What It Is

C-peptide, like the hormone insulin, is produced in the pancreas. Both are released simultaneously from the pancreas when the compound called proinsulin is split into two pieces.

Insulin is responsible for regulating the body's glucose levels. Glucose, the body's main source of energy, is a sugar that comes from foods.

After a meal, our bodies break down the foods we eat into glucose and other nutrients, which are then absorbed into the bloodstream from the gastrointestinal tract. Glucose levels in the blood rise after a meal and trigger the pancreas to make insulin and release it into the blood. When insulin is released, so is C-peptide.

Insulin works like a key that opens the doors to cells and allows the glucose in. Without insulin, glucose can't get into the cells and it stays in the bloodstream. The most common cause of abnormal fluctuations in blood glucose is diabetes.

C-peptide, on the other hand, has no effect on blood sugar. It is, however, useful as a marker of insulin production, since the pancreas typically releases C-peptide and insulin in about equal amounts.

In general, high C-peptide levels are associated with increased insulin production, while low C-peptide levels indicate decreased insulin production.

Why It's Done

The C-peptide test may be ordered to determine how much insulin is being made by the pancreas. This information is useful because:

1. It can help doctors tell the difference between type 1 and type 2 diabetes. In type 1 diabetes, the pancreas produces little or no insulin and little or no C-peptide. In type 2 diabetes, C-peptide levels will typically be normal or high, as the pancreas works harder to overcome insulin resistance (when the tissues become less sensitive to the effects of insulin) by producing more insulin.
2. It can help find the cause of low blood sugar (hypoglycemia), including the incorrect use of diabetes medications.

Preparation

Your doctor will let you know if any special preparations are needed for this test. Sometimes it's necessary for a child to avoid eating or drinking for 8 hours prior to the test; in other cases, doctors may want to check levels at specific times, such as timed intervals after food or glucose is given.

On the day of the test, it may help to have your child wear a short-sleeve shirt to allow easier access for the technician who will be drawing the blood.

The Procedure

A health professional will usually draw the blood from a vein after cleaning the skin surface with antiseptic and placing an elastic band (tourniquet) around the upper arm to apply pressure and cause the veins to swell with blood. A needle is inserted into a vein (usually in the arm inside of the elbow or on the back of the hand) and blood is withdrawn and collected in a vial or syringe.

After the procedure, the elastic band is removed. Once the blood has been collected, the needle is removed and the area is covered with cotton or a bandage to stop the bleeding. Collecting blood for this test will only take a few minutes.

What to Expect

Collecting a blood sample is only temporarily uncomfortable and can feel like a quick pinprick. Afterward, there may be some mild bruising, which should go away in a day or so.

Getting the Results

The blood sample will be processed by a machine. The results are commonly available in a few days.

Risks

The C-peptide test is considered a safe procedure. However, as with many medical tests, some problems can occur with having blood drawn, such as:

• fainting or feeling lightheaded
• hematoma (blood accumulating under the skin causing a lump or a bruise)
• pain associated with multiple punctures to locate a vein

Helping Your Child

Having a blood test is relatively painless. Still, many kids are afraid of needles. Explaining the test in terms your child can understand might help ease some of the fear.

Allow your child to ask the technician any questions he or she might have. Tell your child to try to relax and stay still during the procedure, as tensing muscles and moving can make it harder and more painful to draw blood. It also may help for your child to look away when the needle is being inserted into the skin.

If You Have Questions

If you have questions about the C-peptide test, speak with your doctor. You can also talk to the technician before the procedure.

Reviewed by: Steven Dowshen, MD
Date reviewed: February 2011