C-peptide test: normal range and what the result means
In ng/ml, MedlinePlus gives 0.3–3.3 ng/ml; the exact interval depends on the laboratory and the method
- Updated
- 2026-09-10
- Every number with its source
What C-peptide is
C-peptide is a fragment of protein that detaches from the proinsulin molecule at the same time, and in the same amount, as insulin is produced. Because C-peptide and insulin are made together in the beta cells of the pancreas, the amount of C-peptide in the blood makes it possible to assess how much insulin the body produces on its own — even in someone using insulin injections from outside, because medicinal insulin does not give rise to C-peptide. That is why the test helps to establish whether insulin is lacking because the pancreas is not producing it, which is characteristic of type 1 diabetes; or whether there is enough insulin, or even too much, but the body cannot use it properly, which is characteristic of insulin resistance and type 2 diabetes. A C-peptide test also helps to clarify the cause of hypoglycaemia (too low blood sugar) of unclear origin, and to monitor treatment when a pancreatic tumour — an insulinoma — is suspected.
Normal range (MedlinePlus)
| Group | C-peptide | Source |
|---|---|---|
| Adults In ng/ml, MedlinePlus gives 0.3–3.3 ng/ml; the exact interval depends on the laboratory and the method | 0.2–1.0 nmol/l | MedlinePlus Reviewed 2026-01-25 |
The figures above are for adults. The exact interval depends on the laboratory and the method used, so it is worth looking at the range printed on your own result sheet, and worth discussing with your doctor whether a repeat test is needed.
When the test is ordered
- When the cause of hypoglycaemia (too low blood sugar) of unclear origin needs clarifying
- When signs of hypoglycaemia are felt — trembling, sweating, headache, hunger or nausea, a fast or irregular heartbeat
- When diagnosing or clarifying the type of diabetes — to tell apart whether the pancreas is not producing insulin, or the body cannot use it properly
- When choosing or adjusting the approach to treating diabetes
- When monitoring treatment where a pancreatic tumour (insulinoma) is suspected or diagnosed
How to prepare
- Preparation depends on the reason for the test — ask your doctor whether you need to go without food (fast) beforehand
- Most often you are asked not to eat or drink for 8–12 hours before the blood draw
High C-peptide: what it can mean
A high C-peptide usually shows the pancreas is producing more insulin than normal — this is considered characteristic of insulin resistance, where the body's cells respond more weakly to insulin. Other causes are type 2 diabetes, Cushing's syndrome, a pancreatic tumour (insulinoma), kidney failure or a low potassium level in the blood. Glucose and insulin tests done at the same time help to establish the cause.
- Type 2 diabetes with insulin resistance
- Cushing's syndrome
- Insulinoma — a pancreatic tumour producing too much insulin
- Kidney failure
- A low potassium level in the blood
Low C-peptide: what it can mean
A low or undetectable C-peptide shows the pancreas produces little or no insulin — this is considered characteristic of type 1 diabetes, and sometimes of a late stage of type 2 diabetes as well. Other causes are too large an insulin dose while treating diabetes, a severe infection, Addison's disease or liver disease. A low result together with a raised blood sugar level calls for further assessment.
- Type 1 diabetes, and in some cases type 2 diabetes as well
- Too large an insulin dose while treating diabetes
- A severe infection
- Addison's disease
- Liver disease
What is usually looked at after a low result
- A glucose test at the same time — to assess whether a low C-peptide matches a high blood sugar
- Assessment of the diabetes type together with your doctor
- Liver and adrenal function markers, if another cause is suspected
- A review of treatment (the insulin dose) with your doctor, if treatment is already being given
C-peptide and an insulin test are not the same
An insulin test measures all the insulin present in the blood, including insulin injected from outside. C-peptide is released only with the insulin the body has produced itself, so it makes it possible to assess how the pancreas is working even in people already treated with insulin injections.
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Frequently asked questions
›What is a normal C-peptide level?
Per MedlinePlus, a normal result in adults is 0.2–1.0 nmol/l (or 0.3–3.3 ng/ml). The exact interval depends on the laboratory and the method used, so it is worth looking at the range printed on your own result sheet.
›How does a C-peptide test differ from an insulin test?
An insulin test measures all the insulin present in the blood, including insulin injected from outside. C-peptide is released only with the insulin the body has produced itself, so it makes it possible to assess how the pancreas is working even in people already treated with insulin injections.
›Why is C-peptide tested when diabetes is suspected?
The test helps to tell apart whether insulin is lacking because the pancreas is not producing it (more often type 1 diabetes), or whether the body cannot use it properly even though the pancreas produces enough or even too much (more often type 2 diabetes with insulin resistance).
›What does a low C-peptide mean?
A low or undetectable C-peptide shows the pancreas produces little or no insulin. This is considered characteristic of type 1 diabetes, and it may also point to too large an insulin dose while treating diabetes, a severe infection or some other conditions — the exact cause is established by a doctor.
›What does a high C-peptide mean?
A high C-peptide usually shows the pancreas is producing more insulin than normal. This is considered characteristic of insulin resistance and type 2 diabetes, and less often of a pancreatic tumour (insulinoma) or other hormonal conditions.
›Do I need to fast before a C-peptide test?
Most often you are asked not to eat or drink for 8–12 hours before the test, but the exact preparation depends on the reason for the test — ask your doctor.
Sources
Related markers
This site is not an emergency service. If severe chest or head pain, breathlessness, confusion, difficulty speaking, heavy bleeding or another acute problem has come on suddenly — do not wait for tests, call 112.
This information is not a diagnosis and does not replace a consultation with a doctor. Reference ranges depend on the laboratory, your age and sex — always compare your result with the range printed on your own report.