Detailed explanation of the three diabetes mellitus tests and their clinical significance.
I. Detection methods and clinical significance of insulin antibodies
1. Biochemical characteristics and pathophysiological effects of insulin antibodies
In 1983, Banting found that the presence of anti-insulin substances in the serum of diabetic patients treated with insulin, and later proved that such substances are gamma globulin, named insulin antibodies. The production of insulin antibodies is related to the immunogenicity of insulin preparations, and a large production of insulin antibodies can lead to insulin insensitivity in patients.
Insulin antibodies are mainly IgG, and IgM, IgD and IgE insulin antibodies can be found in a few drug recipients, and IgE antibodies are mainly found in patients who have allergic reactions to insulin.
95-99.9% of insulin antibodies in the serum are bound to insulin, and a small proportion is free. The rate of binding of insulin to antibodies varies depending on the insulin concentration in the blood. The blood includes two types of antibodies: high affinity-low binding capacity and low affinity-high binding capacity. The former has an affinity constant of 109 to 1010 mol/L and a binding capacity of 108 mol/L; the latter has an affinity constant of 106 to 108 mol/L and a binding capacity of 10ˉ7 mol/L. Low affinity antibodies account for more than 90% of the total binding capacity, so they are of greater physiological significance. The physiological roles of insulin antibodies are: neutralizing insulin in blood; delaying insulin degradation and prolonging insulin lan-life; releasing insulin bound to antibodies; playing the role of insulin transporter protein; antigen-antibody complex can activate complement, and long-term effects can cause or aggravate microangiopathy.
2. Detection methods and precautions for insulin antibodies
(1) The main methods are: immunoelectrophoresis, complement binding method, coagulation test, immunoprecipitation method, gel filtration method and radioimmunoassay. The radioimmunoassay is the most commonly used method because of its high sensitivity and simplicity. The unit of binding capacity is usually expressed as U/L or mol/L.
The quantitative determination of insulin antibodies requires the prior removal of insulin from the sample. Insulin is removed by adding HCL to the serum to dissociate the antibody from insulin, and then adsorbing insulin with activated carbon. To remove insulin, add serum to HCL to dissociate the antibody from insulin, then adsorb the insulin using activated charcoal or precipitate the antibody using polyethylene glycol, and the treated palm can be used for antibody dilution tests.
(2) Precautions
In order to avoid the effect of insulin on antibody determination, the recipient blood should be selected in the morning before insulin injection.
3, the clinical significance of insulin antibody determination
(1) Relationship between insulin antibody and stability of diabetes mellitus
Dixon observed 24 cases of onset of 35-year-old stable diabetic patients, the insulin antibody binding capacity of 0.5 to 9.0 U / L; 23 cases of onset of 23-year-old unstable diabetic patients, the insulin antibody binding capacity < 0.5 u / L. This indicates that moderately elevated insulin antibody to regulate the level of free insulin in the blood is beneficial to the stability of the disease. The stabilization of the disease is mainly dependent on the patient's residual islet function, and insulin antibodies can help to maintain the blood free insulin at a more stable level, so that it does not become too high or too low, and thus acts as a buffer. This is of great significance to those with almost complete loss of beta-cell function.
(2) Insulin antibodies and the spontaneous remission period of type 1 diabetes
Type 1 diabetic patients early by insulin treatment for a period of time, beta cell function has a certain degree of recovery, the disease remission, at this time can be less or stop using insulin treatment, known as the “spontaneous remission period”. Insulin antibodies can affect the length of this remission period. Since antibodies can bind and consume endogenous insulin, the relapse rate of patients with antibodies is significantly higher than that of the group without antibodies, i.e., the level of insulin antibodies in patients’ blood is negatively correlated with the length of spontaneous remission.
(3) Insulin antibodies and hypoglycemia
Spontaneous hypoglycemia at night in diabetic patients may be related to insulin antibodies. Insulin antibodies can bind a large amount of insulin in blood, and when blood acidity increases at night and free insulin in blood decreases faster, insulin dissociates from antibodies and is released to cause hypoglycemic syndrome. When the use of high-purity insulin reduces or disappears insulin antibodies, hypoglycemic episodes can also be reduced or stopped.
