Test Quick Guide

The C-peptide blood test helps evaluate insulin production by the beta cells in the pancreas or helps determine the cause of low blood glucose (hypoglycemia). It is commonly used as a diagnostic test for diabetics to determine whether or not your body is producing enough insulin.

The test can also be used if your doctor suspects you have insulin resistance, your diabetes has been incorrectly classified, or you have documented hypoglycemia, and your health practitioner is investigating causes.

About the Test

Purpose of the test

While not a regularly ordered lab test, C-peptide testing can be very useful in certain medical situations. Additionally, with the rising incidence of type 2 diabetes in younger patients and newer medications aimed at preserving insulin production, the test will likely become more commonly used.

The following are some uses of C-peptide testing:

  • To evaluate the cause of documented hypoglycemia. Symptoms of hypoglycemia may be caused by excessive supplementation of insulin, alcohol consumption, inherited liver enzyme deficiencies, liver or kidney disease, or insulinomas. The latter are tumors of the islet cells in the pancreas that can produce uncontrolled amounts of insulin and C-peptide and cause acute episodes of hypoglycemia.
  • To monitor the effectiveness of insulinoma treatment and to detect its recurrence
  • To assist in the management of people with diabetes treated with insulin. Most insulin tests cannot distinguish between the body’s insulin and that given by injections. Still, C-peptide levels reflect only the insulin being produced by the body’s functioning beta cells.

    Additionally, if you are taking insulin, you may produce antibodies against it that could interfere with insulin testing but not with C-peptide testing. Knowing how much C-peptide (and thus insulin) is produced by the beta cells may allow health care practitioners to know when insulin is required and when it may be safe to switch to a different type of treatment.

  • To assist in the classification of diabetes in select cases. Generally, type 1 and type 2 diabetes will be classified without this test. But with the increased prevalence of obesity in teens and the understanding of other types of diabetes, some people with diabetes may be difficult to classify and/or classified incorrectly. Measurement of a C-peptide three to five years from diagnosis can assist in classification.
  • When you have had your pancreas removed or pancreas islet cell transplants intended to restore the ability to make insulin. In this case, C-peptide levels may be used to verify the effectiveness of treatment and the continued success of the procedure.

What does the test measure?

C-peptide is a substance — a short chain of amino acids — released into the blood as an inactive byproduct of the formation of insulin by the pancreas. This test measures the amount of C-peptide in a blood or sometimes urine sample.

Within specialized cells called beta cells in the pancreas, proinsulin, a biologically inactive molecule, is split apart. It forms one molecule of C-peptide and one molecule of insulin, vital for transporting glucose into the body’s cells and required daily.

When insulin is released from the beta cells into the blood in response to increased levels of glucose, equal amounts of C-peptide are also released. Since C-peptide is produced at the same rate as insulin, it is useful as a marker of insulin production.

In particular, C-peptide testing can be used to help evaluate the production of insulin made by the body (endogenous) and to help differentiate it from insulin taken in as diabetic medication (exogenous), which does not generate C-peptide. This test may be done in conjunction with an insulin test or a glucose test.

When should I get this test?

C-peptide levels may be ordered when there is documented acute or recurring hypoglycemia and/or excess insulin is suspected. These tests help distinguish the body’s insulin from outside sources of insulin. Symptoms of hypoglycemia include:

  • Sweating
  • Palpitations
  • Hunger
  • Confusion
  • Blurred vision
  • Fainting
  • In severe cases, seizures and loss of consciousness

However, many of these symptoms can occur with other conditions as well.

A C-peptide test may be ordered periodically when you have been diagnosed with an insulinoma to monitor the effectiveness of treatment and to detect tumor recurrence.

C-peptide levels may be ordered when the health care provider wishes to assess whether a person with diabetes still requires insulin injections or can switch to a different type of medication.

This is more commonly part of the management of type 1 diabetes, but may be useful in knowing when patients with type 2 diabetes have developed an absolute insulin requirement. If a health care provider is suspecting incorrect classification of your diabetes, the test may be ordered.

Rarely, C-peptide levels may be monitored over time when you have had your pancreas removed or have had pancreas islet cell transplants.

Finding a C-peptide Test

How can I get a c-peptide test?

C-peptide testing is usually performed at a doctor’s office or another medical setting like a hospital or lab. The tests are normally ordered by a doctor but may be available without orders from your doctor at a walk-in lab.

Can I take the test at home?

No. The C-peptide test requires special equipment and training to perform.

How much does the test cost?

The cost of a C-peptide test will vary depending on factors such as where the test is done, and whether you have health insurance. When ordered by a doctor, insurance typically covers the test, although you may have to pay a copay or deductible. Your doctor’s office, lab, and health plan can provide information about any out-of-pocket costs that may be your responsibility.

Taking the C-peptide Test

The C-peptide test usually requires a blood sample, which is usually taken from your arm in a doctor’s office, health clinic, hospital, or lab.

Before the test

Fasting for eight to 12 hours before blood testing is usually required. In some cases, your health care practitioner may want to examine C-peptide production after a formal stimulation test in which you will be administered a standardized amount of a meal or substance that stimulates the pancreas.

During the test

A blood sample is taken from a vein in your arm. The person taking the sample may tie a band around your upper arm and will clean the area where the needle will be inserted into your skin. A small amount of blood is drawn into a tube. You may feel a slight sting when the needle enters your skin.

The process of taking a blood sample usually takes less than three minutes.

After the test

At a doctor’s office or lab, you will be asked to apply gentle pressure to the site with a bandage or a piece of gauze after the needle is withdrawn. This will help stop bleeding and may prevent bruising. Next, the site will be bandaged. You may resume your normal activities following the test.

A blood draw is a very low-risk procedure. You may have slight bruising at the site where the blood sample was taken.

C-peptide Test Results

Receiving test results

The doctor who ordered your C-peptide test may share the results with you, or you may be able to access them through an online patient portal. C-peptide results are usually available within a few business days.

Interpreting test results

A high level of C-peptide generally indicates a high level of endogenous insulin production. This may be in response to a high blood glucose caused by glucose intake and/or insulin resistance. A high level of C-peptide is also seen with insulinomas and may be seen with low blood potassium, Cushing’s syndrome, and renal failure.

A low level of C-peptide is associated with a low level of insulin production. This can occur when insufficient insulin is being produced by the beta cells, with type 1 diabetes, for example, or when production is suppressed by treatment with exogenous insulin.

Absent C-peptide means there is an absolute requirement for insulin from an outside source.

When used for monitoring someone diagnosed with an insulinoma, decreasing levels of C-peptide indicate a response to treatment. Increasing levels may indicate that a tumor has come back (recurrence).

And when used for managing your diabetes, the meaning of a value must be considered in the context of your type of diabetes and the treatment you are currently receiving. With all the information, your health care practitioner can determine whether or not a change in treatment is necessary.

You may want to ask your doctor the following questions:

  • Why test for C-peptide rather than insulin?
  • If I have an insulin pump, will I still need a C-peptide test?
  • Are there any changes I should make to my diet/activity level?


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