Insulin testing has several possible uses. Insulin is a hormone that is produced and stored in the beta cells of the pancreas. Insulin helps transport glucose , the body's main source of energy, from the blood to within cells. If a person makes too little insulin or is resistant to its effects, cells starve. If someone makes too much, as may happen with an insulin-producing tumor (insulinoma) for example, then symptoms of low blood glucose (hypoglycemia) emerge.
Insulin testing may be used to help:
Diagnose an insulinoma, verify that removal of the tumor has been successful, and/or to monitor for recurrence
Diagnose the cause of hypoglycemia in an individual with signs and symptoms
Monitor the amount of insulin produced by the beta cells in the pancreas (endogenous); in this case, a C-peptide test may also be done. Insulin and C-peptide are produced by the body at the same rate as part of the conversion of proinsulin to insulin in the pancreas. Both tests may be ordered when a health practitioner wants to evaluate how much insulin in the blood is made by the body and how much is from outside (exogenous) sources such as insulin injections. The test for insulin measures insulin from both sources while the C-peptide test reflects insulin produced by the pancreas.
Determine when a type 2 diabetic might need to start taking insulin to supplement oral medications
Determine and monitor the success of an islet cell transplant intended to restore the ability to make insulin, by measuring the insulin-producing capacity of the transplant
Insulin testing may be ordered with glucose and C-peptide tests. Insulin levels are also sometimes used in conjunction with the glucose tolerance test (GTT). In this situation, blood glucose and insulin levels are measured at pre-established time intervals to evaluate insulin resistance.
Insulin levels are most frequently ordered following a low glucose and/or when someone has acute or chronic symptoms of low blood glucose (hypoglycemia) caused by, for example, an insulinoma. Symptoms of hypoglycemia may include:
In serious cases, seizures and loss of consciousness
These symptoms may indicate low blood glucose but may also be seen with other conditions.
A health practitioner also may order insulin and C-peptide tests after an insulinoma has been successfully removed to verify the effectiveness of treatment and then order the tests periodically to monitor for recurrence.
Periodic testing may also be used to monitor the success of an islet cell transplant by measuring the insulin-producing capacity of the transplant.
Insulin for injection used to come strictly from animal sources (cow and pig pancreas cells). Most insulin used today is synthetic, made by biochemical synthesis to identically match the biological activity of the insulin produced by human cells.
There are different pharmaceutical formulations of insulin with different properties. Some are rapid-release and quick-acting and others are slow-release preparations that act over a prolonged period. Diabetics may take mixtures and/or different types of insulin throughout the day.
Insulin assays are designed to measure endogenous human insulin. However, different assays react variably with exogenous (animal or synthetic) insulin. If someone is receiving insulin, these effects should be clarified with the testing laboratory. If several or periodic insulin assays will be performed, they should be analyzed by the same laboratory to ensure consistency.
The insulin tolerance test (ITT) is not widely used, but is one method for determining insulin sensitivity (or resistance), especially in obese individuals and those with PCOS. This test involves an IV-infusion of insulin, with subsequent measurements of glucose and insulin levels.
If someone has developed antibodies against insulin, especially as a result of taking non-human (animal or synthetic) insulin, these can interfere with insulin testing. In this case, a C-peptide may be performed as an alternative way to evaluate insulin production. Note also that most people with type 1 diabetes will also have autoantibodies against insulin.
This article was last reviewed on June 18, 2014. | This article was last modified on February 24, 2015.
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