Insulin is secreted by the cells in response to elevated blood glucose, certain amino acids, ketones, fatty acids, cortisol, growth hormone, glucagon, gastrin, secretin, cholecystokinin, gastric inhibitory peptide, estrogen, and progesterone. Because of normal feedback mechanisms, high insulin levels inhibit secretion of insulin. Elevated blood levels of somatostatin, epinephrine, and norepinephrine also inhibit insulin secretion.

Abnormally elevated serum insulin levels are seen with insulin- and proinsulin-secreting tumors (insulinomas), with reactive hypoglycemia in developing diabetes mellitus, and with excessive administration of exogenous insulin.

A blood glucose level is usually obtained with the serum insulin determination. Serum insulin levels may also be measured when glucose tolerance tests are performed.

• Administration of insulin or oral hypoglycemic agents within 8 hours of the test may lead to falsely elevated levels.
• Failure to follow dietary restrictions before the test may lead to falsely elevated levels.
• Therapy with drugs containing estrogen and progesterone may produce elevated levels.

• Evaluation of postprandial ("reactive") hypoglycemia of unknown etiology
• Support for diagnosing early or developing non-insulin-dependent diabetes mellitus as indicated by excessive production of insulin in relation to blood glucose levels (best demonstrated with glucose tolerance tests or 2-hour postprandial tests)
• Confirmation of functional hypoglycemia (i.e., no known physiological cause for the hypoglycemia) as indicated by circulating insulin levels appropriate to changing blood glucose levels
• Evaluation of fasting hypoglycemia of unknown etiology
• Support for diagnosing insulinoma as indicated by sustained high levels of insulin and absence of blood glucose-related variations
• Evaluation of uncontrolled insulin-dependent diabetes mellitus
• Differentiation between insulin-resistant diabetes, in which insulin levels are high, and non-insulin-resistant diabetes, in which insulin levels are low
• Support for diagnosing pheochromocytoma as indicated by decreased levels

Nursing Care Before the Procedure

Client preparation is the same as that for the related blood glucose test (e.g., fasting blood glucose, glucose tolerance test) with which the serum insulin determination is performed.

The general procedure is the same as that for the related blood glucose test. Blood samples for serum insulin determinations are obtained in red-topped tubes and then packed in ice. The samples should be handled gently to avoid hemolysis and sent immediately to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for the related blood glucose test.

• Assess the client for signs of hypoglycemia, which may occur in response to fasting or excessive blood glucose load.
• Resume foods and any medications withheld before the test.
• Abnormal values: Note and report decreased or increased levels and relation to type I or II diabetes mellitus, respectively, and response to glucose intake.
• Critical values: Notify the physician at once if fasting level is greater than 30 U/mL. Prepare client for glucose administration.