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Human C-peptide is a 31-amino-acid chain with a molecular mass of approximately 3,020 Da. Metabolically inert, it originates in the pancreatic B cells as a by-product of the enzymatic cleavage of proinsulin to insulin. In this process, insulin and C-peptide are split from the prohormone and secreted into the portal circulation in equimolar concentrations. Within limits, C-peptide levels can serve as a valuable index to insulin secretion. Therefore, low C-peptide levels are to be expected where insulin secretion is diminished, as in insulin-dependent diabetes, or suppressed, as a normal response to exogenous insulin, whereas elevated C-peptide levels may result from the increased B-cell activity observed in insulinomas.
Normal range: 0.9–7.1 ng/mL.
For estimating insulin levels in the presence of antibodies to exogenous insulin
Diagnosis of factitious hypoglycemia due to surreptitious administration of insulin in which high serum insulin levels occur with low C-peptide levels
Evaluation of insulinoma
Monitoring pancreatic and islet cell transplant function
Type 2 DM
Exogenous insulin administration (e.g., factitious hypoglycemia)
Type 1 DM
C-peptide serum levels correlate with insulin levels in blood, except in islet cell tumors and possibly in obese patients.