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Editor: Thiruvengadam Muniraj Updated: 11/10/2022 7:26:19 AM

Amylase is a digestive enzyme predominantly secreted by the pancreas and salivary glands and found in other tissues at very small levels.[1] Amylase was first described in the early 1800s and is considered one of the first enzymes in history to be scientifically investigated. It was initially termed diastaste but was later renamed amylase in the early 20th century.[2]

Amylases' main function is to hydrolyze the glycosidic bonds in starch molecules, converting complex carbohydrates to simple sugars. There are three main classes of amylase enzymes; Alpha-, beta-, and gamma-amylase, and each act on different parts of the carbohydrate molecule.[2] Alpha-amylase can be found in humans, animals, plants, and microbes. Beta-amylase is found in microbes and plants. Gamma-amylase is found in animals and plants.[3] 

In 1908, a study by Wohlgemuth identified the presence of amylase in urine, and this subsequently led to the use of amylase as a diagnostic laboratory test. Amylase is a commonly ordered test along with lipase, especially in suspected acute pancreatitis.[2]

Etiology and Epidemiology

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Elevated amylase can be seen in a variety of conditions, including pancreatic disease, salivary disease, decreased metabolic clearance, intestinal disease, and macroamylasemia.[4] Between 11 and 13 % of patients with non-pancreatic abdominal pain have elevated pancreatic enzymes.[5] In a group of asymptomatic HIV-positive patients, 60 percent had an abnormal amylase or lipase measurement on at least one occasion.[6] Twenty-six (12.5%) of 208 patients with acute abdominal pain unrelated to the pancreas had elevated serum amylase on admission.[5] 

Abnormally elevated amylase levels are seen in 35% of patients with liver disease.[7] Between 16 and 25% of diabetic ketoacidosis cases present elevated levels of amylase.[8][9] In a group of 74 patients with surgically resectable lung cancer,13 showed hyperamylasemia.[10]

Amylase is a heterogeneous calcium-dependent metalloenzyme of molecular weights varying from 54-62 kDa.The small size allows it to be easily filtered through the glomeruli.[1] Amylase is cleared via the kidneys and reticuloendothelial system.[3] It exists as two isoenzymes: pancreatic (P-type) and nonpancreatic (S-type). These isoenzymes are products of two closely linked loci on chromosome 1. Additional amylase heterogeneity is due to allelic variation (S-type, 12 alleles; P-type, 6 alleles).[11] Both also undergo post-translational modification by deamidation, glycosylation, and deglycosylation to form various isoforms. Amylase has a wide tissue distribution, with the highest activities of the P and S-types being found in the exocrine pancreas and salivary glands, respectively.[12]

P-type amylase is synthesized by pancreatic acinar cells and secreted into the intestinal tract via the pancreatic duct system. Its action is favored by the mildly alkaline conditions in the duodenum.[13] The greatest S-type amylase activity is in salivary glands, where it initiates the hydrolysis of starches while food is in the mouth and esophagus. Its action is terminated by acid in the stomach. S-type amylase is also found in extracts of testes, ovaries, fallopian tubes, Mullerian ducts, striated muscle, lungs, and adipose tissue, as well as in semen, colostrum, tears, and milk. About 25% of plasma amylase is excreted by the kidneys, but the majority is reabsorbed in the proximal tubules.[14] The liver is suspected to be the major organ for amylase removal, resulting in a half-life of about ten hours. Serum amylase is tightly regulated in the body.[15] There is a balance between the rate of production and the rate of clearance. Elevated amylase may be due to increased pancreatic or extrapancreatic production or a decreased clearance rate.

Genetic regulation is likely to play a crucial role in the preliminary determination of salivary amylase.[1] In newborns, the predominant amylase isozymes seen in the urine are of salivary origin and later both salivary and pancreas, which increases during development. The functional integrity of amylase is absolutely dependent on the presence of calcium.[16] However, full integrity is displayed only in the presence of any one of various anions, including chloride, bromide, nitrate, or monohydrogen phosphate. Chloride and bromide are the most effective activators. The pH optimum is 6.9 to 7.0.[17]

