C-Peptide Test
A C-peptide test measures the amount of C-peptide in your blood or urine to help determine how much insulin your body is naturally producing.
What is a C-peptide test?
This test measures the level of C-peptide in a sample of your blood or urine. C-peptide is a chain of amino acids that your pancreas makes in the process of making insulin. Insulin is a hormone that your pancreas makes to help blood glucose (blood sugar) get into your cells, where it can be used for energy.
C-peptide and insulin enter your bloodstream at the same time and in equal amounts, but C-peptide stays in your blood longer. It is also unaffected by outside sources of insulin such as the medicines involved in diabetes treatment. In other words, C-peptide can give an accurate picture of how much insulin your body is making naturally.
What is it used for?
A C-peptide test can provide important information to help:
- Find the cause of hypoglycemia (low blood glucose). Some types of hypoglycemia are linked to high insulin levels. By checking your C-peptide levels, your provider can tell whether this is the case.
- Manage diabetes treatment. If you have type 1 or type 2 diabetes, treatment decisions may depend on knowing how much insulin your pancreas is making.
- Monitor treatment for a tumor in your pancreas, called an insulinoma. These tumors make too much insulin and cause low blood glucose.
- Find out whether you have type 1 or type 2 diabetes if your diagnosis is uncertain.
Why do I need a C-peptide test?
You may need a C-peptide test if you have been diagnosed with a pancreatic tumor (insulinoma) or hypoglycemia without a clear cause. Symptoms of hypoglycemia include:
- Shakiness
- Sweating
- Headache
- Hunger or nausea
- An irregular or fast heartbeat (arrhythmia)
- Dizziness
- Fatigue
You may also need the test if you have diabetes and your provider is considering changing your treatment, or if you have type 2 diabetes and your provider wants to see if you need to start taking insulin.
What happens during a C-peptide test?
A C-peptide test usually uses a sample of your blood, but it may also be done on a sample of all your urine collected over a 24-hour period.
During a blood test, a health care professional will take a blood sample from a vein in your arm. This usually takes less than five minutes.
For a 24-hour urine sample test, you will collect all urine in a provided container over 24 hours, keeping it refrigerated or in a cooler with ice, before returning it to your provider or laboratory.
Will I need to do anything to prepare for the test?
For a C-peptide blood test, you may need to fast (not eat or drink) for 8-12 hours before the test. Your provider may also want to test C-peptide after you've eaten something to see how your pancreas responds. Ask your provider for specific instructions.
Are there any risks to the test?
There is very little risk to having a blood test, such as slight pain or bruising at the needle site. There are no known risks to a urine test.
What do the results mean?
In general:
- A high level of C-peptide usually means that your body is making too much insulin, which can be linked to type 2 diabetes, Cushing's syndrome, insulinoma, kidney failure, or low potassium.
- A low level of C-peptide may mean your body isn't making enough insulin, which can be linked to type 1 diabetes, taking too much insulin, severe infection, Addison disease, or liver disease.
Is there anything else I need to know about a C-peptide test?
A C-peptide test is not used to diagnose diabetes. Other tests, such as blood glucose and urine glucose, are used to screen for and diagnose diabetes.