Amylase
Reference values of a-amylase activity: in the blood serum - 25-220 IU / l;in the urine 10-490 IU / l.
a-Amylase belongs to the group of hydrolases that catalyze the hydrolysis of polysaccharides, including starch and glycogen, to simple mono- and disaccharides. The pancreatic and salivary glands are the richest in amylase. Amylase is secreted into the blood mainly from these organs. Human blood plasma contains two types of a-amylases: pancreatic( P-type),
expression pancreatic, and salivary( S-type) produced by the salivary glands.
Under physiological conditions, the activity of this enzyme in the blood serum is represented by pancreatic amylase by 40%, by 60% by salivary amylase.
Determination of a-amylase activity is important in the diagnosis of pancreatic diseases. An increase in the activity of a-amylase in the blood serum in 2 times or more should be regarded as a symptom of a pancreatic lesion. A small hyperamilazemia gives reason to suspect pancreatic pathology, but sometimes it is possible with diseases of other organs.
P-type a-amylase is excreted mainly in the urine, which is considered one of the reasons for the more informative urine amylase than serum in terms of assessing the functional state of the pancreas. It is believed that 65% of the enzyme activity in urine is due to pancreatic amylase. This explains the fact that in acute pancreatitis it is it that increases in serum( up to 89%) and especially in urine( up to 92%), without changes in salivary gland amylase.
In acute pancreatitis, the activity of blood and urine amylase increases 10-30 times. Hyperamilazemia occurs at the onset of the disease( after 4-6 hours), reaches a maximum after 12-24 hours, then rapidly decreases and comes to normal on the 2-6th day. The level of increase in serum amylase activity does not correlate with the severity of pancreatitis [Banks PA, 1982].
The activity of amylase in the urine begins to rise 6-10 hours after an acute attack of pancreatitis and returns to normal after 3 days. In some cases, the activity of amylase in the urine has two increase waves for 3 days. The diagnostic sensitivity of serum amylase detection for acute pancreatitis is 95%, specificity is 88% [Wallach J. M. D. et al., 1996].
Acute pancreatitis can occur without increasing the activity of amylase( in particular, with pancreatonecrosis).In the first day after the onset of the disease, a normal level of urine amylase activity is detected in 25% of patients with abortive pancreatitis, 20% with fat, and 10% with hemorrhagic. More accurate information is obtained by studying the activity of amylase in the daily volume of urine. An important and, in some cases, crucial for the recognition of the recurrent form of acute pancreatitis is a repeated increase in the activity of blood and urine amylase during recurring recurrences of the pain syndrome. With different forms of acute pancreatitis, the dynamics of increasing a-amylase in blood and urine is of different nature. So, for edematous pancreatitis, short-term amylase is characteristic for 1-3 days of the disease;for fatty pancreonecrosis - high and long amylase, and for hemorrhagic pancreatic necrosis - short-term hyperamilazemia on the 3rd day of the disease. Pathogenetically, hyperamilazemia develops as a result of blockade by the edematous interstitial tissue of the excretory ducts of the pancreas and is most typical for fatty pancreocerebral necrosis. With hemorrhagic pancreatic necrosis, a sharp increase in the activity of a-amylase in the blood is noted, followed by a rapid decrease in it, which reflects the progression of necrosis.
Hyperamylasemia and hyperamilazuria are important, but not specific for acute pancreatitis;in addition, the increase in their activity may be short-lived. To increase the informativeness of the obtained results of the study, it is useful to combine the activity of blood and urine amylase with a parallel determination of the creatinine concentration in urine and serum. Based on these data, the index of amylase-creatinine clearance is calculated according to the following formula [Bohger, M.M., 1984]: [(AMXKrC) /( KpMxAC)] x100, where AM is urine amylase;Ac - amylase of blood serum;KrM - creatinine in urine;KrS - kre-atinin in the blood serum. Normally, the amylase-creatinine index is not more than 3, its increase is considered a sign of pancreatitis, as pancreatitis increases the level of true pancreatic amylase, and its clearance is 80% faster than the clearance of saliva amylase. Nevertheless, it was found that in acute pancreatitis the clearance of both P- and S-amylases increases significantly, which is explained as follows. In healthy people, the serum amylase is first filtered in the renal glomeruli, and then reabsorbed by tubular epithelium. In acute pancreatitis, the tubular reabsorption mechanism is inhibited by excessive excretion of P- and S-amylase. Since the amylase activity of the serum in acute pancreatitis is mainly due to P-amylase, then as the clearance of total amylase increases, the clearance of the P-amylase increases. In acute pancreatitis, the activity of serum amylase and the amylase-creatinine clearance is usually increased by suppressing the renal tubular reabsorption of amylase. In diseases under the guise of pancreatitis, serum amylase activity may increase, but the amylase-creatinine clearance remains normal, since there is no tubular defect. It is very important for this study to collect blood and urine at the same time.
