A blood sample is needed.

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.

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.

C-peptide is measured to tell the difference between insulin the body produces and insulin that is injected into the body.

Someone with type 1 or type 2 diabetes may have their C-peptide level measured to see if their body is still producing insulin. C-peptide may also be measured in case of low blood sugar to see if the person's body is producing too much insulin.

A normal result is between 0.3 to 3.3 nanograms per milliliter (ng/mL), or 0.2 to 1.0 nanomoles per liter (nmol/L).

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.

The normal 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.

People with type 2 diabetes, obesity, or insulin resistance may have a high C-peptide level. This means their body is producing a lot of insulin to keep (or try to keep) their blood sugar normal.

There is little risk involved with having your blood taken. 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:

• Bleeding
• Fainting or feeling lightheaded
• Multiple punctures to try to locate veins
• Hematoma (blood buildup under the skin)
• Infection (a slight risk any time the skin is broken)

C-peptide

Atkinson MA, McGill DE, Dassau E, Laffel L. Type 1 diabetes mellitus. In: Melmed S, Auchus RJ, Goldfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 36.

Dhatariyia KK, Umpierrez GE, Crandall JP. