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

The Test Sample

What is being tested?

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

In the pancreas, within specialized cells called beta cells, proinsulin, a biologically inactive molecule, splits apart to form one molecule of C-peptide and one molecule of insulin. Insulin is vital for the transport of glucose into the body's cells and is required on a daily basis. When insulin is required and 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 that is not produced by the body but is taken in as diabetic medication (exogenous) and so does not generate C-peptide. This test may be done in conjunction with an insulin test.

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, all urine produced over a 24-hour time period will be collected.

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?

Fasting for 8 to 10 hours before blood testing is usually required.