In patients with chronic pancreatitis, the activity of amylase in the blood and urine increases( in 10-88% and 21-70% of patients, respectively) during the exacerbation of the process and when there are obstacles to the outflow of pancreatic juice( inflammation, edema of the pancreas head and compression of ducts, scar stenosispapilla of the duodenum, etc.).In the sclerotic form of pancreatitis, hyperamilazemia is also determined by the degree of impaired ductility and the functional capacity of the remaining part of the gland. To increase the sensitivity of the study of the activity of blood and urine amylase in chronic pancreatitis, A.I.Khazanov( 1997) recommends that they be analyzed on the first day of their stay in the hospital, then at least two times after instrumental research( fibrogastroduodenoscopy, X-ray examination of the stomach and intestines, etc.), as well as at the time of pain in the abdomen. At the same time, the sensitivity of the test rises from 40 to 75-85%.
In chronic pancreatitis with fibrotic changes in the pancreas, exacerbations, often pronounced and common, are accompanied by a relatively small increase in the activity of amylase.
Due to impaired functional ability of the pancreas, hyperamilazemia can often be absent in acute purulent pancreatitis( with extensive "total" pancreatic necrosis necrosis).
In pancreatic cancer, the activity of amylase in the blood and urine may increase, but often remains within normal limits or even decreases.
Evaluation of the results of the study of the activity of amylase in blood and urine is complicated by the fact that the enzyme is also found in salivary glands, large intestine, skeletal muscles, kidneys, lungs, ovaries, fallopian tubes, prostate gland. Therefore, the activity of amylase can be increased in a number of diseases having a similar pattern with acute pancreatitis: acute appendicitis, peritonitis, perforated ulcer of the stomach and duodenum, intestinal obstruction, cholecystitis, mesenteric vascular thrombosis, as well as pheochromocytoma, diabetic acidosis,for heart defects, after liver resection, for taking large doses of alcohol, for taking sulfonamides, for morphine, for thiazide diuretics, for oral contraceptives. The increase in amylase activity in these diseases is due to a number of reasons and is reactive in most cases. Due to the large reserves of amylase in acinar cells, any violation of their integrity or the slightest difficulty in the outflow of the secretion of the pancreas can lead to a significant ingress of amylase into the blood. In patients with peritonitis, an increase in amylase activity may reflect the multiplication of amylase-forming bacteria. Usually, the activity of a-amylase with these diseases increases in blood 3-5 times.
A decrease in the activity of a-amylase in the blood is possible with thyrotoxicosis, myocardial infarction, pancreatic necrosis.
Reference values of pancreatic a-amylase activity: in the blood serum - 30-55% of total amylase( average 43%) or 17-115 IU / l;in urine, 60-70% of the total amylase( an average of 65%).
Serum contains up to 3 isoenzymes of a-amylase, the main ones are P- and S-types, that is, pancreatic and salivary glands. Pancreatic amylase is better excreted in the urine than the isoenzyme of the salivary glands. An increase in the activity of salivary amylase is noted with stomatitis, Parkinsonism, a decrease in mental agitation or depression, with an anacid state of gastric secretion.
The main value of determining P-type a-amylase is that the increase in its activity is highly specific for pancreatic diseases. Pancreatic α-amylase increases with acute pancreatitis. The activity of total amylase in this case is increased due to the pancreatic fraction. The diagnostic sensitivity of the pancreatic fraction of amylase in the blood serum for acute pancreatitis is 92%, specificity 85% [Wallach J. M. D. et al., 1996].
Determination of the activity of the pancreatic fraction of a-amylase is particularly important in chronic pancreatitis in patients with a normal level of total amylase. In patients with chronic pancreatitis pancreatic amylase is 75-80% of total blood amylase. Increase in pancreatic amylase indicates an exacerbation of chronic pancreatitis, and a decrease in exocrine pancreatic insufficiency with atrophy of acinar tissue and organ fibrosis in patients suffering from this disease for a long time.
The activity of pancreatic a-amylase, in addition to the diagnosis of acute pancreatitis, is also determined after surgery on the abdominal organs for the purpose of early diagnosis of the complication development - postoperative pancreatitis. Pancreatic a-amylase in the urine rises with acute pancreatitis, and makes up the bulk of the total amylase, as it is excreted better in the urine than the salivary fraction.
Activity of the pancreatic fraction of a-amylase, unlike the general one, does not increase with parotitis, diabetic ketoacidosis, lung cancer, acute gynecological diseases. However, the test can be false-positive in other diseases that do not affect the pancreas.
Amylase deficiency is the norm in the first half of the child's life, which is why small children are unable to digest starch. The normal level of activity of pancreatic amylase is reached by 9 months. The cases of late formation of the activity of this enzyme can be genetically conditioned, while the type of inheritance is autosomal dominant, which is assumed in connection with family accumulation of pathology. Clinically, the disease manifests itself in excess of starch in food: there is a frequent loose, voluminous, mushy or watery stool with an acidic odor. Despite the high caloric content of the diet, the child does not add to the mass. The pancreatic juice of such children is transparent, the activity of the enzyme amylase is either absent or significantly reduced. The pronounced multiplication of bacteria in the small intestine, characteristic of this disease, worsens the absorption of other nutrients. The appointment of a starched diet leads to the disappearance of all symptoms and helps to restore body weight.
It should be remembered physiological absence or insufficient activity of pancreatic amylase in children of the first year of life and not to introduce into their nutrition an excess of flour porridge, often causing the development of diarrhea and malabsorption syndrome.