The C-Peptide Test for Diabetes
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What Is the C-Peptide Test for Diabetes?

What Is the C-Peptide Test for Diabetes?
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Published on February 14, 2025
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The C-peptide test is a way of estimating how much of the hormone insulin your body creates. This test can help doctors determine what type of diabetes you have. It can also help show how far your diabetes has progressed or how effective your diabetes treatment has been.

Diabetes 101: What Are the Different Types of Diabetes?

Diabetes 101: What Are the Different Types of Diabetes?

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What Is C-Peptide?

C-peptide is a protein that is created in the pancreas as a byproduct of insulin production.

Insulin is an essential hormone that helps to regulate blood sugar levels. One of the biggest differences between type 1 and type 2 diabetes is that people with type 1 always create too little insulin, whereas people with type 2 sometimes may have elevated insulin levels, especially when they have only recently developed diabetes. Measuring insulin levels can help determine what type of diabetes a patient has.

However, it’s not easy to measure insulin directly; insulin concentrations in the bloodstream change quickly and dramatically because the hormone is rapidly cleared by the liver. Researchers have learned the concentration of C-peptide, a surrogate marker for natural insulin production, provides a more reliable and stable measurement.

The C-peptide test is useful even in people who use insulin to treat their diabetes, because injected insulin carries no C-peptide. The test is still a reliable indication of how much natural insulin production is occurring within the pancreas.

Who Typically Needs a C-Peptide Test?

Doctors sometimes order a C-peptide test for people with newly-diagnosed diabetes to help identify what type of diabetes they have, especially for people who did not fit the classic archetypes of type 1 or type 2 diabetes.

The test can also be helpful for people with diabetes who are not responding well to their prescribed treatment. Because both major forms of diabetes share similar symptoms, it can be easy for clinicians to make a misdiagnosis. Up to 40 percent of adults who develop type 1 diabetes after age 30 are initially misdiagnosed with type 2 diabetes.

People who have been told that they have type 1 diabetes are also sometimes misdiagnosed. A pilot program of routine C-peptide testing resulted in about 7 percent of adults being reclassified into a different form of diabetes.

 In some cases, a C-peptide test will help identify the presence of a rarer form of diabetes, such as latent autoimmune diabetes in adults (LADA) or maturity-onset diabetes of the young (MODY).
The C-peptide test may also be useful for people with longstanding diabetes. In both type 1 and type 2 diabetes, insulin production tends to decline over the years. A C-peptide test can help reveal how much natural insulin production an individual has left, which may help a doctor refine their diabetes treatment regimen.

The C-peptide test can also be used to identify other conditions that create a hormone imbalance, and is sometimes used when people without diabetes experience hypoglycemia. A C-peptide test can help identify the following conditions:

How Is the C-Peptide Test Performed?

C-peptide is generally measured with a blood test.

A fasting blood draw is usually preferable because the results are easier to interpret than those of a random blood draw, which can be strongly affected by what you’ve eaten recently. Your medical provider may schedule your blood draw for the morning and ask you to not eat or drink anything before your test.

In some cases, especially if results of a first test are unclear, your doctor may run a “stimulated” C-peptide test. You’ll be instructed to consume a particular amount of glucose, or receive a small injection of glucagon, a hormone that increases blood sugar levels. This will show how well your insulin production can temporarily ramp up in response to rising blood sugar concentrations.

C-peptide can also be measured with a urine test. This can be a single test, or it may require you to collect urine over 24 hours.

What’s Next: C-Peptide Test Interpretation

The results of your C-peptide test will help your healthcare provider determine how much natural baseline insulin your body makes.

C-peptide results are reported with a reference range, which will vary from one lab to another. Your C-peptide may be high, low, or normal. In new patients, high levels tend to indicate type 2 diabetes, and low levels tend to indicate type 1 diabetes.

 Nevertheless, this measurement can be difficult to interpret, and any result should be discussed with the healthcare professional who ordered your test and will help interpret it in the context of your blood sugar level.

High C-Peptide Levels

New patients with type 2 diabetes typically have C-peptide measurements on the high end of normal, or above the normal range. That’s because the insulin resistance characteristic of type 2 diabetes forces the pancreas to work harder and produce extra insulin.

Type 2 diabetes, however, is a progressive disease. Insulin resistance increases over time, and eventually, the overworked pancreas begins to lose its ability to produce insulin. The longer you have had type 2 diabetes, the more your C-peptide levels are likely to decline. After years or decades of life with type 2 diabetes, C-peptide may eventually be so low as to be undetectable.

