What is C-Peptide ?

In this article, you will learn about the lab testing of C-peptide as well as why it is of importance. Many frequent questions regarding this test will be answered as well as information you may not have realized you need.

Also, a better understanding of the physiology of c peptide will be discussed to help give a foundation to your learning. This article is for knowledge base only and you should always seek the assistance of medical personnel to understand and read lab values.

For further research on the subject, the following link has many short videos to help you understand the various medical vocabulary used in this article


Physiology of C Peptide

It is important to understand the physiology of any lab result so that you can have a fundamental understanding of why it may be needed in certain diagnosis.

  • It is an amino acid polypeptide
  • It is produced by beta cells in the pancreases
  • Its role is to connect the A-chain in the insulin protein chain to its B-chain.
  • The c-peptide is removed after proinsulin is placed into the vesicles of the Golgi apparatus.
    o    This action creates the bond between the A and B chain(1,3)
  • It is eliminated by the kidneys, not the liver as in insulin elimination.
  • The half-life (the amount of time in the body before being eliminated is thirty minutes. (4)

c-peptide protein is made up of 31 amino acids.figure


Image 1: In this image, you can see how the c-peptide protein is made up of 31 amino acids. This is a self-made depiction of c-peptide. (5)

Photo Source : upload.wikimedia.org

What is the C Peptide lab test and what does it check for?

Diagnostic purposes

  • Assists in diagnosis between Type 1 and Type 2 diabetes – Shows how much natural insulin is being produced.
  • Assists in diagnosis between hypoglycemia and diabetes
  • Diagnosis  of Factitious hypoglycemia
  • Diagnosis of gastronomes due to Multiple Endocrine Neoplasm Syndromes.
  • Ruling out of Polycystic Ovarian Syndrome. (1,2)

When certain symptoms are noted in patients which may indicate low blood sugar. These symptoms include and are not limited to:

  • Sweating
  • Hunger
  • Fainting
  • Blurred vision
  • Palpitations
  • Seizures or loss of consciousness (4)

Treatment purposes

  • It is used to better understand how your treatment is working and guide further treatment.
  • Although it does not check blood sugar it confirms if and how much insulin your body is making.
  • Some patients start showing antibodies to insulin, therefore, this test is done to check function. (2,4,8,13,16)

The process is a simple blood sample taken by a lab assistant.

Are there any special considerations before getting the test done?

You will need to ask the doctor if he prefers:

  • That you eat before the test
  • That you do not eat 8-10 hours before.
  • This depends on why the doctor is ordering the test.

Always be sure your doctor and or lab assistant is aware of any medications you are currently taking.

  • Hold medications only if instructed by your doctor.
  • The only medications that may be held are oral Type 2 diabetic medications.  (2,5,6,15)

This test is not done in all laboratories and you may want to check ahead of time before going in to get the test done. (4)

  • For the actual test, the only discomfort is the needle stick to draw the blood sample (4,5,6,11,12)

What is a C Peptide bond ?

The peptide bond is what was explained in the physiology of c-peptide. It is basically a bond made between two amino acids in a peptide chain. In this instance, the bond is between the A and B chain. (11)

a peptide bond which is simply connecting two amino acids in the chain.figure

Figure 2 : A peptide bond which is simply connecting two amino acids in the chain.

Image Source : upload.wikimedia.org

What are the C Peptide Normal ranges

Normal levels consist of the following:

  • Some place the rage at 0.5-2.0 nanograms (2)
  • Others place the range at 0.8-3.1 nomograms or 0.26-1.03 nomograms
  • The range of results depends on why your doctor has ordered the test.

Lower levels means you are not making enough insulin

Could prove:

  • Type 1 or 2 diabetes
  • Poorly functioning pancreas
  • Low blood sugar
  • Shrinking in insulinoma (discussed in more detail further on) (2)

Higher levels means you are making too much insulin

Could prove:

  • Insulin resistance
  • Tumor or insulinoma (discussed in more detail further on)
  • Kidney disease
  • High doses of type 2 diabetic medications or need for reduction in dosage (2)

How does C Peptide relate with diabetes?

The c peptide shows the amount of insulin being made in the body. For those who have diabetes and are using insulin, the c-peptide test can help the doctor to understand how well the treatment is helping.

With type 2 diabetes the body makes too much insulin and the patient is given oral medications to help decrease insulin production. The c-peptide test can help your doctor to see if your dosage should be increased or decreased because they can see how much natural insulin your body is making. (2,4,6,9,11)

What is C Peptide insulinoma?

An insulinoma is simply a tumor found in the pancreas. Specifically, it is found in the cells which produce insulin called islets of Langerhans. Insulinomas cause low blood glucose levels because it blocks the production of said insulin. (17)

Are there any risks to the test?

The chance of risk is extremely low. In the case of any risk, it will be the same as for any blood draw and may include the following.

  • Hematoma or accumulation of blood under the skin. This will appear as a bruise.
  • Localized pain around the blood draw site.
  • Fasting or light-headedness. (8)

the blood draw is being done with a butterfly needle.photo
Picture 3 : The blood draw is being done with a butterfly needle.

Figure Source : www.google.com.mx

What is C Peptide insulin resistance?

When a patient has been administered insulin the body sometimes creates antibodies which fight against the insulin received. A high level of c-peptide when patient had been fasting can show insulin resistance.  (4,14)


  1. https://en.wikipedia.org/wiki/C-peptide
  2. http://www.webmd.com/diabetes/what-is-c-peptide-test#1
  3. https://labtestsonline.org/understanding/analytes/c-peptide/tab/test/
  4. https://medlineplus.gov/ency/article/003701.htm
  5. http://emedicine.medscape.com/article/2087824-overview
  6. http://www.diabetes.co.uk/c-peptide-test.html
  7. http://www.medicinenet.com/script/main/art.asp?articlekey=12467
  8. http://kidshealth.org/en/parents/test-cpeptide.html
  9. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3748788/
  10. https://www.ncbi.nlm.nih.gov/pubmed/403392
  11. https://en.wikipedia.org/wiki/Peptide_bond
  12. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0052847
  13. https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/protein2.htm
  14. https://www.ncbi.nlm.nih.gov/pubmed/10780930
  15. https://www.khanacademy.org/test-prep/mcat/biomolecules/amino-acids-and-proteins1/v/peptide-bond-formation-and-cleavage
  16. http://www.jbc.org/content/278/17/14798.full
  17. https://labtestsonline.org/glossary/insulinoma/

Published by Dr. Rajesh MD under Diabetes Information.
Article was last reviewed on January 8th, 2022.

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