Insulin, Free and Total; Serum LabCorp

Free and total insulin is a test which measures the fraction of insulin that is bound by antibodies, as well as to measure total insulin following the removal of antibodies by special preparation. Most patients receiving insulin will eventually form antibodies that bind to insulin. These antibodies may or may not affect the activity and metabolism of insulin. Free insulin represents a portion of total insulin unbound by insulin binding antibodies in the circulation. This fraction serves as a measure of biologically active insulin, and also provides an indication of the true relationship between insulin dose and blood glucose in diabetic patients. Free insulin levels inversely correlate with periods of hypoglycemia or hyperglycemia in diabetic patients, where total insulin can remain fairly constant. Sequential free insulin measurements are useful in the evaluation of possible abnormal insulin absorption following subcutaneous injection. In these instances, the presence of insulin-binding antibodies can alter the pharmacokinetics of insulin and lead to an abnormal period of hyperglycemia in response to a delayed increase into he free fraction. Total insulin measures the sum of the antibody-bound and the free insulin fraction. This reflects both the insulin-binding capacity of circulating insulin-binding antibodies and the patient insulin dose. In patients with significant levels of antibodies, the bound fraction of insulin accounts for less than 90% of the total insulin. With constant insulin therapy, the total insulin levels are normally quite stable.