C-peptide test

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).

A health professional uses a needle to take a blood sample, usually from the arm.

Your doctor will give you instructions about eating and drinking before this 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.

Normal

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

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 it may mean 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 can't be measured. The blood glucose level will be high, and insulin will be needed in order for the person to survive.