The function analyte amylase is an endoglycosidase enzyme of the hydrolase class that catalyzes the hydrolysis of 1,4-α-glucosidic linkages between adjacent glucose units in complex carbohydrates.[18] Straight-chain (linear) polyglucans, such as amylose, and branched polyglucans, such as amylopectin and glycogen, are hydrolyzed at different rates.[19] In the case of amylose, the enzyme splits the chains at alternate α-1,4- hemiacetal (-C-O-C-) links, forming maltose and some residual glucose. In the case of branched polyglucans, maltose, glucose, and a residue of limit dextrins are formed. The enzyme does not attack the α-1,6-linkages at the branch points.[2]

Either serum or heparinized plasma can be used as a sample. However, one study found that heparinized plasma samples gave significantly higher results than serum samples when measured using dry-film technology.[20] Because amylase has an absolute requirement for calcium ions, chelating anticoagulants such as citrate, oxalate, and EDTA cannot be used to collect plasma to be used for amylase measurements. Urine specimens with no preservatives in random or timed collections are also valid specimens. Amylase is occasionally measured in the ascitic, peritoneal, or pleural fluid, where its presence can indicate pancreatitis or the presence of a tumor. Serum shows no loss of amylase activity for four days at room temperature, for two weeks at 5 degrees C, for one year at −28 degrees C, or five years at −75 degrees C. Urine specimens should be analyzed within 12 hours at room temperature or within five days at 5 degrees C, and urine should not be frozen.[21]

For many years, amylase has been primarily used for diagnosing acute pancreatitis. Amylase can be measured with a blood test or urine test. The urine test may be performed by a clean catch or 24-hour urine collection.[3] The normal range of serum amylase differs from laboratory to laboratory. It is clinically important to differentiate pancreatic amylase from other amylase isoforms. An elevated amylase with normal lipase may be suggestive of a problem outside the pancreas.[15]

The lipase-to-amylase ratio may help to distinguish gallstone-induced pancreatitis from alcoholic pancreatitis. Gallstones cause higher increases in amylase, and alcohol causes higher increases in lipase. A lipase-to-amylase ratio > 2 has 91% sensitivity and 76% specificity for alcoholic pancreatitis, and a ratio > 5 has 31% sensitivity and close to 100% specificity for alcoholic pancreatitis.[22] Increased ALT to 3 times normal is highly specific to gall stones pancreatitis. Measurement of both serum amylase and lipase increases specificity compared to either test alone but does not significantly improve sensitivity.[23]

The amylase test is performed on semi-automatic or fully automated analyzers which are based on the principle of photometry. Photometry is the measurement of light absorbed in the ultraviolet (UV) to visible (VIS) to infrared (IR) range. This measurement is used to determine the amount of an analyte in a solution or liquid. Photometers utilize a specific light source and detectors that convert light passed through a sample solution into a proportional electrical signal. These detectors may be photodiodes, photoresistors, or photomultipliers. Photometry uses Beer–Lambert’s law to calculate coefficients obtained from the transmittance measurement.[24] A correlation between absorbance and analyte concentration is then established by a test-specific calibration function to achieve highly accurate measurements.[25]

P-type amylase can be differentiated from S-type amylase by selective inhibition of S-type by a wheat germ inhibitor, temperature inhibition, immunoprecipitation, or immunoinhibition by a monoclonal antibody. However, only the methods based on selective inhibition by monoclonal antibodies have shown sufficient precision, reliability, practicability, and analytical speed to allow reliable measurement of P-type amylase.[26] The amylase isoforms can be separated by isoelectric focusing, ion-exchange chromatography, or gel/capillary electrophoresis by electrophoretic endosmosis.[27]

Interfering Factors

Amylase assays are generally not prone to interference from hemoglobin, bilirubin, or triglycerides. Collecting specimens in tubes containing oxalate, citrate, or EDTA may result in falsely decreased values due to the chelation of necessary amylase cofactors.[3] Medications, including aspirin, morphine, antiretrovirals, and estrogen-containing medication, can affect serum levels of amylase.[28]

Increased amylase activity in serum can result from a condition known as macroamylasaemia – a state in which amylase forms macromolecular complexes, usually with immunoglobulin (IgA or IgG in most cases), but also as self-polymerization or association with other proteins.[29] These complexes generally retain enzymatic activity but cannot be filtered by renal glomeruli; this leads to delayed clearance and increased serum amylase activity.[30] This benign condition has been reported in as many as 1.5% of hospitalized patients, accounting for as much as 28% of chronic, otherwise unexplained hyperamylasaemia.[31] Macroamylasaemia has a disease associated with autoimmunity, malignancy, cardiovascular disease, diabetes mellitus, and malabsorptive disorders. Macroamylasemia should be considered in an asymptomatic patient with elevated serum amylase.[32] There is no required treatment for the condition.[33] 