(4) Insulin antibodies and gestational diabetes
The level of insulin antibodies decreases after a diabetic woman becomes pregnant. This is due to the increase of maternal estrogen and progesterone which inhibit antibody production. It has been reported that maternal insulin antibodies are deposited in the pancreas of the fetus through the placenta, which leads to hereditary diabetes. Gestational diabetics should apply pure insulin to reduce insulin antibody levels.
(5) Insulin antibodies and late comorbidity of diabetes mellitus
Insulin antibodies may promote or aggravate diabetic microangiopathy. Insulin antibody IgG has been shown to precipitate in the glomeruli of experimental animals, resembling glomerulonodular sclerosis. It has been clinically proven that patients with high levels of insulin antibodies, diabetic nephropathy and proliferative retinopathy occur earlier.
Insulin antibodies and insulin resistance
II. Insulin
Insulin is secreted by pancreatic beta cells, it contains 51 amino acids and has a molecular weight of 5800. insulin is closely related to sugar, fat and protein metabolism. It promotes the uptake and utilization of glucose, or the formation of hepatic glycogen or fat by the liver and peripheral tissues, or enters the triglyceride cycle to produce energy by oxidation. It stimulates lipid synthesis, inhibits lipolysis and ketone body production. It also promotes protein synthesis and inhibits protein catabolism.
[Reference value
The mean fasting plasma insulin is 100.45±62.423 pmol/L (radioimmunoassay). Its level is related to body weight. It is 96.863±63.858pmol./L for non-overweight adults; 108.343±58.835pmol/L for overweight adults; 146.37±134.89pmol/L for obese adults. (The weight standard is according to the Shanghai Diabetes Research Collaborative Group. Not overweight person means: subject’s weight/ideal weight 1.20).
【Clinical significance】.
Diabetes mellitus typing and diagnosis: Diabetes mellitus is divided into two main categories. type I is also known as insulin-dependent type (IDDM for short). Fasting plasma insulin is extremely low, and the rise and fall after oral glucose test (OGTT) or bun meal is extremely slow, or even no change at all. It indicates that its pancreatic β-cell function is extremely poor; Type II is also called non-insulin-dependent type (abbreviated as NIDDM). This type includes two types: insulin-requiring and non-insulin-requiring. It is characterized by fasting hyperinsulinemia; after OGTT trial meal, insulin release is delayed and the peak is shifted back, but the total release is not low, and about 12% of patients show low response or delayed response, indicating that type II diabetes is fundamentally different from type I. It is presumed that the pathogenesis of high responders is based on insulin antibodies; low responders are based on inadequate β-cell secretion and the factor of insulin resistance. type I and type II need insulin therapy, due to long-term insulin injections, their serum insulin antibody positivity rate is as high as 96% and 87%, respectively. This interferes with the results of insulin radioimmunoassay, so this assay has gradually been replaced by C-peptide.
Diagnosis and prognosis of liver disease: The liver is an important organ for the stabilization of blood glucose level and insulin degradation and metabolism in the fiber organism, so glucose metabolism disorders can occur in liver disease, and glucose metabolism disorders are related to the degree of liver damage, most significantly in patients with hepatic sclerosis. Blood insulin measurement is valuable for judging the severity of liver damage, rational treatment and estimating prognosis, and fasting blood insulin levels in patients with hepatic sclerosis are 2 to 3 times higher than normal. Serum insulin levels at all time phases of oral glucose test were 1.3,3.6 and 8 times higher than normal at 60,120 and 180 minutes, respectively, compared with the normal group. In patients with acute viral hepatitis and chronic hepatitis, although fasting blood insulin levels were normal, the values of oral glucose test in all time phases were also higher than those of the normal group. The phenomenon of decreased glucose tolerance and increased plasma insulin levels in liver disease suggests that the uptake of insulin in the portal vein by the liver is reduced when hepatocytes are damaged. The treatment of such patients should try to avoid those measures that can aggravate the disorder of glucose metabolism and β-cell burden.
