What is a C-peptide test?
This test measures the level of C-peptide in your blood or urine. C-peptide is a substance made in the pancreas, along with insulin. Insulin is a hormone that controls the body’s glucose (blood sugar) levels. Glucose is your body’s main source of energy. If your body doesn’t make the right amount of insulin, it may be a sign of diabetes.
C-peptide and insulin are released from the pancreas at the same time and in about equal amounts. So a C-peptide test can show how much insulin your body is making. This test can be a good way to measure insulin levels because C-peptide tends to stay in the body longer than insulin.
Other names: insulin C-peptide, connecting peptide insulin, proinsulin C-peptide
What is it used for?
A C-peptide test is often used to help tell the difference between type 1 and type 2 diabetes. With type 1 diabetes, your pancreas makes little to no insulin, and little or no C-peptide. With type 2 diabetes, the body makes insulin, but doesn’t use it well. This can cause C-peptide levels to be higher than normal.
The test may also be used to:
Why do I need a C-peptide test?
You may need a C-peptide test if your health care provider thinks you have diabetes, but is unsure whether it is type 1 or type 2. You may also need a C-peptide test if you have symptoms of low blood sugar (hypoglycemia). Symptoms include:
What happens during a C-peptide test?
A C-peptide test is usually given as a blood test. During a blood test, a health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.
C-peptide can also be measured in urine. Your health care provider may ask you to collect all urine passed in a 24-hour period. This is called a 24-hour urine sample test. For this test, your health care provider or a laboratory professional will give a container in which to collect your urine and instructions on how to collect and store your samples. A 24-hour urine sample test generally includes the following steps:
- Empty your bladder in the morning and flush that urine away. Record the time.
- For the next 24 hours, save all your urine passed in the container provided.
- Store your urine container in the refrigerator or a cooler with ice.
- Return the sample container to your health provider’s office or the laboratory as instructed.
Will I need to do anything to prepare for the test?
You may need to fast (not eat or drink) for 8–12 hours before a C-peptide blood test. If your health care provider has ordered a C-peptide urine test, be sure to ask if there are any specific instructions you need to follow.
Are there any risks to the test?
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
There are no known risks to a urine test.
What do the results mean?
A low level of C-peptide can mean your body isn’t making enough insulin. It may be a sign of one of the following conditions:
It may also be a sign that your diabetes treatment is not working well.
A high level of C-peptide can mean your body is making too much insulin. It may be a sign of one of the following conditions:
- Type 2 diabetes
- Insulin resistance, a condition in which the body doesn’t respond the right way to insulin. It causes the body to make too much insulin, raising your blood sugar to very high levels.
- Cushing’s syndrome, a disorder in which your body makes too much of a hormone called cortisol.
- A tumor of the pancreas
If you have questions about your results, talk to your health care provider.
Is there anything else I need to know about a C-peptide test?
A C-peptide test can provide important information about the type of diabetes you have and whether or not your diabetes treatment is working well. But it is not used to diagnose diabetes. Other tests, such as blood glucose and urine glucose, are used to screen and diagnose diabetes.