Insulin and C-Peptide

The Insulin and C-Peptide test (C-peptide and insulin panel) is used to assess pancreatic beta-cell activity and native insulin secretion, whether or not a patient receives exogenous insulin therapy. When considered with concurrent plasma glucose and C-peptide, results support the workup of hypoglycemia and help distinguish endogenous hyperinsulinemia (eg, insulinoma) from exposure to injected insulin. C-peptide provides an antibody-independent indicator of endogenous insulin synthesis in the presence of insulin autoantibodies, assists in evaluating insulin resistance, and informs treatment choices in type 2 diabetes, including whether to initiate insulin or other antihyperglycemic agents. The combination may also be used to monitor function of transplanted islet tissue.

Insulin is produced by pancreatic beta cells and is released in response to rising blood glucose. It drives glucose uptake in insulin-sensitive tissues and modulates hepatic glycogen synthesis and lipogenesis. When insulin secretion is inadequate or its action is impaired, hyperglycemia develops with the chronic metabolic complications characteristic of diabetes mellitus. Type 1 diabetes reflects deficient insulin production, whereas type 2 diabetes is defined by insulin resistance. Excess insulin (hyperinsulinemia) lowers plasma glucose and can precipitate hypoglycemia; autonomous secretion from an insulinoma can produce pronounced elevations. Dysglycemia is also seen with metabolic syndrome, disorders of the adrenal and pituitary axes, and polycystic ovary syndrome. Insulin autoantibodies can interfere with some insulin immunoassays; in such cases, measuring C-peptide provides an alternative marker that reflects endogenous insulin secretion.