Insulin testing: Preparation, Reference Range and Understanding the Result
Insulin testing is used to assist in identifying causes of hypoglycemia, especially upon signs and symptoms of hypoglycemia which could be neurohypoglycopenic and autonomic.
Hypoglycemia is a condition in which plasma glucose levels is less then 55 mg/dL. A 72-hour fasting test is performed in this case. Insulin testing is also used to diagnose early type 2 diabetes, in which there is a relatively increased production of insulin with a concurrent increase in blood glucose levels.
In addition, insulin testing is used to help differentiate type 1 and type 2 diabetes.
Insulin is an anabolic hormone which is responsible for promoting glucose uptake, glycogenesis, lipogenesis, and protein synthesis of skeletal muscle and fat tissue through the tyrosine kinase receptor pathway. It also helps in the regulation of plasma glucose homeostasis, as it counteracts glucagon and other catabolic hormones such as, epinephrine, glucocorticoid, and growth hormone. Insulin secretion is induced by elevated plasma glucose levels. Insulinoma is an example of hypersecretory state, in which insulin is secreted at a high rate independent from the plasma glucose level. Here even in hypoglycemic state, high insulin levels are found. Oral administration of glucose is more effective in increasing insulin secretion than intravenous glucose .
A 72-hour fasting test is used to identify causes of hypoglycemia. The patient is instructed to fast, and plasma glucose, insulin, proinsulin, and C-peptide levels are measured every 6 hours until the plasma glucose level is less than 65 mg/dL. Once the plasma glucose level is less than 65 mg/dL, the testing frequency is increased to every 1 to 2 hours. When plasma glucose levels are less than 45 mg/dL accompanied by signs and symptoms of hypoglycemia, a blood sample is collected and tested for glucose, insulin, proinsulin, C-peptide, beta-hydroxybutyrate, and sulfonylurea levels. The response of the blood glucose level is measured after giving 1 mg of intravenous glucagon to the patient.
Insulin level should be less then 25 mIU/L when done
on fasting. After 30 minutes of glucose administration it should be 30
to 230 mIU/L. 1 hour after glucose administration it should be 18 to
276 mIU/L and 16 to 166 mIU/ after 2 hour. 3 hours after glucose
administration it should return to normal range that is less then 25
Looking in to the result of insulin test and C-peptide test it is easier to diagnose certain medical condition. High insulin and C-peptide levels indicate a condition called insulinoma, insulin secretagogue and congenital hyperinsulinism in which mutation in insulin-secreting gene happens.
High insulin levels and low C-peptide levels indicates excessive insulin administration. An extremely elevated insulin level and high insulinÃ¢â¬âtoÃ¢â¬âC-peptide ratio is an indication of autoimmunity to insulin or insulin receptor. This is common in patients receiving insulin or those who have autoimmune diseases such as systemic lupus erythematosus (SLE) or Hashimoto thyroiditis. The cause is due to dissociation of postprandial insulin that is bound antobodies after 1 hour. Low insulin and C-peptide levels and high growth factor 2 level indicates nonÃ¢â¬âbeta cell tumors.
Conditions associated with increased insulin resistance include:
In all these cases beta cell compensates via hypersecretion of insulin.
Conditions associated with beta-cell destruction include the following:
Insulin levels may be falsely elevated by the following:
Insulin levels may be falsely decreased by the following: