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.
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.
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.
How is the test used?
While not a regularly ordered lab test, C-peptide testing can be very useful in certain medical situations. Additionally, with the rising incidence of type 2 diabetes in younger patients and newer medications aimed at preserving insulin production, the test will likely become more important.
The following are some uses of C-peptide testing:
- To evaluate the cause of documented hypoglycemia Symptoms of hypoglycemia may be caused by excessive supplementation of insulin, alcohol consumption, inherited liver enzyme deficiencies, liver or kidney disease, or by insulinomas. Insulinomas are tumors of the islet cells in the pancreas that can produce uncontrolled amounts of insulin and C-peptide and can cause acute episodes of hypoglycemia. • To monitor the effectiveness of insulinoma treatment and to detect its recurrence
- To assist in management of people with diabetes treated with insulin. Most insulin tests are not able to distinguish between the body’s own insulin and that given by injections, but C-peptide levels reflect only the insulin being produced by the body’s own functioning beta cells. Additionally, people taking insulin may produce antibodies against insulin that could interfere with insulin testing, but not with C-peptide testing. Knowing how much C-peptide (and thus insulin) is being produced by the beta cells may allow healthcare practitioners to know when insulin is absolutely required and when it may be safe to switch to a different type of treatment.
- To assist in classification of diabetes in select cases Generally, type 1 and type 2 diabetes will be classified without this test. However, with increased prevalence of obesity in teens and the understanding of other types of diabetes, some people with diabetes may be difficult to classify and/or classified incorrectly. Measurement of a C-peptide 3-5 years from diagnosis can assist in classification.
- When you have had your pancreas removed or have had pancreas islet cell transplants, intended to restore the ability to make insulin, C-peptide levels may be used to verify the effectiveness of treatment and continued success of the procedure.
When is it ordered?
C-peptide levels may be ordered when there is documented acute or recurring low blood glucose (hypoglycemia) and/or excess insulin is suspected. C-peptide tests help distinguish the body's own insulin from outside sources of insulin. Symptoms of hypoglycemia include:
- Blurred vision
- In severe cases, seizures and loss of consciousness
However, many of these symptoms can occur with other conditions as well.
A C-peptide test may be ordered periodically when you have been diagnosed with an insulinoma to monitor the effectiveness of treatment and to detect tumor recurrence.
C-peptide levels may be ordered when the healthcare provider wishes to assess whether a person with diabetes still requires insulin injections or can switch to a different type of medication. This is more commonly part of the management of type 1 diabetes, but may be useful in knowing when patients with type 2 diabetes have developed an absolute insulin requirement. If a healthcare provider is suspecting incorrect classification of a person’s diabetes, the test may be ordered.
Rarely, C-peptide levels may be monitored over time when you have had your pancreas removed or have had pancreas islet cell transplants.
What does the test result mean?
A high level of C-peptide generally indicates a high level of endogenous insulin production. This may be in response to a high blood glucose caused by glucose intake and/or insulin resistance. A high level of C-peptide is also seen with insulinomas and may be seen with low blood potassium, Cushing syndrome, and renal failure.
A low level of C-peptide is associated with a low level of insulin production. This can occur when insufficient insulin is being produced by the beta cells, with diabetes for example, or when production is suppressed by treatment with exogenous insulin.
Absent C-peptide means there is an absolute requirement for insulin from an outside source.
When used for monitoring someone diagnosed with an insulinoma, decreasing levels of C-peptide indicate a response to treatment. Levels that increase may indicate that a tumor has come back (recurrence).
When used for managing your diabetes, the meaning of a value must be considered in context of your type of diabetes and the type of treatment you are currently receiving. With all the information, your healthcare practitioner can determine whether or not a change in treatment is necessary.
Is there anything else I should know?
C-peptide testing has become more widely available, though there is still some method variability. If a series of C-peptide tests are going to be performed, they should be done at the same laboratory using the same method.
Even though they are produced at the same rate, C-peptide and insulin leave the body by different routes. Insulin is processed and eliminated mostly by the liver, while C-peptide is removed by the kidneys. Since the half-life of C-peptide is longer than insulin, its levels in the blood are higher than insulin, making it a more reliable measure of insulin production.
Can I do a C-peptide test at home like I can when I check my blood glucose?
If I need to go on the insulin pump, will I need a C-peptide test?
What else is known about C-peptide?
Researchers are starting to better understand the role of C-peptide in the body. Some studies have been conducted to evaluate the use of C-peptide in conjunction with insulin for those with diabetes. Results have been promising, showing decreased diabetic complications with improvements in kidney function, blood flow, and nerve function. However, further studies are needed.