c-peptide is a peptide that is produced along with insulin, and is often used to evaluate the secretion and reserve function of pancreatic islet cells. Insulinogen is cleaved under the action of protein hydrolase, and c-peptide of equal molecular weight is formed along with insulin. Therefore, the level of c-peptide can be used to assess the production of insulin, and thus assist in the diagnosis of diabetes mellitus typing. The c-peptide rises to its peak 30-60 minutes after a meal, with a peak value 5-6 times the basal value. The measurement is not affected by serum insulin antibodies or exogenous insulin. If the fasting level increases, it suggests the possible existence of pancreatic islet β-cell tumors or cirrhosis; a decrease in the level of fasting status is mostly suggestive of diabetes mellitus; if the c-peptide is not high but insulin increases, it suggests that the exogenous insulin is used in too large a quantity. In conclusion, it is recommended to consult a medical professional for a standardized interpretation of the results.