Test Overview

A C-peptide test measures the level of this peptide in the blood. It is generally found in amounts equal to insulin. C-peptide is found in amounts equal to insulin because insulin and C-peptide are linked when first made by the pancreas. Insulin helps the body use and control the amount of sugar (glucose) in the blood. Insulin allows glucose to enter body cells where it is used for energy. The level of C-peptide in the blood can show how much insulin is being made by the pancreas. C-peptide does not affect the blood sugar level in the body. See a picture of the pancreas.

A C-peptide test can be done when diabetes has just been found and it is not clear whether type 1 diabetes or type 2 diabetes is present. A person whose pancreas does not make any insulin (type 1 diabetes) has a low level of insulin and C-peptide. A person with type 2 diabetes has a normal or high level of C-peptide.

A C-peptide test can also help find the cause of low blood sugar (hypoglycemia), such as excessive use of medicine to treat diabetes or a noncancerous growth (tumor) in the pancreas (insulinoma). Because man-made (synthetic) insulin does not have C-peptide, a person with a low blood sugar level from taking too much insulin will have a low C-peptide level. An insulinoma causes the pancreas to release too much insulin, which causes blood sugar levels to drop (hypoglycemia). A person with an insulinoma will have a high level of C-peptide in the blood.

Why It Is Done

A C-peptide test is done to:

• Help tell the difference between type 1 diabetes and type 2 diabetes.
• Find the cause of low blood sugar (hypoglycemia).
• Check to see whether a tumor of the pancreas (insulinoma) was completely removed.

How To Prepare

You may be asked to stop eating and drinking for 8 hours before having this blood test.

Insulin and some oral medicines used to treat type 2 diabetes can change the test results. Your doctor may ask you to stop these medicines before your blood test.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

How It Is Done

The health professional drawing your blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure to the site and then a bandage.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Risks

There is very little chance of a problem from having blood sample taken from a vein.

• You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
• In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

Results

A C-peptide test measures the level of this peptide in the body.

Normal

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

The level of C-peptide in the blood must be read with the results of a blood glucose test. Both these tests will be done at the same time.

High values

• High levels of both C-peptide and blood glucose are found in people with type 2 diabetes or insulin resistance (such as from Cushing's syndrome).
• A high level of C-peptide with a low blood glucose level may mean that an insulin-producing tumor of the pancreas (insulinoma) is present or that the use of certain medicines such as sulfonylureas (for example, glyburide) is causing the high level.
• If C-peptide levels are high after an insulinoma is taken out, it may mean that the tumor has returned or that the tumor has spread to other parts of the body (metastasized).

Low values

• Low levels of both C-peptide and blood glucose are found in liver disease, a severe infection, Addison's disease, or insulin therapy.
• A low level of C-peptide with a high blood glucose level is found in people with type 1 diabetes.
• Complete removal of the pancreas (pancreatectomy) causes a C-peptide level so low it cannot be measured. The blood glucose level will be high, and insulin will be needed in order for the person to survive.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

• Taking medicines, such as insulin, or sulfonylurea medicines for type 2 diabetes.
• Having kidney failure. Both insulin and C-peptide are removed from the body by the kidneys. C-peptide levels may be high in a person with kidney failure.
• Being obese. More insulin is made in obese people and can cause high levels of C-peptide.

What To Think About

• A C-peptide test must be done at the same time as a blood glucose test. For more information, see the topic Blood Glucose.
• A person with new type 2 diabetes often has a normal or high level of C-peptide in the blood. Over time, a person with type 2 diabetes may develop a low level of C-peptide.
• To help tell the difference between type 1 and type 2 diabetes and to help guide treatment, most doctors look at a person's age, weight, and how long symptoms have been present. In rare cases, a C-peptide stimulation test may be done to help tell the difference between the two types of diabetes. During a C-peptide stimulation test, a blood sample is taken to measure C-peptide. Then a shot of a hormone to increase blood sugar (glucagon) is given into a vein in the arm. Another blood sample is taken. In people with type 1 diabetes, C-peptide levels will be low because the pancreas cannot make any insulin in response to the glucagon. In people with type 2 diabetes, C-peptide levels will be higher than the first blood test because the pancreas is making more insulin in response to the glucagon.

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