A high C-peptide measurement may also indicate conditions other than diabetes, such as:

Low C-Peptide Levels

New patients with type 1 diabetes typically have C-peptide levels that are on the low end of normal, or below the normal range, indicating too little insulin secretion. People who develop type 1 diabetes as adults typically have a slower disease onset, and usually have more capacity to secrete insulin both when they are diagnosed and in the following years. In cases of severe onset, such as people diagnosed with diabetic ketoacidosis, there may be no evidence of C-peptide at all.

A C-peptide measurement, however, should not be used alone to diagnose type 1 diabetes. The official diagnostic criteria require both high blood sugar levels and the presence of the autoimmune markers that cause type 1 diabetes.

For people with longstanding type 1 diabetes, even very small amounts of measurable C-peptide are a good sign, and are associated with better blood sugar control, lower insulin requirements, and a lower risk of hypoglycemia.

A low C-peptide measurement can also indicate the presence of Addison’s disease or liver disease.

The Takeaway

  • C-peptide measurement is a reliable way of estimating a patient’s natural capacity to produce insulin.
  • The test helps identify different types of diabetes in the newly diagnosed, and for people who are not responding well to their diabetes treatment, the test may help reveal a misdiagnosis.
  • C-peptide tests can also be used to identify other conditions that impact hormone production, such as Addison’s disease or Cushing’s syndrome.
  • In every form of diabetes, insulin production declines with age; a C-peptide test can help a doctor understand how far your condition has progressed, potentially letting them refine your diabetes management approach.

Resources We Trust

EDITORIAL SOURCES
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Resources
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  3. Misdiagnosis of type 1 and type 2 diabetes in adults. The Lancet Regional Health - Europe. June 2, 2023.
  4. Foteinopoulou E et al. Impact of routine clinic measurement of serum C‐peptide in people with a clinician‐diagnosis of type 1 diabetes. Diabetic Medicine. November 1, 2020.
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  6. C-Peptide Test. Cleveland Clinic. October 3, 2022.
  7. C-Peptide (Urine). University of Rochester Medical Center.
  8. Iqbal S et al. The Predictive Ability of C-Peptide in Distinguishing Type 1 Diabetes From Type 2 Diabetes: A Systematic Review and Meta-Analysis. Endocrine Practice. May 2023.
  9. Arya P et al. C-peptide Level in Patients With Uncontrolled Type 2 Diabetes Mellitus on Oral Anti-diabetic Drugs. Cureus. March 24, 2024.
  10. Grönberg A et al. Higher Risk of Severe Hypoglycemia in Children and Adolescents With a Rapid Loss of C-Peptide During the First 6 Years After Type 1 Diabetes Diagnosis. BMJ Open Diabetes Research & Care. November 15, 2022.
  11. American Diabetes Association Professional Practice Committee. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes—2024. Diabetes Care. December 11, 2023.
  12. Taylor PN et al. C-Peptide and Metabolic Outcomes in Trials of Disease Modifying Therapy in New-Onset Type 1 Diabetes: An Individual Participant Meta-Analysis. The Lancet Diabetes & Endocrinology. December 2023.
  13. Lanctôt SO et al. Fasted C-Peptide Distribution and Associated Clinical Factors in Adults With Longstanding Type 1 Diabetes: Analysis of the Canadian Study of Longevity in Type 1 Diabetes. Canadian Journal of Diabetes. March 2024.
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Anna L. Goldman, MD

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Anna L. Goldman, MD, is a board-certified endocrinologist. She teaches first year medical students at Harvard Medical School and practices general endocrinology in Boston.

Dr. Goldman attended college at Wesleyan University and then completed her residency at Icahn School of Medicine at Mount Sinai Hospital in New York City, where she was also a chief resident. She moved to Boston to do her fellowship in endocrinology at Brigham and Women's Hospital. She joined the faculty after graduation and served as the associate program director for the fellowship program for a number of years.

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Ross Wollen

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Ross Wollen joined Everyday Health in 2021 and now works as a senior editor, often focusing on diabetes, obesity, heart health, and metabolic health. He previously spent over a decade as a chef and craft butcher in the San Francisco Bay Area. After he was diagnosed with type 1 diabetes at age 36, he quickly became an active member of the online diabetes community, eventually becoming the lead writer and editor of two diabetes websites, A Sweet Life and Diabetes Daily. Wollen now lives with his wife and children in Maine's Midcoast region.

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