C-peptide is a secretory product of pancreatic beta cells, which shares a common precursor with insulin – insulinogen. One molecule of insulinogen is cleaved into one molecule of insulin and one molecule of C peptide under special action, so in theory C peptide and insulin are equivalent secretion, the physiological function of free C peptide in blood is not very clear, but C peptide is not destroyed by liver, and the half-life is obviously longer than insulin, so the measurement of C peptide level can better reflect the function of synthesis and release of insulin by β cell. For patients who have been treated with insulin, the insulin antibodies produced in the body can interfere with insulin determination; meanwhile, the insulin determination by the current immunoassay method cannot distinguish whether it is endogenous or exogenous insulin, which makes it difficult to understand the function of β-cells, while there is a fairly stable proportional relationship between C-peptide and insulin, and it is not interfered by insulin antibodies, and the injected exogenous insulin does not contain C-peptide. The measurement of C peptide level in blood can respond to the level of endogenous insulin, which can understand both the function of β-cells. 1.The practice and precautions of C peptide release test are the same as glucose tolerance test and insulin release test, and its clinical significance is: (1) The measurement of C peptide helps clinical typing of diabetes and helps to understand the islet function of patients. (2) Because C-peptide is not interfered by insulin antibodies, C-peptide concentration can be measured directly in patients receiving insulin therapy to determine the function of pancreatic β-cells in patients. (3) The cause of hypoglycemia can be identified. If C-peptide exceeds normal, it can be considered as the result of excessive insulin secretion, and if C-peptide is lower than normal, it is due to other reasons. (4) C peptide measurement helps to diagnose islet cell tumor and judge the effect of insulinoma surgery. High C peptide level in blood of insulinoma, if C peptide level in blood is still high after surgery, it means there is residual tumor tissue, if C peptide level keeps increasing in follow-up, it reveals the possibility of tumor recurrence or metastasis.