C-peptide testing can be used for a few different purposes. C-peptide is a substance produced by the beta cells in the pancreas when proinsulin splits apart and forms one molecule of C-peptide and one molecule of insulin. Insulin is the hormone that is vital for the body to use its main energy source, glucose. Since C-peptide and insulin are produced at the same rate, C-peptide is a useful marker of insulin production.

The following are some purposes of C-peptide testing:

• A C-peptide test is not ordered to help diagnose diabetes, but when a person has been newly diagnosed with diabetes, it may be ordered by itself or along with an insulin level to help determine how much insulin a person's pancreas is still producing (endogenous insulin).
• In type 2 diabetes, the body is resistant to the effects of insulin (insulin resistance) and it compensates by producing and releasing more insulin, which can also lead to beta cell damage. Type 2 diabetics usually are 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, because of the beta cell damage, type 2 diabetics may make very little insulin and require injections. Any insulin that the body does make will be reflected in the C-peptide level; therefore, the C-peptide test can be used to monitor beta cell activity and capability over time and to help a health practitioner determine when to begin insulin treatment.
• People who are on insulin therapy, regardless of the source of the insulin, may develop antibodies to insulin.These typically interfere with tests for insulin, making it nearly impossible to directly evaluate endogenous insulin production. In these cases, C-peptide measurement is a useful alternative to testing for insulin.
• C-peptide measurements can also be used in conjunction with insulin and glucose levels to help diagnose the cause of documented hypoglycemia and to monitor its treatment.  Symptoms of hypoglycemia may be caused by excessive supplementation of insulin, alcohol consumption, inherited liver enzyme deficiencies, liver or kidney disease, or by insulinomas. 
• The C-peptide test may be used to help diagnose Insulinomas. These are tumors of the islet cells in the pancreas that can produce uncontrolled amounts of insulin and C-peptide and can cause acute episodes of hypoglycemia.C-peptide tests may be used to monitor the effectiveness of insulinoma treatment and to detect recurrence.
• Sometimes a C-peptide test may be used to help evaluate a person diagnosed with metabolic syndrome, a set of risk factors that includes abdominal obesity, increased blood glucose and/or insulin resistance, unhealthy blood lipid levels, and high blood pressure (hypertension).
• Rarely, when someone has had his pancreas removed or has had pancreas islet cell transplants, intended to restore the ability to make insulin, C-peptide levels may be used to verify the effectiveness of treatment and continued success of the procedure.

C-peptide levels may be ordered when a person has been newly diagnosed with type 1 diabetes as part of an evaluation of the person's "residual beta cell function."  

With type 2 diabetes, the test may be ordered on a regular basis when a health practitioner wants to monitor the status of a person's beta cells and insulin production over time and to determine if/when insulin injections may be required.

C-peptide levels may be done when there is documented acute or recurring low blood glucose (hypoglycemia) and/or excess insulin is suspected. Symptoms of hypoglycemia include:

• Sweating
• Palpitations
• Hunger
• Confusion
• Blurred vision
• Fainting
• In severe cases, seizures and loss of consciousness

However, many of these symptoms can occur with other conditions as well. 

When a person has been diagnosed with an insulinoma, a C-peptide test may be ordered periodically to monitor the effectiveness of treatment and to detect tumor recurrence.

Rarely, C-peptide levels may be monitored over time when someone has had his pancreas removed or has had pancreas islet cell transplants.

A high level of C-peptide generally indicates a high level of endogenous insulin production. This may be in response to a high blood glucose caused by glucose intake and/or insulin resistance. A high level of C-peptide is also seen with insulinomas and may be seen with low blood potassium, Cushing syndrome, and renal failure.

When used for monitoring, decreasing levels of C-peptide in someone with an insulinoma indicate a response to treatment; levels that are increasing may indicate a tumor recurrence.

A low level of C-peptide is associated with a low level of insulin production.This can occur when insufficient insulin is being produced by the beta cells, with diabetes for example, or when production is suppressed by treatment with exogenous insulin.

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

Even though they are produced at the same rate, C-peptide and insulin leave the body by different routes. Insulin is processed and eliminated mostly by the liver, while C-peptide is removed by the kidneys.  Since the half-life of C-peptide is about 30 minutes compared to insulin's 5 minutes, normally there will be about 5 times as much C-peptide in the blood as insulin.