C-Peptide Test

What is a C-peptide test?

A C-peptide test measures the amount of C-peptide in the blood or urine. It can help healthcare providers determine what type of diabetes a person has: Type 1 or Type 2. It also can reveal how well diabetes treatments are working. Another use for C-peptide is to determine if your pancreas is making insulin.

The test is sometimes called:

• Connecting peptide insulin.
• Insulin C-peptide.
• Proinsulin C-peptide.

What is C-peptide?

C-peptide is a byproduct the pancreas releases into the body when it makes insulin. The pancreas produces about the same amount of insulin and C-peptide.

Insulin is a hormone that regulates the amount of sugar (glucose) in the blood. Glucose is the human body’s main source of energy. Insulin allows cells in the muscles, fat and liver to absorb glucose to be used as energy.

In people with Type 1 diabetes, the pancreas isn’t able to make insulin. They need insulin injections so the body can use glucose. People with Type 2 diabetes make insulin, but their bodies either don’t produce enough insulin or resist the effects of insulin. Some people with Type 2 diabetes need medications to help their bodies use glucose. Over time, some people with type 2 diabetes are no longer able to make insulin.

Because C-peptide is a byproduct of insulin production, this test can reveal how much insulin the body is producing. When people take insulin, their bodies don’t make or release C-peptide. This test can tell the difference between insulin made by the body (endogenous) and insulin from medication (exogenous).

What C-peptide level is related to diabetes?

A C-peptide test can’t determine whether a person has diabetes. It merely indicates if your body has the ability to make its own insulin from your pancreas.

To diagnose diabetes, a healthcare provider must order a blood glucose test or glycosylated hemoglobin test (A1c).

Why is a C-peptide test performed?

Your healthcare provider might recommend a C-peptide test for several reasons:

• Help classify the type of diabetes you have: Type 1 or Type 2 (in conjunction with other tests).
• Confirm that diabetes treatments are working effectively.
• Determine whether you’re producing enough insulin or whether you need insulin injections or a pump.
• Investigate the causes of low blood sugar (hypoglycemia).
• Distinguish between insulin the body produces and insulin that’s is injected into your body.
• Rarely, check the status of pancreatic cancer.

Who performs a C-peptide test?

A C-peptide test is usually ordered by a primary care provider or endocrinologist (specialist in glands and hormones). The test is often performed by a nurse or lab technician.

How do you test for C-peptide?

A C-peptide test usually requires a blood sample. In an office, clinic or lab, a healthcare provider will insert a thin needle into a vein in your arm. The needle will collect a small amount of blood into a tube. You might feel a slight sting when the needle goes in.

The test also can be conducted on a urine sample. Often, the test requires all urine from a 24-hour period. Your healthcare provider will give you a container, as well as instructions on collecting and returning the sample. You may have to keep the container in a cold place such as a refrigerator or cooler. Providers order this test less often.

Do I need to fast for a C-peptide test?

The blood test may require you to fast (avoid food and drink, other than water) for eight to 12 hours. Sometimes, your healthcare provider might want to test C-peptide after you eat something to stimulate the pancreas. The C-peptide is usually done alongside a glucose level.

Your healthcare provider may also want you to stop taking certain medications before the test. Ask your healthcare provider for specific instructions.

What are the risks of this test?

Blood tests involve minimal risk, and urine tests have no known risks.

When will I get C-peptide test results?

Results of a C-peptide test can take one to five days, depending on the healthcare provider and lab.

What is a normal result for a C-peptide test?

A normal result of a C-peptide test ranges from 0.5 ng/mL to 2.0 ng/mL (or 0.17 to 0.83 nmol/L). These values may differ slightly from lab to lab.

What do C-peptide test results mean?

A low level of C-peptide may be considered normal if your blood sugar is low and you haven’t eaten recently. It can also mean that your body isn’t making enough insulin or that you’re taking insulin injections. This can be related to one of the following medical conditions, especially if you have high blood sugar:

• Type 1 diabetes.
• Type 2 diabetes (if your pancreas is no longer producing insulin).

Low C-peptide also might indicate that diabetes treatment isn’t working well enough.

Other medical conditions that can cause low C-peptide with low blood sugar levels include:

• Addison disease, a disorder in which your adrenal glands make too little of the hormones cortisol and aldosterone. This is usually accompanied with a low blood sugar result.
• Liver disease.
• Insulin administration by injection/inhalation (exogenous insulin).

A high level of C-peptide might mean your body is making too much insulin. That can point to one of the following medical conditions:

• Cushing syndrome, when your body produces too much of the hormone cortisol.
• Insulin resistance.
• Low levels of potassium in the blood (hypokalemia).
• Type 2 diabetes.
• Pancreatic tumor (insulinoma).
• Kidney failure.

If the test reveals no C-peptide, then you need insulin replacement (usually in the form of insulin injections or possibly, an insulin pump).

A note from Cleveland Clinic

A C-peptide test measures the amount of C-peptide in the blood or urine. It’s often used to differentiate between Type 1 and Type 2 diabetes. It also can assess how well diabetes treatment is working or help diagnose other conditions. If you’re having a C-peptide test, talk to your healthcare provider about specific instructions.