Test Summary

The determination of insulin in serum is primarily used for the diagnosis of glycemic disorders in diabetic and pre-diabetic patients in the assessment of insulin resistant syndromes. Insulin is synthesized by the pancreatic beta cell as a precursor, proinsulin. Proinsulin is processed to insulin and C-peptide, a contiguous peptide between the insulin A and B chains, as it passes through the cell. The C-peptide in the proinsulin ensures correct folding and processing of proinsulin as it passes through the cell. Both insulin and C-peptide are released together from the beta cells in response to increased glucose levels. Because of differences in half-life and hepatic clearance, peripheral blood levels of C-peptide and insulin are no longer equimolar but remain highly correlated. A steady-state plasma glucose test in individuals undergoing an insulin suppression test to assess insulin resistance found that the combination of insulin and C-peptide was a better indicator of insulin resistance than either one individually.

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