C-peptide: A Marker of Insulin Production

What is C-peptide

C-peptide is a protein produced alongside insulin in the pancreas. Every time the pancreas produces one molecule of insulin, one molecule of C-peptide is released at the same time. This means the level of C-peptide in the blood directly reflects how much insulin your pancreas is producing on its own.

C-peptide is especially useful when there is a need to determine whether the body is producing too much, too little, or enough insulin — particularly in the diagnosis and monitoring of diabetes.

Reference Ranges

Fasting C-peptide reference ranges:

• Normal: 0.8–3.9 ng/mL
• Low: below 0.8 ng/mL
• High: above 3.9 ng/mL

It is important that blood is drawn fasting (at least 8 hours without food), as C-peptide naturally rises after eating. The laboratory's reference range always takes priority.

Why C-peptide May Be Low

A low C-peptide indicates that the pancreas is producing too little insulin. This is characteristic of:

• Type 1 diabetes — autoimmune destruction of pancreatic beta cells. C-peptide helps confirm the diagnosis and distinguish it from type 2.
• Latent Autoimmune Diabetes in Adults (LADA) — a slowly progressing form of type 1 diabetes.
• Long-standing type 2 diabetes — after many years, the pancreas may become exhausted and insulin production decreases.

A very low or undetectable C-peptide means the body is producing almost no insulin of its own and is dependent on external insulin.

Why C-peptide May Be High

A high C-peptide indicates increased insulin production. The most common causes are:

• Insulin resistance — the body produces more insulin to compensate for reduced cellular sensitivity. This is characteristic of the early stages of type 2 diabetes, obesity, and metabolic syndrome.
• Insulinoma — a rare benign pancreatic tumour that produces insulin autonomously.
• Cushing's syndrome — excess cortisol increases insulin resistance.

A high C-peptide combined with high insulin and normal or high glucose indicates insulin resistance — a signal that it is worth discussing lifestyle changes with your doctor.

What to Do Next

If C-peptide is low and glucose is high, the doctor will evaluate the type of diabetes and the treatment strategy. If C-peptide is high, it is usually recommended to check fasting glucose, HbA1c, and insulin — this helps assess the degree of insulin resistance.

Important to know: C-peptide does not respond to injected insulin, making it the best way to assess remaining pancreatic function in people who are already using insulin.

Monitoring Over Time

C-peptide trends are particularly important in diabetes management. A declining C-peptide over months or years may indicate progressive beta cell loss. In the QbiLabs chart, you can track this trend alongside changes in glucose and HbA1c.

Questions for Your Doctor

1. Does my C-peptide level match my diabetes type diagnosis? 2. Does a high C-peptide indicate insulin resistance, and what lifestyle changes can I make? 3. Should I repeat the C-peptide test to see the trend? 4. Should I also test insulin alongside C-peptide?

*This is not medical advice. Consult your doctor regarding any health decisions.*