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Also known as: Insulin 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 hypoglycaemia (low blood sugar)

When to Get Tested?

If you have diabetes mellitus and your doctor wants to see if you are producing any of your own insulin, if you are thought to be insulin resistant (when your body does not respond properly to insulin), or to help establish if it is time to add insulin injections to your current treatment. Your doctor also may request a C-peptide blood test if you have an episode of low blood sugar, called hypoglycaemia.

Sample Required?

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

Test Preparation Needed?

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

The Test Sample

What is being tested?

This test measures the amount of C-peptide in a blood or urine sample. C-peptide and the hormone insulin are created from a larger molecule called proinsulin, and stored in the beta cells of the pancreas. When insulin is released into the bloodstream to help transport glucose into the body's cells (to be used for energy), equal amounts of C-peptide also are released. This makes C-peptide useful as a marker of insulin production and indeed better in some cases than insulin as it is more stable in the blood.

C-peptide can be used to help evaluate the production of endogenous insulin (insulin made by the body's beta cells) and to help differentiate it from exogenous insulin (insulin that is not produced by the body, e.g. injected insulin). This differentiation can be used to help diagnose and monitor a variety of conditions.

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?

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

The Test

Common Questions

Ask a Laboratory Scientist

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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.