C-peptide Also known as: Insulin C-peptide; Connecting peptide insulin; Proinsulin C-peptide

C-peptide
Also known as: Insulin C-peptide; Connecting peptide insulin; Proinsulin C-peptide
Formal name: C-peptide
Related tests: Insulin; Glucose
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The Test
How is it used?
When is it ordered?
What does the test result mean?
Is there anything else I should know?
How is it used?
C-peptide is used to monitor insulin production and to help determine the cause of low blood sugar (hypoglycemia). The test is not ordered to help diagnose diabetes, but when a person has been newly diagnosed with diabetes, it may be ordered by itself or along with an insulin level to help determine how much insulin a person's pancreas is still producing.

Type 1 diabetes is an autoimmune process that often starts in early childhood and involves the almost complete destruction of the beta cells over time. Eventually, little or no insulin is produced, leading to a complete dependence on exogenous insulin.

In type 2 diabetes, there is insulin resistance and a compensatory increase in insulin production and release that can also lead to beta cell damage. Type 2 diabetics usually are treated with oral drugs to stimulate their body to make more insulin and/or to cause their cells to be more sensitive to the insulin that is already being made. Eventually, because of the beta cell damage, type 2 diabetics may make very little insulin and require injections. Any insulin that the body does make will be reflected in the C-peptide level; therefore, the C-peptide test can be used to monitor beta cell activity and capability over time and to help the doctor determine when to begin insulin treatment.

Patients who are or were on insulin therapy, regardless of the source of the insulin, may develop antibodies to insulin. These typically interfere with tests for insulin, making it nearly impossible to directly evaluate endogenous insulin production. In these cases, C-peptide measurement is a useful alternative to testing for insulin.

C-peptide measurements can also be used in conjunction with insulin and glucose levels to help diagnose the cause of documented hypoglycemia and to monitor its treatment. 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. C-peptide tests may be ordered periodically to monitor the effectiveness of insulinoma treatment and to detect recurrence.

Sometimes a C-peptide test may be used to help evaluate a person diagnosed with metabolic syndrome, a set of risk factors that includes abdominal obesity, increased blood glucose and/or insulin resistance, dyslipidemia, and hypertension.

When is it ordered?
C-peptide levels may be ordered when a person has been newly diagnosed with type 1 diabetes, as part of an evaluation of the person's "residual beta cell function." With type 2 diabetes, the test may be ordered when a doctor wants to monitor the status of a person's beta cells and insulin production over time and to determine if/when insulin injections may be required.

C-peptide levels may be done when there is documented acute or recurring hypoglycemia. Symptoms include:
Sweating
Palpitations
Hunger
Confusion
Blurred vision
Fainting
In severe cases, seizures and loss of consciousness

However, many of these symptoms can occur with other conditions as well.

The C-peptide test may be used to help separate excessive insulin production from excessive administration and to help diagnose insulinomas. When a person has been diagnosed with an insulinoma, a C-peptide test may be ordered periodically to monitor the effectiveness of treatment and to detect tumor recurrence.

Rarely, when someone has had his pancreas removed or has had pancreas islet cell transplants, intended to restore the ability to make insulin, C-peptide levels may be monitored over time to verify the effectiveness of treatment and continued success of the procedure.

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What does the test result mean?

High levels of C-peptide generally indicate high levels of endogenous insulin production. This may be in response to high levels of blood glucose caused by glucose intake and/or insulin resistance. High levels of C-peptide are also seen with insulinomas and may be seen with hypokalemia, pregnancy, Cushing's syndrome, and renal failure.

Decreasing levels of C-peptide in someone with an insulinoma indicate a response to treatment; levels that are increasing may indicate a tumor recurrence.

Low levels of C-peptide are associated with low levels of insulin production. This can occur when insufficient insulin is being produced by the beta cells or when production is suppressed by exogenous insulin or with suppression tests that involve substances such as somatostatin.

Is there anything else I should know?
C-peptide testing is not widely used and may not be available in every laboratory. 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 about 30 minutes compared to insulin's 5 minutes, normally there will be about 5 times as much C-peptide in the bloodstream as insulin.