C-peptide
Made during insulin production; shows how much insulin the pancreas produces itself
Also known as
Definition
C-peptide is a short chain of amino acids formed in the pancreas when insulin is made. C-peptide and insulin enter the bloodstream at the same time and in equal amounts, but C-peptide stays in the blood longer. Insulin from outside the body, such as diabetes medication, does not affect the value. It therefore shows how much insulin the body makes on its own. It is usually measured in blood, less often in a 24-hour urine sample.
| Parameter | Value |
|---|---|
| Unit | ng/ml |
| Reference Range | Typical adult range: 0.3–3.3 ng/ml. The value depends on blood glucose and on whether it was measured fasting or after a meal. Labs differ and some use other units or samples; the range on your own report is what counts. Discuss it with your doctor. |
Agree on your personal target range with your doctor.
Conversion: 1 ng/ml ≈ 0.331 nmol/l (molar mass about 3,020 g/mol). 0.3–3.3 ng/ml therefore corresponds to about 0.1–1.1 nmol/l, or roughly 100–1,090 pmol/l. Compare trends in the same unit.
↓ What a low value means
A low C-peptide level can mean the pancreas makes little or no insulin. This can be linked to type 1 diabetes and in some cases type 2 diabetes, and also to too much insulin from outside the body, which can suppress the body's own production, to severe infections, Addison disease or liver disease. A low value can be normal when fasting, because blood glucose and insulin are low then as well; it stands out mainly when blood glucose is high at the same time. Discuss any deviation with your doctor.
↑ What a high value means
A high C-peptide level usually shows that the body is making a lot of insulin. This can be linked to insulin resistance, obesity and type 2 diabetes, where the pancreas releases more insulin to keep blood glucose normal. Other possible links are Cushing's syndrome, kidney failure, low blood potassium and, rarely, an insulin-producing tumor of the pancreas (insulinoma). Interpretation depends on blood glucose, medicines and other values; discuss any deviation with your doctor.
When to test
C-peptide is measured to find the cause of low blood glucose (hypoglycemia), to tell type 1 from type 2 diabetes when the type is unclear, to estimate the body's own insulin production in known diabetes, and to monitor after treatment of an insulinoma. Fasting for 8–12 hours may be required for the blood test; sometimes it is measured after a meal on purpose. The test is not used to diagnose diabetes.
Frequently asked questions
Why measure C-peptide instead of insulin? +
C-peptide is released in the same amount as insulin, but stays in the blood longer and is not distorted by insulin from outside the body. It therefore shows how much insulin the pancreas makes on its own, even during insulin treatment.
Can C-peptide diagnose diabetes? +
No. Tests such as blood glucose or urine glucose are used to detect diabetes. C-peptide can, however, help tell type 1 from type 2 diabetes when the type is unclear.
Do I need to fast for the test? +
Fasting for 8–12 hours is often required. Sometimes C-peptide is measured after a meal on purpose to see the pancreas when it is active. Ask your practice about the instructions and whether medicines can affect the result.
Is a low fasting value a concern? +
Not necessarily. Without a recent meal, blood glucose and insulin are low, and so is C-peptide. A low value stands out mainly when blood glucose is high at the same time. Interpretation belongs with your doctor.
C-peptide
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.