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What Is C-Peptide?

By Jillian O Keeffe
Updated May 17, 2024
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C-peptide is a molecule that is naturally present in the body as a part of insulin production. Levels of this molecule change when a person suffers from diabetes or another disease that alters insulin production. Doctors may analyze blood samples for C-peptide levels in order to help diagnose one of these conditions. The concentration of the molecule directly reflects the concentration of insulin in the body.

A healthy person produces insulin as part of a mechanism to transport and use glucose in the blood. Glucose is the common form of energy that the human body makes from food as a raw material. Insulin, therefore, is essential to normal functioning of the body. People who suffer from diabetes have problems with their insulin production or with the efficiency of the insulin.

Insulin is a hormone, and is made by specialized cells in the pancreas. Before they release the insulin into the blood, they make an initial form of the hormone, called preproinsulin, a larger molecule than insulin. Enzymes inside the cell have to chop up preproinsulin into three pieces before the cell can send it out into the blood. Two of these pieces, from each end of the preproinsulin molecule, stick together to make insulin in its final form. The center portion that is cut out is C-peptide, and this floats off by itself.

The pancreas releases insulin after this process, and the organ also releases C-peptide. Scientists do not know whether C-peptide has any useful role in the body, or if it is simply a byproduct of the insulin production process. Within medical diagnostics, however, the molecule is used to assess how much insulin an individual patient produces. This is possible because each C-peptide molecule represents one preproinsulin, and in turn is equal to one insulin molecule.

Only natural insulin production produces the peptide, not artificial insulin injections. Once doctors know how much C-peptide is in the blood, they know how much insulin a person makes. Type 1 diabetes is an illness where patients do not make a normal level of insulin, and so the peptide test can help diagnose this condition. Other diseases, such as a cancer of the pancreas, can affect the levels of insulin made, and the test can help to identify this. Even if a person is receiving medication to alter insulin levels, sometimes the medicine needs to be tweaked, and one of these tests can identify whether more or less of the medication is optimal for the patient.

Typically, the test requires a sample of blood from a patient, as insulin and C-peptide are both found in blood. This may be sampled through a vein, or from a tiny puncture wound for very young kids. The blood is then analyzed by a laboratory and the results returned to the doctor. Possible side effects of the sampling can include dizziness, pain at the site of sampling and a small chance of infection.

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