The circulating pancreatic amylase is higher in female subjects with O blood type than those with A blood type (lower pancreatic amylase in A blood type).[34] Psychosocial stress contributes to elevated salivary amylase even in a healthy population, which likely leads to elevated total serum amylase. However, whether psychosocial stress has a long-term effect on serum amylase has not been confirmed in clinical studies.[35]

In pancreatitis with associated hypertriglyceridemia, serum amylase levels may be erroneously normal. This is attributed to an inhibitor associated with triglyceride elevations that interferes with the assay for the enzyme. Diluting the serum can reduce the activity of the inhibitor such that a recalculation of serum amylase can uncover the true serum amylase levels.[36]

Results, Reporting, and Critical Findings

Reference intervals of amylase differ between the various available assays method because of differences in substrates used and reagent preparations.[37] A patient's blood test values should be interpreted based on the reference value of the laboratory in which the test is done. It is recommended that each laboratory establish its own reference interval based on its methodology.[38] A significant proportion of subjects of African and Asian origin have an S-type amylase activity above the reference interval derived from white populations. This can result in an apparently elevated total amylase that is non-pathological.[39]

Blood amylase activity of newborns is approximately 18% of adults. There are no significant differences between males and females in the serum activity of amylase. In healthy adults, P-AMY represents approximately 40% to 50% of total amylase activity in serum. Serum P-AMY activity is not demonstrable in most children younger than six months, but activity rises slowly after that to reach adult concentrations at five years of age, reflecting the postnatal development of the exocrine pancreatic function.[40] As a consequence, the use of this enzyme for the diagnosis of acute pancreatitis in young children should be avoided.[41]

Currently, there is no internationally established reference range for amylase levels. The reference range can be as wide as 20 to 300 U/L. However, elevated amylase levels of more than three times the upper limit of normal strongly support the diagnosis of acute pancreatitis.[42] Less than this is often associated with other conditions. Abnormally low amylase levels are not common but can be observed in cystic fibrosis, chronic pancreatitis, diabetes mellitus, obesity, and smoking.[43] Clinicians should be aware of such causes to help to interpret low amylase activity in patients.[44]

A finding of persistently raised total amylase and normal lipase should raise the possibility of macroamylasemia. Amylase creatinine clearance ratio (ACCR) or polyethylene glycol (PEG) precipitation are useful screening tests for macroamylase.[45] ACCR is easily calculated from paired random urine and serum amylase and creatinine measurements. A reduced ACCR of less than 1% is suggestive of macroamylasaemia, but each laboratory should investigate the transferability of the expected values to its own patient population and, if necessary, determine its own reference ranges.[46] An ACCR of greater than 5% suggests acute pancreatitis.[47] However, the ACCR is also known to be increased in diabetic ketoacidosis, renal disease, and after surgery.[48]

Clinical Significance

Amylase is primarily used in diagnosing pancreatic diseases. Amylase is a commonly measured enzyme due to the availability of inexpensive, easily automated methods. Although amylase is a sensitive indicator of acute pancreatitis, it is not specific as it can be elevated in several conditions unrelated to the pancreas.[49] Pancreatitis can be defined by two out of the three following criteria: abdominal pain, serum amylase and/or lipase levels more than three times the upper limit of normal, and abdominal imaging supporting characteristic findings of pancreatitis. Therefore, its clinical significance has been questioned.[50] In cases of elevated levels of amylase with little support for pancreatitis, alternative causes of hyperamylasaemia should be considered. [51]

Amylase is not useful in predicting the severity of an acute pancreatic episode or monitoring the condition. The magnitude of the increase in serum enzyme activity is not related to the severity of pancreatic involvement; however, the greater the rise, the greater the probability of acute pancreatitis. The lack of specificity of total amylase measurement has led to an interest in the direct measurement of P-type amylase instead of total enzyme activity for the differential diagnosis of patients with acute abdominal pain.[19] By applying the best decision limit (an activity equal to threefold the URL), the specificity of P-type amylase for diagnosing acute pancreatitis is greater than 90%. Sensitivity in late detection of this condition is also notably improved with P-AMY. P-type amylase values remain increased in 80% of patients with uncomplicated pancreatitis one week after onset when only 30% still show increased total amylase activity.[52] This long-standing increase in P-type amylase activity in serum also makes redundant the traditional measurement of total amylase in urine—a test performed to achieve better diagnostic sensitivity in the late phase of pancreatitis.[53]

