C-peptide is a product that is created when the hormone insulin is produced and released into the body. The insulin C-peptide test measures the amount of this product in the blood.
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 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 the body produces and insulin someone injects into their body.
Someone with type 1 or type 2 diabetes may have their C-peptide level measured to see if their body is still producing any insulin. It may also be measured in cases of low blood sugar to see if the person's body is producing too much insulin.
A normal result 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 provider 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 them.
What Abnormal Results Mean
C-peptide level is based on blood sugar level. C-peptide is a sign that your body is producing insulin. A low level (or no C-peptide) indicates that your pancreas is producing little or no insulin.
- A low level may be normal if you have not eaten recently. Your blood sugar and insulin levels would naturally be low then.
- A low level is abnormal if your blood sugar is high and your body should be making insulin at that time.
Veins and arteries vary in size from one person 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 lightheaded
- Hematoma (blood buildup under the skin)
- Infection (a slight risk any time the skin is broken)
Atkinson MA. Type 1 diabetes mellitus. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 13th ed. Philadelphia, PA: Elsevier; 2016:chap 32.
Hall JE. Insulin, glucagon, and diabetes mellitus. In: Hall JE, ed. Guyton and Hall Textbook of Medical Physiology. 13th ed. Philadelphia, PA: Elsevier; 2016:chap 79.
Polonsky KS, Burant CF. Type 2 diabetes mellitus. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 13th ed. Philadelphia, PA: Elsevier; 2016:chap 31.