C-peptide

What is being analysed?
The test measures C-peptide levels in a blood or urine sample. C-peptide and insulin come from a proinsulin molecule and are stored in the beta cells of the pancreas. Insulin is released into the bloodstream, to help transport glucose into cells (as a source of energy), in the same amount as C-peptide. C-peptide is therefore a useful indicator of insulin production.
C-peptide can be used to assess the body’s own insulin production (insulin produced by the pancreatic beta cells) and to distinguish it from injected insulin, to help diagnose and monitor various conditions.

How is the sample collected for testing?
The blood sample is taken from a vein. If a 24-hour urine sample is requested, all the urine passed over 24 hours must be collected.

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
When type 1 or type 2 diabetes has recently been diagnosed, C-peptide helps establish how much of the body’s own insulin is still being produced and whether there is insulin resistance.
Type 1 diabetes is an autoimmune disease that often appears early, in childhood, and progressively destroys the pancreatic beta cells. Insulin (or C-peptide) production may therefore be low or absent, causing complete dependence on injected insulin.
In type 2 diabetes, often called “adult-onset diabetes”, a combination of factors leads to reduced insulin production and increased insulin resistance, as well as damage to some beta cells. Type 2 diabetes is treated with medicines that stimulate insulin production and/or increase cells’ sensitivity to the insulin already produced. If the body’s own insulin production is very low, treatment with insulin injections is needed. Insulin and C-peptide are produced in equal amounts, so C-peptide can be used to monitor beta cell function over time and help decide when to start insulin injections.
Measuring C-peptide together with insulin and glucose can help diagnose the causes of hypoglycaemia and monitor its treatment. Symptoms of hypoglycaemia can be due to too much insulin, alcohol, liver or kidney disease, or insulinomas (tumours of the pancreatic cells that produce uncontrolled amounts of insulin and C-peptide).

When is it requested?
C-peptide may be requested when diabetes is diagnosed as part of the assessment of “residual beta cell function” (how much insulin the beta cells are producing). In type 2 diabetes it may be requested to monitor beta cell activity and insulin production over time and to decide whether/when to start insulin treatment. Blood and urine C-peptide can be monitored to assess kidney function, by observing how quickly the peptide is cleared from the circulation.
C-peptide may be requested in acute or recurrent hypoglycaemia with symptoms such as sweating, palpitations, hunger, confusion, visual disturbances and fainting (although these symptoms can have other causes). The C-peptide test can help distinguish excessive insulin production from excessive insulin administration and help diagnose insulinoma.
After pancreatectomy or a pancreatic islet transplant (to restore insulin production), monitoring C-peptide concentrations can help check the effectiveness of treatment and the continued success of the operation.

What does the test result mean?
High C-peptide concentrations generally indicate high insulin levels. These increases may be linked to excessive insulin production in response to high blood glucose due to excessive intake and/or insulin resistance (in insulin resistance the body’s cells cannot use insulin to transport glucose inside them. The cells therefore become “hungry” for glucose and send signals to the body to make more insulin).
High C-peptide concentrations are also found with insulinomas, low potassium, pregnancy, Cushing’s syndrome and kidney disease. During a glucose tolerance test, a temporary rise in C-peptide levels often occurs.
Low C-peptide concentrations are found when the beta cells produce less insulin or when production is suppressed by injected insulin. Diuretics and alcohol can sometimes also lower levels.

Is there anything else I should know?
Although C-peptide and insulin are produced in equal amounts, they are metabolised by different routes. Insulin is metabolised and cleared by the liver, whereas C-peptide is cleared by the kidneys. C-peptide stays in the circulation longer than insulin, so C-peptide concentrations are normally about 5 times those of insulin. When the liver and kidneys do not metabolise C-peptide and insulin efficiently, C-peptide results become difficult to interpret. This test can therefore provide important information about the beta cells and insulin production, but it is not perfect.
Fasting is required before a C-peptide test if the result will be used to evaluate hypoglycaemia.

1. Can C-peptide be measured at home?
2. If I use an insulin pump, do I need a C-peptide test?
3. What else is C-peptide used for?

1. Can C-peptide be measured at home?
No. Unlike glucose, measuring C-peptide requires suitable equipment and expertise.

2. If I use an insulin pump, do I need a C-peptide test?
Possibly. An insulin pump is usually recommended when no insulin is being produced and repeated blood glucose checks and repeated insulin injections are needed. A C-peptide test may sometimes be requested during the initial assessment to establish how well the beta cells are working and how much insulin they produce.

3. What else is C-peptide used for?
Research is under way to better understand the role of C-peptide. Clinical trials are in progress in Europe in which C-peptide is given to people with type 1 diabetes. The results look promising, showing fewer diabetic complications and improved kidney function, blood circulation and nerve function. However, further studies are needed.