Amylase inhibitors such as acarbose have been used in treating type 2 diabetes and have been shown to reduce hemoglobin A1C and peak postprandial glucose. Acarbose has also been shown to improve the remission of dumping syndrome in bariatric patients. The drug has also been shown to improve the risk of cardiovascular disease by slowing down the thickening of carotid arteries.[54] Elevated amylase can be seen in a wide variety of conditions. It is important for clinicians to have a clear, stepwise approach when hyperamylasemia is found. This will help avoid unnecessary hospitalization and delayed or inappropriate treatment.[55] Biliary tract diseases, such as cholecystitis, cause up to a fourfold increase in serum P-type amylase activity due to primary or secondary pancreatic involvement.[56]

Various intra-abdominal events can lead to a significant increase in serum P-type amylase activities, up to a fourfold increase and sometimes beyond. Such increases may be caused by leakage of P-type amylase from the intestine into the peritoneal cavity and then into circulation.[57] In renal insufficiency, serum amylase activity is increased in proportion to the extent of renal impairment (usually, no more than five times the URL).[58]

Cases of amylase-producing multiple myeloma have been described. Increased amylase activity is due to salivary-type hyperamylasemia in most patients (sialyl salivary type).[59] A common feature of the myeloma cell lines associated with hyperamylasemia is a translocation of chromosome 1, which harbors the gene for amylase. The link does not appear to be immunoglobulin class-specific. The onset of hyperamylasemia is reported to be associated with a rapid disease progression, extensive bone destruction, and increased mortality, hence serum amylase activity may be a useful prognostic ‘tumor marker’ (the activity decreases in response to treatment and increases at times of relapse) in patients with multiple myeloma.[60][61] The amylase isoenzyme in cases of ruptured ectopic pregnancy is not well characterized. In severe cases presenting late, the increased isoenzyme may be P-AMY (from pancreatic involvement related to peritonitis), even though S-AMY is present in the fallopian tube.[62]

Some patients with phaeochromocytoma/paraganglioma were found to have hyperamylasemia, usually the salivary isotype. Here, hyperamylasemia may be related to the hypertensive crisis and vasoconstriction leading to tissue hypoxia rather than being a result of tumor secretion and is often transient.[63] Salivary-type hyperamylasemia has also been observed in various conditions without salivary gland disorders, such as diabetic ketoacidosis, pneumonia, and postoperative states in a wide variety of surgical interventions, including extra abdominal procedures such as post-coronary bypass.[64]

Benign pancreatic hyperenzymemia, a syndrome first described by Gullo, is characterized by raised serum amylase, pancreatic isoamylase, lipase, and trypsin activities in asymptomatic subjects with no evidence of pancreatic disease by imaging. The syndrome occurs sporadically or in a familial form, and amylase activity fluctuates significantly with occasional transient normalization in some cases.[65] CFTR, SPINK1, and PRSS1 gene mutations do not seem to have a role in the etiology of the condition, and benign pancreatic hyperenzymemia cannot be explained by mutations in genes whose variants are known to be associated with pancreatitis or by mutations in other PRSS1/SPINK1 genes.[66] It was found that about one-third of patients with chronic nonpathological pancreatic hyperenzymemia had abnormally high fecal calprotectin concentrations and they recommended evaluating the finding for the possible link between intestinal ecology and pancreatic enzyme alteration.[67]

Damage of salivary glands, leading to salivary hyperamylasemia, has been seen in trauma or surgery to the salivary gland, radiation to the neck area involving the parotid gland and subsequently causing duct obstruction or calculi of the salivary glands.[14] Another cause of subclinical damage to the salivary gland is chronic alcoholism and anorexia nervosa. Salivary amylase activity is three times higher than normal in 10% of patients with alcoholism; this may also be related to chronic liver disease.[68]

Hyperamylasemia in anorexia nervosa is associated with vomiting, and indeed the finding of raised salivary amylase may provide a clue to concealed vomiting.[69] However, pancreatitis may occur in these patients, particularly during the course of refeeding and so measurement of plasma lipase and/or amylase isoenzymes may be justified to differentiate pancreatitis from salivary hyperamylasemia.[70]

