C-Peptide Test: Normal Range & What It Tells You

What Is C-Peptide?

C-peptide is released in equal amounts to insulin when your pancreas produces it. Measuring C-peptide shows how much insulin your body is actually making, even if you are taking insulin injections.

Normal C-Peptide Range

Why It's Ordered

• Distinguishing Type 1 from Type 2 diabetes
• Evaluating unexplained low blood sugar (hypoglycaemia)
• Assessing pancreatic insulin-producing capacity
• Monitoring insulinoma (insulin-producing tumour)

What Low C-Peptide Means

Low levels suggest the pancreas is making little or no insulin: typical in Type 1 diabetes or advanced Type 2 diabetes.

What High C-Peptide Means

High levels suggest the pancreas is overproducing insulin, seen in early Type 2 diabetes, insulin resistance, or rarely an insulinoma.

Frequently Asked Questions

How C-peptide is used in practice

The main clinical value of C-peptide is distinguishing between types of diabetes when the picture is unclear, for example in a younger adult who could have late-onset type 1 or early type 2 diabetes. A low C-peptide alongside high glucose indicates the pancreas is producing little insulin, supporting type 1; a normal or high result with high glucose suggests insulin resistance, supporting type 2.

It is also central to investigating unexplained low blood sugar. If someone has hypoglycaemia with a high C-peptide, their body is producing excess insulin, which raises the possibility of an insulin-producing tumour or the effect of certain diabetes tablets. If C-peptide is low while insulin is high, this suggests insulin has come from an injection rather than from the pancreas.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

1. Biochemistry, Hormones. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK541112

References

Sources cited on this page. PubMed links open the original abstract.

1. Jones AG, Hattersley AT. The clinical utility of C-peptide measurement in the care of patients with diabetes. Diabet Med. 2013;30(7):803–817. PMID 23738576 · doi:10.1111/dme.12159

What C-peptide measures and why it matters

C-peptide (connecting peptide) is a 31-amino acid fragment cleaved from proinsulin when the pancreas produces insulin. One molecule of C-peptide is produced for every molecule of insulin. Because C-peptide is released in equimolar amounts with insulin, its blood level is a direct measure of how much insulin the pancreas is making.

The key clinical advantage of C-peptide over measuring insulin directly: injected insulin does not contain C-peptide, and the liver does not clear C-peptide as rapidly as insulin. This means:

• C-peptide accurately reflects endogenous (the body's own) insulin production, even in patients taking exogenous insulin injections
• C-peptide has a longer half-life in blood (approximately 30 minutes versus 4 minutes for insulin), making it easier to measure reliably

Clinical uses of C-peptide testing

• Distinguishing type 1 from type 2 diabetes: Type 1 diabetes destroys insulin-producing beta cells – C-peptide falls to undetectable or very low levels (below 0.2 nmol/L). Type 2 diabetes involves insulin resistance with initially preserved or high C-peptide production. This distinction matters when insulin dependence is unclear – particularly in Latent Autoimmune Diabetes in Adults (LADA), which presents like type 2 diabetes but has low C-peptide and positive islet antibodies (anti-GAD, anti-ZnT8).
• Hypoglycaemia investigation: C-peptide is critical in the hypoglycaemia workup. During a hypoglycaemic episode, an elevated C-peptide alongside high insulin suggests the insulin is coming from the patient's own pancreas – raising suspicion for insulinoma (insulin-secreting pancreatic tumour) or sulfonylurea misuse. Suppressed C-peptide with high insulin in hypoglycaemia indicates exogenous insulin administration – factitious hypoglycaemia (someone self-administering insulin without medical indication).
• Monitoring residual beta-cell function: In type 1 diabetes, residual C-peptide production (even at low levels) is associated with better glycaemic control, fewer hypoglycaemic episodes, and reduced long-term complications. Clinical trials of beta-cell preservation therapies (teplizumab, baricitinib) use C-peptide as a primary endpoint.
• Post-pancreatectomy assessment: After pancreatic surgery, C-peptide confirms whether sufficient beta-cell mass remains to avoid insulin dependence.

How C-peptide is measured – practical points

C-peptide is measured on a fasting blood sample or during a mixed-meal stimulation test (a standardised liquid meal that provokes maximal insulin secretion). A random or fasting C-peptide below 0.2 nmol/L in a person with diabetes suggests complete beta-cell failure (type 1 / LADA). A stimulated C-peptide above 0.6 nmol/L indicates significant residual insulin secretion. Results must always be interpreted alongside blood glucose at the time of sampling – a C-peptide of 0.5 nmol/L is appropriately suppressed with a glucose of 3.0 mmol/L but inappropriately elevated at hypoglycaemic levels.