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C-peptide: Function, Normal Values & Diagnosis

C-peptide is a biomarker used to assess the body's own insulin production. It plays a key role in the diagnosis and monitoring of diabetes mellitus.

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Things worth knowing about "C-peptide"

C-peptide is a biomarker used to assess the body's own insulin production. It plays a key role in the diagnosis and monitoring of diabetes mellitus.

What is C-peptide?

C-peptide (connecting peptide) is a short protein fragment produced during the synthesis of insulin in the pancreas. Insulin is initially formed as an inactive precursor called proinsulin. Through enzymatic cleavage, one molecule of insulin and one molecule of C-peptide are released simultaneously in equal amounts into the bloodstream. This makes C-peptide a reliable indicator of the body's own insulin production.

Unlike insulin, C-peptide is not broken down by the liver, so it remains detectable in the blood for longer and provides a more stable measure of the function of the insulin-producing beta cells in the pancreas.

Medical Importance and Clinical Use

C-peptide measurement is used in a variety of clinical situations:

Distinguishing type 1 from type 2 diabetes: In type 1 diabetes, insulin production by the beta cells is severely reduced or completely absent, resulting in very low or undetectable C-peptide levels. In type 2 diabetes, insulin production is initially preserved or even elevated, so C-peptide levels may be normal or elevated.
Monitoring diabetes progression: C-peptide helps physicians assess whether residual beta cell function remains in a diabetic patient, influencing treatment decisions such as whether insulin therapy is required.
Detection of insulin-producing tumors (insulinoma): Unexplained elevation of C-peptide may indicate an insulinoma, a rare pancreatic tumor that produces excessive amounts of insulin.
Post-surgical or post-transplant evaluation: After pancreatic surgery or transplantation, C-peptide levels can reflect the remaining or transplanted beta cell function.
Differentiation of hypoglycemia causes: In a patient with low blood sugar, C-peptide can help determine whether hypoglycemia is caused by excessive endogenous insulin production or by covert external insulin administration. In the latter case, C-peptide levels are low because no endogenous insulin is being produced.

Reference Values and Interpretation

Fasting C-peptide reference ranges typically fall between 0.5 and 2.0 nmol/L (approximately 1.5–6.0 ng/mL), though exact values may vary by laboratory. Both low and elevated values can be clinically significant:

Low C-peptide: Indicates reduced beta cell function, as seen in advanced type 1 diabetes or following pancreatectomy.
Elevated C-peptide: May indicate insulin resistance, type 2 diabetes, insulinoma, or impaired renal clearance of C-peptide.

C-peptide values must always be interpreted in the context of current blood glucose levels and the overall clinical picture of the patient.

How the Test is Performed

C-peptide is measured from a simple blood sample, typically taken in a fasting state or following a stimulation test (e.g., glucagon stimulation or a mixed meal test) to assess maximum beta cell capacity. A 24-hour urine collection for C-peptide is also possible and can provide additional insight into average insulin secretion over time.

References

Jones AG, Hattersley AT: The clinical utility of C-peptide measurement in the care of patients with diabetes. Diabetic Medicine, 2013;30(7):803–817. PubMed PMID: 23413806.
American Diabetes Association: Standards of Medical Care in Diabetes. Diabetes Care, 2024;47(Suppl 1). Available at: www.diabetesjournals.org
World Health Organization (WHO): Classification of Diabetes Mellitus. Geneva: WHO, 2019. Available at: www.who.int

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