Hyperamylasemia may be associated with various tumors caused by either an ectopic production of the enzyme by the tumors or perhaps an inflammatory response by the tumor cells resulting in the marked release of the enzyme normally produced in these tissues into the bloodstream.[71] The raised isoenzyme is almost exclusively salivary type in ovarian, lung cancer, multiple myeloma, and pheochromocytoma.[72] Amylase-producing tumors of the lung are rare and may comprise in total only 1 to 3% of all lung carcinomas, and in these cases, the salivary amylase isotype is generally found. Amylase-producing lung carcinomas are mostly adenocarcinomas, but hyperamylasemia has also been reported in small cell carcinoma.[73] Amylase activity has been suggested as a useful tumor marker for monitoring the patient’s treatment in amylase-producing lung carcinoma.[74] One study reported that 39% of patients with ovarian carcinoma had hyperamylasaemia, which is salivary-type dominant, and that salivary amylase may be useful in the evaluation of radiotherapy effectiveness in this context.[75]

Gut diseases, including mucosal inflammatory disease of the small intestine, mesenteric infarction, intestinal obstruction, appendicitis, and peritonitis, usually result in increased P-type isoamylase because of increased absorption of amylase from the intestinal lumen.[76] Gut perforation is associated with leakage of intestinal contents into the peritoneum, causing inflammation and absorption of amylase across the inflamed peritoneum. This can result in hyperamylasemia. Acidosis, which can be due to (1) ketoacidosis that results in increased S-type and P-type isoamylases or (2) nonketotic acidosis that results in increased S-type isoamylase, can cause hyperamylasemia.[77] Amylase increases may occur postoperatively, resulting in increased S-type and P-type isoamylases; however, an increase in salivary amylase is more common.[78] This may occur after extracorporeal circulation or nonabdominal surgery (e.g., 30% of patients undergoing cardiac surgery have elevated S-type isoamylase).[79]

Rare cases of hyperamylasemia have been reported in association with systemic lupus erythematosus (SLE), as well as with ciprofloxacin treatment.[80] Other causes of hyperamylasemia include pneumonia (increased salivary amylase), cerebral trauma, burns, abdominal aortic aneurysms (increased pancreatic amylase), drugs (increased salivary and/or pancreatic amylase), anorexia nervosa and bulimia (increased salivary amylase), non-pathologic (increased salivary and/or pancreatic amylase), and organophosphate poisoning. Postprocedure balloon-assisted enteroscopy has also been associated with elevated amylase levels; measure pancreatic amylase levels rather than total amylase levels following these procedures.[3][81] Elevated pancreatic enzymes can be found in critically injured trauma patients, even in the absence of true pancreatitis.[82] 

Quality Control and Lab Safety

For non-waived tests, laboratory regulations require, at the minimum, analysis of at least two levels of control materials once every 24 hours. If necessary, laboratories can assay QC samples more frequently to ensure accurate results. Quality control samples should be assayed after calibration or maintenance of an analyzer to verify the correct method performance.[83] To minimize QC when performing tests for which manufacturers’ recommendations are less than those required by the regulatory agency (such as once per month), the labs can develop an individualized quality control plan (IQCP) that involves performing a risk assessment of potential sources of error in all phases of testing and putting in place a QC plan to reduce the likelihood of errors.[84] 

Westgard multi-rules are used to evaluate the quality control runs. In case of a rule violation, proper corrective and preventive action should be taken before patient testing is performed. The criteria for acceptable performance for amylase assay by the Clinical Laboratory Improvement Amendments (CLIA) and College of American Pathologists (CAP) proficiency program is within ± 30% of the mean value of laboratory peer groups.[85]

Enhancing Healthcare Team Outcomes

Interprofessional healthcare team members must communicate effectively when laboratory results point towards a non-pancreatic cause. Knowing the different conditions that may affect amylase levels is also important.[86] Lipase is typically preferred instead of amylase due to its higher specificity. Lipase typically stays elevated for up to two weeks, while amylase concentrations remain elevated for up to five days. Therefore, amylase is not as clinically useful as lipase if there is a delay between symptom onset and the time the patient seeks medical attention.[18] 

The 2013 American College of Gastroenterology mentions that co-ordering lipase and amylase is neither cost-effective nor treatment advantageous. It also states that ordering amylase alone is unreliable and does not increase diagnostic efficiency compared to lipase.[12] If there is access to lipase testing, adding amylase increases the cost to the patient and has little value in supporting the diagnosis of pancreatitis.[87]



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