Insulin C-Peptide Test: Purpose, Preparation, Procedure and Understanding the Result

The C-peptide test is done to monitor and treat diabetes. It shows how well your body makes insulin, which moves sugar or glucose from your blood into your cells.

March 3, 2019

The connecting peptide or C-peptide test is done to monitor and treat diabetes. It shows how well your body makes insulin, which moves sugar or glucose from your blood into your cells. C-peptide is a byproduct created when insulin is produced. Measuring the amount of C-peptide in blood indicates how much insulin is being produced.
Insulin is the hormone that is responsible for lowering blood glucose levels which is produced by beta cells in the pancreas. Our body will break down the food we eat into glucose and other nutrients, which is then absorbed from the blood by beta cells.
The test can help your doctor decide whether you need to take insulin to control your condition if enough of insulin is not produced by the pancreas or to check your dosage if you already take it. Both Type 1 diabetes, when the immune system attacks and destroys cells in the pancreas, and  type 2 diabetes, when your body doesn't produce or use insulin properly can be diagnose by this test.

Purpose of Insulin C-Peptide Test:

The insulin C-peptide test is used to monitor insulin production in the body. It can be used to:

  • determine the cause of hypoglycemia, also called low blood sugar in which the amount of insulin in your body will be more then required
  • measure how much insulin the pancreas is producing in a person when newly diagnosed with type 1 diabetes
  • provide information about how well the beta cells in the pancreas are working
  • distinguish between type 1 and type 2 diabetes, if it is not sure which type of diabetes you have

This test is also done if you experience the symptoms of hypoglycemia, even if you don't have type 1 or type 2 diabetes. . In this case, the body may be producing too much insulin. Symptoms of hypoglycemia include:

  • fainting
  • heart palpitations
  • sweating
  • excessive hunger
  • nervousness or irritability
  • blurred vision
  • seizures and/or loss of consciousness
  • confusion

Preparation of Insulin C-Peptide Test:

Fasting for 10 to 12 hours before the test is required. During this period you should not eat or drink anything except water. You may also need to stop taking certain medications which can be advised by your doctor.

Procedure of Insulin C-Peptide Test:

A band will be tied by a technician around your arm so that the veins of the hand are more prominent to prick the injection. The vein is punctured with a fresh, disposable syringe and the blood is withdrawn after the site is cleaned with an alcohol swab. A cotton swab will be given to you after drawing the blood to place on the place where the injection was pricked. You will be asked to apply pressure on the cotton in order to stop the bleeding. You might notice a bruise in the place where the injection was pricked on your skin. This is normal and should fade away in a few days.
Before you undergo this test, you should inform the the lab technician if you suffer from a bleeding or clotting disorder or are taking medicines like aspirin, warfarin or other blood-thinning agents, as in such cases the bleeding might take a while to stop.

Understanding the Result:

A normal C-peptide range is 0.5 to 2.0 ng/mL (nanograms per milliliter).

What does it mean if your C peptide is high? 

A high level of C-peptide can mean that you may have:

  • insulin resistance which means your body doesn't use it as well as it should
  • a tumor, called an insulinoma
  • kidney disease as it is excreted by the kidney

An high level may also be due to taking too much of a certain medication to treat type 2 diabetes.

What does it mean if your C peptide is low?

A low C-peptide level mean that:
  • you may have type 1 or type 2 diabetes
  • your pancreas doesn't work as well as it should, or it is making less insulin
  • you have low blood sugar
  • treatment has shrunk your insulinoma