By Susan George Former Head of Department (Biochemistry)
This test measures the level of C-peptide in a blood sample. C-peptide and insulin are protein chains created by the activation and division of proinsulin (an inactive precursor to insulin). C-peptide and insulin are created and stored in the beta cells of the pancreas. When insulin is required and released into the bloodstream to help transport glucose into the body's cells (to be used in energy production), equal amounts of C-peptide also are released into the portal blood supply that passes through the liver. This makes C-peptide useful as a marker of insulin production. The liver extracts a considerable and variable amount of insulin; however, almost all of the C-peptide emerges from the liver and enter the systemic circulation. C-peptide has a half-life of about 35 minutes as compared to the half-life of insulin which is approximately 5 to 10 minutes.
How is it used?
C-peptide can be used to help evaluate the production of endogenous insulin (insulin made in and by the body's beta cells) and to help differentiate it from exogenous insulin (insulin that is not produced by the body and not accompanied by C-peptide). This differentiation can be used to help diagnose and monitor a variety of conditions.
C-peptide testing can be done when diabetes has been newly diagnosed and it is not clear whether type 1 diabetes or type 2 diabetes is present. A person whose pancreas is unable to produce any insulin (type 1 diabetes) usually has a decreased or undetectable level of C-peptide. A person with type 2 diabetes has a normal or increased level of C-peptide because the pancreas is still producing insulin. C-peptide testing together with fasting blood glucose can also help determine the cause of hypoglycemia. There are several causes of hypoglycemia, including the excessive use of medication to treat diabetes or the presence of a non-cancerous growth (tumour) in the pancreas called an insulinoma. Because man-made (synthetic) insulin does not contain C-peptide, a person with a low blood sugar level from the inappropriate use of insulin will have a low C-peptide level. An insulinoma causes the pancreas to release excessive amounts of insulin, which causes blood sugar levels to decrease (hypoglycemia). A person with an insulinoma will have a high level of C-peptide in the blood.
What Affects the Test
Factors that can interfere with your test and the accuracy of the results include:
- Medication: These include insulin, sulphonylurea medication for type 2 diabetes, corticosteroids, diuretics, birth control pills, desferrioxamine, rifampicin, terbutaline, and propranolol.
- Alcohol use.
- Kidney failure. Since both insulin and C-peptide are removed from the body by the kidneys, C-peptide levels may be increased in a person with kidney failure.
- Obesity, which can cause increased C-peptide levels because of the increased production of insulin associated with obesity.
- Recent radioactive scans within 1 week of a C-peptide test.
C-peptide: 298-1324 pmol/L
How is the sample collected for testing?
Blood sample is collected into a 10mL plain tube.
Serum has to be separated and frozen.