C-Peptide (Blood)

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

This blood test looks at how well your pancreas makes insulin. It's used to help diagnose blood sugar disorders, such as low blood sugar (hypoglycemia) and diabetes.

Your body needs the hormone insulin to move sugar through your bloodstream to your cells for energy. A healthy pancreas makes equal amounts of insulin and the protein C-peptide. C-peptide and insulin enter your bloodstream at the same time and in equal amounts. C-peptide stays in your blood longer than insulin, but C-peptide doesn't affect blood sugar levels. So by measuring your C-peptide, your health care provider can also learn about your insulin level. 

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

If you have diabetes, the C-peptide test can show how well your treatment is working. Most insulin tests can't tell the difference between your body’s insulin and insulin given by injections. But the C-peptide test can. It shows how much insulin your pancreas is making. So it can be used to make decisions about your treatment. Your health care provider may order this test to check if certain medicines are helping your body make more insulin. These may include glucagon-like peptide 1 analogs (GLP-1) or DPP IV inhibitors.

If your diabetes diagnosis is not certain after 3 years, a C-peptide test may be needed.

The C-peptide test may be done to find the cause of low blood sugar. These may include:

• Liver or kidney disease.
• Malnutrition.
• Side effects from certain diabetes medicines, including insulin.
• Too much alcohol use without eating.

The C-peptide test may also be done to check the activity of tumors that make insulin. If you have been diagnosed with a tumor that makes insulin, your provider will use this test to check how well your treatment is working. They will also use this test to keep track of your progress and to find out if the tumor has returned.

Your health care provider also might order:

• Blood glucose test. This measures the amount of glucose (sugar) in your blood.

• Glucagon test. This measures the level of another hormone made by the pancreas. Glucagon helps raise blood sugar.

• A1c test. This is also known as glycosylated hemoglobin blood test. This is a measure of your blood glucose levels over the past 3 months. It shows how well your diabetes is being controlled.

• Insulin assay. This is a test that directly measures your insulin levels.

• Liver and kidney function tests. These are to rule out liver and kidney diseases.

Test results may vary depending on your age, gender, health history, and other things. Your test results may be different depending on the lab used. They may not mean you have a problem. Ask your health care provider what your test results mean for you.

Test results are given in nanograms per milliliter (ng/mL). Normal results are within the range of 0.5 to 2.7 ng/mL but can vary depending on the lab that is used for testing.

A high level of C-peptide could mean a number of conditions. It may mean you have a kidney problem. Or you may have an insulinoma. This is a tumor that grows in the cells in the pancreas that make insulin. Or you may need to adjust the amount of insulin you take. Other causes include:

• A low level of potassium in your blood.
• Cushing syndrome.

A level of C-peptide that's lower than normal means that your body isn't making enough insulin or that your pancreas isn't working correctly. Other causes include:

• Taking too much insulin to treat diabetes. This may block your pancreas from making insulin on its own.
• A severe infection.
• Addison disease.
• Liver disease.

If you have been treated for an insulin-producing tumor in your pancreas, a decrease in your C-peptide levels means your treatment is working. An increase in your C-peptide may mean that your tumor is back.

The test needs a blood sample, which is drawn through a needle from a vein in your arm.

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.

Taking insulin for your diabetes can raise your C-peptide levels. Your C-peptide level can also change if your kidneys aren't working correctly. The timing of your most recent meal may affect your C-peptide level.

Follow your health care provider's directions for not eating or drinking anything before the test. The test may also be done after you've eaten. Tell your provider about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drug use. Do NOT stop taking any medicine without first talking to your provider. Talk to your provider about any foods you need to stay away from for this test.