What is being tested?

This test measures the level of C-peptide in a blood or urine sample. C-peptide and the hormone insulin are created from a larger molecule called proinsulin and stored in the beta cells of the pancreas. When insulin is released into the bloodstream to help transport glucose into the body's cells (to be used for energy), equal amounts of C-peptide also are released. This makes C-peptide useful as a marker of insulin production.

C-peptide can be used to help evaluate the production of endogenous insulin (insulin made by the body's beta cells) and to help differentiate it from exogenous insulin (insulin that is not produced by the body, e.g. injected insulin, which does not contain C-peptide). This differentiation can be used to help diagnose and monitor a variety of conditions.

How is it used?

When a patient has newly diagnosed type 1 or type 2 diabetes, C-peptide can be used to help determine how much insulin the patient's pancreas is still producing and whether or not that insulin is being used effectively.

Type 1 diabetes is an autoimmune process that often starts in early childhood and involves the destruction of the beta cells of the pancreas over time. Eventually, little or no insulin (or C-peptide) is produced, leading to a complete dependence on injected insulin.

In type 2 diabetes, often called ‘adult-onset diabetes’, a combination of factors leads to decreased insulin production and increased insulin resistance (when the body does not respond to insulin), along with some beta cell damage. Type 2 diabetics are usually treated with oral drugs to stimulate their body to make more insulin and/or to cause their cells to be more sensitive to the insulin that is already being made.

Eventually, type 2 diabetic patients may make very little insulin and require injections. Any insulin that the body does make will be reflected in their C-peptide level; therefore, the C-peptide test can be used to monitor beta cell function over time and to help the doctor determine when to start insulin injections.

C-peptide measurements can also be used with insulin and glucose levels to help diagnose the cause of hypoglycaemia (low blood glucose) and to monitor its treatment. Symptoms of hypoglycaemia may be caused by taking too much insulin, alcohol consumption, liver or kidney disease, or insulinomas (tumours of the cells in the pancreas that can produce uncontrolled amounts of insulin and C-peptide).

When is it requested?

C-peptide levels may be requested in a person with newly diagnosed diabetes, as part of an evaluation of their ‘residual beta cell function’ (how much insulin the beta cells are making). With type 2 diabetes, the test may be requested if a doctor wants to monitor how  a patient's beta cells are performing and producing insulin over time and to determine if/when insulin injections may be required.

C-peptide levels may be measured when there is sudden or recurring hypoglycaemia. Symptoms include sweating, palpitations, hunger, confusion, visual problems and seizures, although these symptoms also can occur with other conditions. The C-peptide test may be used in these circumstances to help determine the source of excess insulin, i.e. whether it is being produced in your body or coming from excessive injection of insulin.

When a person has been diagnosed with an insulinoma (a rare tumour of pancreatic beta cells that makes insulin in an uncontrolled manner and cause frequent hypoglycaemia), a C-peptide test may be requested to monitor the effectiveness of treatment and to detect tumour recurrence.

If you have had your pancreas removed or are one of the few patients to have had a pancreas transplant (in order to restore your ability to make insulin), your C-peptide levels may be monitored to verify the effectiveness of treatment and the continued success of the procedure.

What does the result mean?

High levels of C-peptide generally indicate high levels of insulin. This may be due to excessive insulin production by the body, a response to high levels of blood glucose caused by glucose intake or insulin resistance, when the body's cells do not respond normally to insulin and so the body makes more insulin in an attempt to compensate.

High levels of C-peptide also are seen with insulinomas (a tumour of the pancreas that causes excess insulin to be produced) and may also be seen with hypokalaemia, pregnancy, Cushing's syndrome and kidney disease. During a glucose tolerance test (GTT), there will often be a temporary increase in C-peptide levels.

Low levels of C-peptide are seen when insufficient insulin is being produced by the beta cells or when production is suppressed by injected insulin. Diuretics and alcohol can also cause low levels in some cases.

Is there anything else I should know?

Even though they are produced at the same rate, C-peptide and insulin leave the body by different routes. Insulin is processed and eliminated by the liver, while C-peptide degrades and is removed by the kidneys. Since C-peptide remains in the body for much longer than insulin, normally there will be about 5 times as much C-peptide in the bloodstream as insulin.

If a person's liver and kidneys are not clearing insulin and C-peptide efficiently then this can make results of the C-peptide test difficult to interpret. Therefore the test can give your doctor important information about your beta cells and insulin production, but it is not perfect.

You will need to fast before a C-peptide blood test if the results will be used to evaluate hypoglycaemia.

C-peptide testing may not be available in every laboratory. If a series of C-peptide tests are going to be performed they should be done by the same laboratory using the same method.

Common questions

• Can I do a C-peptide test at home?

No. The C-peptide test requires special equipment and training to perform. It is not done in every laboratory and often must be sent to a specialised laboratory.

• If I need to go on an insulin pump, will I need a C-peptide test?

You may. Insulin pumps are usually recommended for those who are not producing their own insulin and who have to check their blood sugar and inject themselves with insulin several times a day. Sometimes a C-peptide test will be requested during an initial evaluation to check the function of your beta cells and to see if you are still producing some insulin.

More information

RCPA Manual: C-peptide