C Peptide

Insulin is initially synthesized as preproinsulin in the islet cells of the pancreas and is then cleaved to proinsulin, which is packaged into secretory granules. Proinsulin is enzymatically cleaved into equimolar amounts of insulin and C-peptide in these granules. An increase in blood glucose concentration is the primary stimulus for insulin secretion.  Insulin secretion involves fusion of the secretory granules with the cell membrane and exocytosis of insulin, C-peptide, and proinsulin.

C-peptide (connecting peptide) is the 31-amino-acid polypeptide in the middle of the proinsulin molecule. C-peptide is cleared from the circulation by the kidneys. It has a circulating half-life of 30 minutes, which is longer than the 5 to10 minute half life for insulin. Because of the differences in half-lives, the molar ratio of circulating insulin to C-peptide is usually <1, despite equimolar secretion.

Standards of Care in Diabetes by the American Diabetes Association recommends measuring C-peptide when the initial clinical presentation is not clearly diagnostic for type 1 versus type 2 diabetes. When a patient is receiving insulin therapy and islet cell autoantibodies are not present, C-peptide testing can differentiate type 1 from type 2 diabetes. C-peptide levels below 0.6 ng/mL are indicative of type 1 diabetes and levels above 1.8 ng/mL are indicative of type 2. Intermediate levels (200-600) may occur in people with monogenic diabetes, which can be confirmed by genetic testing. 

C-peptide and insulin are both elevated in patients with insulinoma, which is a rare tumor that produces excess insulin and hypoglycemia. The diagnosis of insulinoma should be considered in patients presenting with Whipple’s triad, which comprises symptoms of hypoglycemia, documented low plasma glucose levels, and resolution of symptoms following glucose administration.

Laboratory tests play an important role in the initial diagnosis of insulinoma. The diagnosis depends on demonstrating elevated serum insulin and C-peptide levels when the patient has symptoms of hypoglycemia and plasma glucose concentration is low. 

Diagnostic criteria for Insulinoma include: 

• Glucose concentration of <55 mg/dL
• Insulin level ≥3 µU/mL
• C-peptide level ≥0.6 ng/mL
• Proinsulin level ≥5 pmol/L
• Beta-hydroxybutyrate level ≤2.7 mmol/L
• Sulfonylurea screen negative 

Altogether, these findings indicate that hypoglycemia is mediated by hyperinsulinemia. 

Discordant levels of serum insulin and serum C-peptide can occur in patients with anti-insulin autoantibodies and in patients with factitious hypoglycemia due to self-administration of insulin or other hypoglycemic medications. In patients with insulin autoantibodies, serum insulin levels are increased because of the prolonged half-life of insulin bound to autoantibody. 

The diagnosis of factitious hypoglycemia can usually be established by documenting the presence of hypoglycemia and by simultaneously measuring plasma insulin, C peptide, proinsulin, and insulin secretagogues such as sulfonylureas. Factitious hypoglycemia results in elevated serum insulin levels and low or undetectable C-peptide levels because C-peptide is not present in pharmaceutical insulin. 

Reference range is usually 1 – 4.0 ng/mL

Specimen requirement for C-peptide is a red top tube of blood collected from a fasting patient.  

References

American Diabetes Association Professional Practice Committee, Diagnosis and Classification of Diabetes: Standards of Care in Diabetes-2025. Diabetes Care 2025;48(Suppl 1):S27-S49. 

Zhuo F, Menon G, and Anastasopoulou C. Insulinoma, 2025. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK544299/.