If you have diabetes mellitus and your doctor wants to see if you are producing any of your own insulin, if you are thought to be insulin resistant (when your body does not respond properly to insulin), or to help establish if it is time to add insulin injections to your current treatment. Your doctor also may request a C-peptide blood test if you have an episode of low blood sugar, called hypoglycaemia.
Fasting for 8 to 10 hours before blood testing is usually required.
This test measures the amount 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 and indeed better in some cases than insulin as it is more stable in the blood.
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). This differentiation can be used to help diagnose and monitor a variety of conditions.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm. If a 24-hour urine sample is required, you will be asked to save all of your urine over a 24 hour time period.
Is any test preparation needed to ensure the quality of the sample?
Fasting for 8 to 10 hours before blood testing is usually required.
How is it used?
When a patient has newly diagnosed diabetes, C-peptide can be used to help determine the type of diabetes, 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. People with type 2 diabetes 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, people with type 2 diabetes may make very little insulin and require injections. Any insulin that the body does make will be reflected in their C-peptide concentration; therefore, the C-peptide test can be used to monitor beta cell function over time and to help your doctor determine when you need to start taking 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, glucose-lowering drugs, some endocrine conditions 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 may be requested if you have newly diagnosed diabetes, as part of an evaluation of your "residual beta cell function" (how much insulin your beta cells are making) and to help differentiate the type of diabetes. With type 2 diabetes, the test may be requested if your doctor wants to monitor how your beta cells are performing and producing insulin over time and to determine if/when insulin injections may be required.
C-peptide 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.
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 concentration may be monitored to verify the effectiveness of treatment and the continued success of the procedure.
What does the test result mean?
High concentrations of C-peptide within the bloodstream generally indicate high concentrations of insulin. This may be due to excessive insulin production by the body, a response to high concentrations 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 concentrations 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 an oral glucose tolerance test (OGTT), there will often be a temporary increase in C-peptide concentration.
Low concentrations 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 C-peptide 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 mainly 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. Alternatively, an appropriate blood sample taken during a hypoglycaemic episode may suffice.
Can I do a C-peptide test at home?
No. The C-peptide test requires special equipment and training to perform and interpret. 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 glucose 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.