What is a C-peptide test?

This test measures the level of C-peptide in a sample of your blood or urine (pee). Measuring C-peptide is an accurate way to find out how much insulin your body is making.

Insulin is a hormone that your pancreas makes. It helps blood glucose (blood sugar) get into your cells, where it can be used for energy. This helps keep your blood glucose at healthy levels.

During the process of making insulin, your pancreas produces C-peptide. C-peptide and insulin enter your bloodstream at the same time and in equal amounts. C-peptide doesn't affect your blood glucose levels, but it stays in your blood longer than insulin, so it's easier to measure accurately.

A C-peptide test can provide important information to help understand, monitor, and/or treat disorders that involve how well your body makes insulin, such as hypoglycemia (low blood sugar) and diabetes.

Other names: insulin C-peptide, connecting peptide insulin, proinsulin C-peptide

What is it used for?

A C-peptide test may be used to help:

• Find the cause of hypoglycemia (low blood sugar). Some types of hypoglycemia are linked to high insulin levels. A C-peptide test can tell you whether too much insulin is involved in your condition. Possible causes of hypoglycemia include:
  • Liver or kidney disease
  • Malnutrition
  • Drinking too much alcohol without eating
  • A tumor in your pancreas (uncommon)
  • Side effects from certain diabetes medicines, including insulin.
• Manage diabetes treatment. If you have type 1 or type 2 diabetes, treatment decisions may depend on knowing how much insulin your pancreas is producing. A C-peptide test can provide an accurate measurement, even if you take insulin for diabetes. That's because your C-peptide levels depend on how much insulin your pancreas makes. They aren't affected by insulin that you take.
• Monitor treatment for a tumor in your pancreas, called an insulinoma (uncommon). These tumors make too much insulin and cause low blood sugar. They are almost always benign (not cancer) and can usually be removed with surgery.
• To find out whether you have type 1 or type 2 diabetes if your diagnosis is uncertain. Usually, your health care provider can diagnose which type of diabetes you have. But in certain cases, it can be hard to tell for sure. If your diabetes diagnosis is uncertain after 3 years, a C-peptide test may be needed.

Why do I need a C-peptide test?

You may need a C-peptide test if:

• You have been diagnosed with hypoglycemia (low blood sugar) but don't know what's causing it.
• You have diabetes and you:
  • Take insulin, and your provider is considering changing your treatment.
  • Have type 2 diabetes, and your provider wants to see if you need to start taking insulin.
  • Have hypoglycemia, which could be caused by taking too much diabetes medicines.
• You have been diagnosed with a pancreatic tumor (insulinoma). A C-peptide test can help monitor your condition and treatment.

What happens during a C-peptide test?

A C-peptide test usually uses a sample of your blood. But the test may also be done on a sample of all your urine collected over a 24-hour period.

During a blood test, a health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

For a 24-hour urine sample test, you will be given a special container to collect your urine and instructions on how to collect and store your samples. Your provider will tell you what time to start. The test generally includes the following steps:

• To begin, urinate in the toilet as usual. Do not collect this urine. Write down the time you urinated.
• For the next 24 hours, collect all your urine in the container.
• During the collection period, store the urine container in a refrigerator or in a cooler with ice.
• 24 hours after starting the test, try to urinate if you can. This is the last urine collection for the test.
• Return the container with your urine to your provider's office or the laboratory as instructed.

Will I need to do anything to prepare for the test?

For a C-peptide blood test, you may need to fast (not eat or drink) for 8–12 hours before the test. Ask your provider if there are any specific instructions you need to follow before either a blood or a urine test.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly. There are no known risks to a urine test.

What do the results mean?

The meaning of your results depends on your age, health, the medicines you take, and the results of other tests, such as a blood glucose test. In general:

• A high level of C-peptide usually means that your body is making too much insulin. Conditions that cause high insulin levels include:
  • Type 2 diabetes
  • Cushing's syndrome
  • Insulinoma, a tumor in your pancreas
  • Kidney failure
  • A low level of potassium in your blood
• A low level of C-peptide may mean your body isn't making enough insulin. Conditions that cause low insulin levels include:
  • Type 1 diabetes and, in some cases, type 2 diabetes
  • Taking too much insulin to treat diabetes (Taking more insulin than you need 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.

If you have questions about your results, talk with your provider.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a C-peptide test?

A C-peptide test is not used to diagnose diabetes. Other tests, such as blood glucose and urine glucose, are used to screen for and diagnose diabetes.