C-Peptide (Blood)

Does this test have other names?

Connecting peptide insulin, insulin C-peptide, proinsulin C-peptide 

What is this test?

This blood test looks at how well your body's makes insulin. It's used to help diagnose blood sugar disorders.

Your body needs the hormone insulin to move sugar through your bloodstream to your cells for energy. A healthy pancreas produces equal amounts of insulin and the protein C-peptide, so by measuring your C-peptide, your health care provider can also find out a lot about your insulin level. 

Why do I need this test?

Measuring C-peptide can show whether you have type 1 or type 2 diabetes. In type 1 diabetes, your body doesn't produce any insulin. In type 2 diabetes, either your body doesn't produce enough insulin or your cells ignore it.

If you have diabetes, the C-peptide test can show how well your treatment is working.

The C-peptide test may also be done to find the cause of low blood sugar or to monitor the activity of tumors that secrete insulin.

What other tests might I have along with this test?

Your doctor also might order:

  • Blood glucose test to measure the amount of glucose (sugar) in your blood

  • Glucagon test to measure levels of another hormone secreted by the pancreas. (Glucagon can increase blood sugar.)

  • Glycosylated hemoglobin blood test, an indirect measure of the state of your blood glucose levels

  • Insulin assay, a test that directly measures your insulin levels

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

Test results are given in nanograms per milliliter (ng/mL). Normal results are within the range of 0.5 to 2.0 ng/mL.

A high level of C-peptide could mean a number of conditions, including a kidney problem or an insulinoma, a tumor of the insulin-producing cells in the pancreas. It could also mean you need to adjust the amount of insulin you take.

A level of C-peptide that's lower than normal means your body isn't producing enough insulin or that your pancreas isn't working properly.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm. It's done after fasting for eight to 10 hours, but may be repeated after you've eaten.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Taking insulin for your diabetes can increase your C-peptide levels. Your C-peptide level can also be affected if your kidneys aren't working properly.

How do I get ready for this test?

You will probably need fast for eight to 10 hours before you have this test. Ask your health care provider for specific instructions.  Be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.