Insulin and C-Peptide, Serum is a comprehensive test that measures both insulin and C-peptide levels to evaluate pancreatic beta cell function and endogenous insulin production. Insulin is the primary hormone regulating glucose metabolism, while C-peptide is a byproduct of insulin synthesis that provides a reliable marker of endogenous insulin secretion. This combined test is particularly valuable for distinguishing between type 1 and type 2 diabetes, as C-peptide levels remain detectable in type 2 diabetes but are typically absent in type 1 diabetes. The test also helps evaluate insulin resistance, assess beta cell reserve, and monitor patients on insulin therapy. C-peptide measurements are unaffected by exogenous insulin administration, making them essential for evaluating patients who may be taking insulin injections.