C-peptide

Do you have diabetes but it is unclear whether it is type 1 or type 2? Or does your blood sugar occasionally fall too low for no clear reason? C-peptide shows how much insulin your pancreas produces itself, even if you are treated with insulin.

Who the test is for

• people with diabetes when it is unclear whether it is type 1, type 2 or latent autoimmune diabetes in adults (LADA);
• people with long-standing diabetes when a doctor assesses whether to start or change insulin;
• people treated with insulin when the amount still produced by the pancreas needs to be known;
• people without diabetes whose sugar repeatedly falls too low;
• after pancreatic surgery or with pancreatic disease, as assessed by a doctor.

What the test reveals

Pancreatic beta cells produce proinsulin, which splits into insulin and C-peptide. Both enter blood in equal amounts, but C-peptide persists longer and fluctuates less, making it a good indicator of the body’s own insulin secretion.

Injected insulin preparations contain no C-peptide. C-peptide therefore shows pancreatic function even in people treated with insulin, while measuring insulin is not useful in them. Differences between the tests are described in The significance of measuring C-peptide and insulin.

The procedure and preparation

Blood is taken from a vein and collection takes a few minutes. Unless your doctor requests otherwise, come in the morning fasting after 8 to 12 hours without food; plain water is permitted. C-peptide rises after meals, so fasting values are compared. The same sample is usually also used to measure glucose.

Do not stop diabetes treatment yourself; agree with your doctor how to take it on the collection day. If taking biotin (for example hair and nail supplements), mention this during collection.

What your result means

The reference interval depends on method and is printed on the report. C-peptide is always interpreted alongside glucose from the same sample, and a doctor assesses it alongside HbA1c and other tests.

• Low C-peptide with raised glucose: the pancreas secretes too little insulin, typical of type 1 or long-standing type 2 diabetes. This helps a doctor decide whether insulin is needed.
• Low C-peptide with low glucose: the expected response of a healthy pancreas. If insulin is high as well, externally administered insulin may cause low sugar.
• Raised fasting C-peptide: most often accompanies insulin resistance, excess weight and early type 2 diabetes. It is also raised with impaired kidney function because it is excreted by the kidneys.
• High C-peptide with low glucose: the pancreas secretes too much insulin, for example due to some medicines or a rare insulin-secreting tumour, requiring further investigation.

If someone with low blood sugar is confused, cannot eat safely or loses consciousness, call 194 or 112 immediately.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed under Contact). Prices for all tests and panels are available in the price list. If a doctor also requests insulin, both tests use the same collection.

Frequently asked questions

What does raised C-peptide mean?

Most often that the pancreas is secreting more insulin to keep glucose normal, accompanying insulin resistance and excess weight. It can also be raised in kidney disease. A doctor assesses its meaning for you alongside glucose and HbA1c.

Can C-peptide identify the type of diabetes?

It helps but is insufficient alone. In type 1, C-peptide becomes very low over time, while in type 2 it is initially normal or raised. A doctor usually also requests antibodies to pancreatic beta cells.

Is C-peptide useful if I am treated with insulin?

Yes, particularly then. Injected insulin contains no C-peptide, so testing shows how much insulin the pancreas still produces itself.

Do I need to fast for C-peptide?

Yes, unless a doctor requests otherwise, such as a measurement after a meal or during low blood sugar. C-peptide rises after eating, so the result could not be compared with the reference interval.