Most patients receiving insulin will eventually form antibodies that bind insulin. These antibodies may or may not affect the activity and metabolism of insulin. Laboratory tests may be used to identify patients with insulin antibodies. Patient antibodies may affect the normal insulin measurements. Free and total insulin is a test to measure the fraction of insulin that is bound by antibodies and to measure total insulin after the removal of antibodies by special preparation. 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 provides an indication of the true relationship between insulin dose and blood glucose in diabetic patients. Levels of free insulin correlate inversely with periods of hypoglycemia or hyperglycemia in diabetic patients, where total insulin can remain fairly constant. Sequential free insulin measurements are useful to evaluate possible abnormal insulin absorption after subcutaneous injection. In this case, 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 in the free fraction. Total insulin measures the sum of the antibody-bound and the free insulin fraction. It 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 >90% of the total insulin. With constant insulin therapy, the total insulin levels are normally quite stable.