III. Clear C-peptide measurement
C-peptide and insulin are isomolecular peptides that are split from insulinogen and are not inactivated by liver enzymes, and their off-life is 10-11 minutes, so their blood concentration can better reflect the reserve function of pancreatic β-cells. C-peptide measurement also has the advantage of not being affected by external insulin.
Reference value
Radioimmunoassay: 0.4±00.20nmol/L in normal adults.
Pathological variation
Elevated: In mild diabetic patients, fasting glucose is not much elevated and C-peptide is mostly higher than normal. In patients with insulinoma, serum C-peptide is mostly increased if insulin antibodies are present in the blood. If serum C peptide can still be measured after total pancreas resection in patients with pancreatic tumor, it indicates that the surgery failed to remove all the pancreatic tissue. If the serum C-peptide is still detectable after total pancreatic resection, it indicates that the pancreatic tissue has not been completely removed. Later, it becomes positive again, suggesting tumor recurrence or metastasis.
Decrease: In heavy diabetic patients with fasting glucose >200mg/dl, serum C-peptide is decreased, and in ketoacidosis, serum C-peptide level is extremely low.
Diagnostic value of C-peptide measurement in hypoglycemic syndrome: Generally, plasma insulin measurement is used to identify hypoglycemic syndrome. For diabetes mellitus that has been treated with insulin, C-peptide measurement is sometimes necessary to determine the patient’s endogenous insulin level. Diabetic patients with insulinoma need to be distinguished from diabetic patients with hypoglycemia due to liver and kidney failure. In hypoglycemia caused by overuse of exogenous insulin or forgetting to eat, C-peptide is constantly reduced. This is because exogenous insulin inhibits the secretion of beta cells.
The significance of C-peptide in liver disease: In cirrhosis, plasma insulin tends to increase, which is due to the decrease of insulin uptake and degradation by the liver, but fasting blood sugar is normal, C-peptide is normal, and the peripheral blood C-peptide/insulin ratio decreases because the liver does not uptake C-peptide, and similar phenomenon occurs after test meals.
Application of C peptide in islet transplantation and pancreas transplantation: Islet transplantation or pancreas allograft is a new method to treat type I diabetes. Whether its beta cells can secrete insulin can be observed periodically by C-peptide measurement.
Diagnosis of insulinoma: Insulinoma is a pancreatic β-cell tumor, the amount of insulin contained in the tumor is equivalent to 4 to 40 times of the normal pancreas of the same volume, and the amount of secretion is 2 to 6 times, other relevant experimental data are as follows.
During the attack, the blood essence can be as low as 30mg% to 35mg%; during the attack, the fasting blood sugar is not less than 60mg%.
At the 2nd hour of OGTT, blood glucose drops again and lasts for 3 to 4 hours.
Methylsulfonylurea test: intravenous injection of methylsulfonylurea 1 gram, blood sugar began to fall after 2 minutes, and reached the lowest level (below 25mg%) in 30 to 45 minutes, and often did not exceed 40mg% in 90 to 180 minutes.
Leucine test: After intravenous injection of leucine, if the insulin level in the blood is significantly increased, it has the significance of confirming the diagnosis.
Diagnosis of hyperthyroidism: In the early stage of hyperthyroidism, the β-cell function of the patient is still normal, but because thyroxine can cause hyper-metabolism and accelerated insulin degradation, the fasting insulin level of the patient is higher than normal; after taking glucose, insulin release is significantly higher than normal; but the glucose tolerance test is similar to that of normal people. Long-term severe hyperthyroidism can lead to pancreatic islet beta cell failure and development of diabetes.
In patients with acromegaly, insulin in the blood is increased.
Blood insulin is increased in patients with Cushing’s his syndrome, spontaneous or steroid-induced.
Patients with dystrophic myasthenia gravis.