C-peptide
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- Also Known As:
- Insulin C-peptide
- Connecting Peptide Insulin
- Proinsulin C-peptide

This page was fact checked by our expert Medical Review Board for accuracy and objectivity. Read more about our editorial policy and review process.
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At a Glance
Why Get Tested?
To help evaluate insulin production by the beta cells in the pancreas or to help determine the cause of low blood glucose (hypoglycemia)
When To Get Tested?
When you have diabetes and your health practitioner wants to determine if you are producing enough of your own insulin or there is an absolute need for insulin injections or an insulin pump; when your health practitioner suspects that you have insulin resistance or your diabetes has been incorrectly classified; when you have documented hypoglycemia and your health practitioner is investigating causes.
Sample Required?
A blood sample is obtained by inserting a needle into a vein. If a 24-hour urine sample is required, all urine produced over a 24-hour time period will be collected.
Test Preparation Needed?
Fasting for 8 to 10 hours before blood testing is usually required. In some cases, your healthcare practitioner may want to examine C-peptide production after a formal stimulation test in which you will be administered a standardized amount of a meal or substance that stimulates the pancreas.
You may be able to find your test results on your laboratory’s website or patient portal. However, you are currently at Testing.com. You may have been directed here by your lab’s website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab’s website or portal, or contact your healthcare practitioner in order to obtain your test results.
Testing.com is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.
The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.
If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.
Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called “normal” values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.
While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are “within normal limits.”
For more information, please read the article Reference Ranges and What They Mean.
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 sometimes a urine sample.
In the pancreas, within specialized cells called beta cells, proinsulin, a biologically inactive molecule, is split 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 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 taken in as diabetic medication (exogenous) which not generate C-peptide. This test may be done in conjunction with an insulin test or a glucose test.
Common Questions
View Sources
Sources Used in Current Review
Current review performed by Jennifer L. Powers, DABCC, Assistant Director, Core Lab for Clinical Studies, Washington University in St. Louis
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