A C-peptide test measures the level of this peptide in the blood. It is generally found in amounts equal to insulin. 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 an illustration 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:

• 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

Do not eat or drink for 8 hours before your blood test.

Insulin and some oral medicines used to treat type 2 diabetes can change the test results. Your health professional 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.