Insulin C-peptide is a blood test that measures the amount of C-peptide, a by-product created when the hormone insulin is produced.

Blood is typically drawn from a vein, usually on the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

The preparation for the test depends on the reason for the C-peptide measurement. Ask your health care provider if you should not eat (fast) before the test. The health care provider may advise you to stop taking medications that can interfere with test results.

For infants and children, the preparation you can provide for this test depends on your child's age and experience. For specific information regarding how you can prepare your child, see the following topics:

• Infant test or procedure preparation (birth - 1 year)
• Toddler test or procedure preparation (1 - 3 years)
• Preschooler test or procedure preparation (3 - 6 years)
• Schoolage test or procedure preparation (6 - 12 years)
• Adolescent test or procedure preparation (12 - 18 years)

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

C-peptide is measured to tell the difference between insulin produced by the body and insulin injected into the body. When the pancreas produces insulin, it starts off as a large molecule. This molecule splits into two pieces: insulin and C-peptide. The function of C-peptide is not known.

The C-peptide level may be measured in a patient with type 2 diabetes to see if any insulin is still being produced by the body. It may also be measured in cases of hypoglycemia (low blood sugar) to see if the person's body is producing too much insulin.

0.5 to 2.0 ng/mL (nanograms per milliliter)

Normal ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

C-peptide values are interpreted based on the blood sugar level. C-peptide is a sign that the body is producing insulin. Low values (or no insulin C-peptide) indicate that your pancreas is producing little or no insulin.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Buse JB, Polonsky KS, Burant CF. Type 2 Diabetes Mellitus. In: Kronenberg HM, Melmed S, Polonsky KS, Larsen PR. Kronenberg: Williams Textbook of Endocrinology. 11th ed. Philadelphia, Pa:Saunders Elsevier;2008:chap 30.

Eisenbarth S, Polonsky KS, Buse JB. Type 1 Diabetes Mellitus. In: Kronenberg HM, Melmed S, Polonsky KS, Larsen PR. Kronenberg: Williams Textbook of Endocrinology. 11th ed. Philadelphia, Pa:Saunders Elsevier;2008:chap 31.