C-Peptide

C-peptide is a chemical compound that is formed along with insulin as a result of the cleavage of the proinsulin molecule. C-peptide and insulin are released into the blood in equivalent amounts. However, C-peptide is more informative for the diagnosis of diabetes due to several of its features: first, it has low biological activity, so its concentration is more stable than insulin levels. C-peptide does not enter the body externally, so the study helps to determine the amount of endogenous insulin even while taking insulin preparations. If a patient is taking insulin-containing preparations, it is impossible to determine the amount of insulin that is produced in the body without cessation of therapy. C-peptide helps to solve this problem.

The peptide is also used to diagnose insulinoma: a test is taken in the morning, then insulin is injected intravenously at the rate of 0.1 units/kg body weight. Then after 1 hour, the blood is taken again: if the level of C-peptide decreases by less than 50%, it indicates the presence of insulinoma. ("Laboratory Methods of Diagnosis", Prof. A.A.Kishkun, 2007).