This kit is used for quantitative determination of C-peptide (CP) in human serum in vitro. C-peptide is a connecting peptide between proinsulin α and β chains, which is composed of 31 amino acids with a molecular weight of 3021. In islet β cells, the single chain polypeptide proinsulin membrane-dependent protease is cut into equimolar insulin (INS) and C-peptide. Its main function is to regulate the insulin synthesis process: during the synthesis of INS, it regulates the disulfide bond connection between α and β chains, promotes the folding of proinsulin molecules, and promotes the proinsulin decomposition into ins and CP. Because CP is almost not absorbed by liver, muscle and adipose tissue, metabolism clearance is slow. The level of CP in peripheral blood is higher and more stable than INS, which can more accurately reflect the secretion of islet β cells. It is used for the diagnosis and classification of diabetes mellitus, and also for the guidance of insulin therapy in diabetes patients.