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Also known as: Insulin C-peptide; Connecting peptide insulin; Proinsulin C-peptide
Formal name: C-peptide
Related tests: Insulin; Glucose

At a Glance

Why Get Tested?

To monitor insulin production by the beta cells in the pancreas and to help determine the cause of hypoglycemia

When to Get Tested?

When you have diabetes and your doctor wants to determine if you are producing enough of your own insulin or if it is time to supplement oral medication with insulin injections or an insulin pump; when your doctor suspects you have insulin resistance; and when you have documented hypoglycemia

Sample Required?

A blood sample drawn from a vein in your arm and sometimes a 24-hour urine sample

Test Preparation Needed?

Ask your doctor if you need to fast before having this test performed.

The Test Sample

What is being tested?

This test measures the amount of C-peptide in a blood or urine sample. C-peptide is a substance (a short chain of amino acids) that is produced when proinsulin, an inactive molecule, splits apart to form two molecules. One molecule is C-peptide and the other is insulin.  C-peptide is produced at the same rate as insulin, making it useful as a marker of insulin production.

Insulin is vital for the transport of glucose into the body's cells and is required on a daily basis. It is produced and stored in the beta cells of the pancreas. When insulin is required and released from the beta cells into the bloodstream in response to increased levels of glucose, equal amounts of C-peptide are also released.

C-peptide can be used to help evaluate the production of insulin made by the body (endogenous) and to help differentiate it from insulin that is not produced by the body but is taken in as diabetic medication (exogenous) and so does not generate C-peptide.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.  If a 24-hour urine sample is required, you will be asked to save all of your urine over a 24-hour time period.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

Usually you will be asked to fast for 8 to 10 hours before having a C-peptide blood test.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 314-315.

Clarke, W. and Dufour, D. R., Editors (© 2006). Contemporary Practice in Clinical Chemistry: AACC Press, Washington, DC. Pp 290-291.

Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 186-189.

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