C-peptide

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.  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 level; 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).

C-peptide levels 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).  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 levels in blood and urine may also be monitored to check your kidney function, by looking at how quickly C-peptide is cleared from the bloodstream.

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.

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.

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.

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.