The connecting peptide, or C-peptide, is a short 31-amino-acid protein that connects insulin’s A-chain to its B-chain in the proinsulin molecule
Patients with diabetes may have their C-peptide levels measured as a means of distinguishing type 1 diabetes from type 2 diabetes
Measuring C-peptide can help to determine how much of their own natural insulin a person is producing as C-peptide is secreted in equimolar amounts to insulin.
C peptide also has a half-life three to four times that of insulin
Liver metabolizes a large and variable amount of insulin secreted into the portal vein but does not metabolize C-peptide
A very low C-peptide confirms Type 1 diabetes and insulin dependence
REFERENCES
Chapter 18: Disorders of the Endocrine Pancreas. Pathophysiology of Disease: An Introduction to Clinical Medicine, 7e. Janet L. Funk, MD