Insulin C-peptide is a blood test that measures the amount of C-peptide, a byproduct created when the hormone insulin is produced.
How the test is performed
A blood sample is needed. For information on how this is done, see: Venipuncture
How to prepare for the test
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. Your health care provider may ask you to stop taking medications that can interfere with the test results.
For infants and children, the preparation you can provide for this test depends on your child's age and experience. For information on how to prepare your child, see the following topics:
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.
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 ng/mL (nanograms per milliliter)
The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some laboratories use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
What abnormal results mean
C-peptide values are 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.
What the risks are
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: Kronenberg HM, Melmed S, Polonsky KS, Larsen PR. 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. Williams Textbook of Endocrinology. 11th ed. Philadelphia, Pa: Saunders Elsevier; 2008:chap 31.
Shehzad Topiwala, MD, Chief Consultant Endocrinologist, Premier Medical Associates, The Villages, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.