Insulin C-peptide is a blood test that measures the amount of C-peptide, a breakdown product that is created when the hormone insulin is produced and released.
How the Test is Performed
A blood sample is needed.
How to Prepare for the Test
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. Your health care provider may ask you to stop taking medicines that can affect the test results.
How the Test will Feel
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.
Why the Test is Performed
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 nanograms per milliliter (ng/mL)
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
People with type 2 diabetes, obesity, and insulin resistance may have a high C-peptide level that is normal for their degree of insulin resistance.
What Abnormal Results Mean
C-peptide level is based on blood sugar level. C-peptide is a sign that the body is producing insulin. A low level (or no insulin C-peptide) indicates 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:
- 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: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 31.
Eisenbarth S, Buse JB. Type 1 diabetes mellitus. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 32.
Khan MI, Weinstock RS. Carbohydrates. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 16.
Update Date 5/10/2014
Updated by: Brent Wisse, MD, Associate Professor of Medicine, Division of Metabolism, Endocrinology & Nutrition, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.