 12 hours, so the concentration can normalise within 24 hours. Amylase levels of more than three times the upper limit of normal strongly support the diagnosis of acute pancreatitis. Levels less than this are often associated with other conditions. Amylase activity is inhibited by acarbose, used in the treatment of T2DM.
Causes of a raised serum amylase:
- pancreatitis
- > 500: possibile
- > 750: reasonably likely
- > 1000: unlikely to be anything else
- perforated bowel
- acute cholecystitis
- small intestinal obstruction
- mesenteric thrombosis
- dissecting aneurysm
- ectopic pregnancy
Care must be exercised when interpreting a serum amylase because it may return to normal within 24 hours of onset of the episode of pancreatitis. A urinary amylase will help in these situations. For the most part, a serum lipase has replaced serum amylase as the diagnostic test for acute pancreatitis. Only order amylase where non-pancreatic disease is suspected.
References:
Akinfemiwa O, Muniraj T. “Amylase.” [Updated 2021 Mar 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK557738/.
Basnayake C, Ratnam D. Blood tests for acute pancreatitis. Aust Prescr 2015;38:128-30. https://doi.org/10.18773/austprescr.2015.043.
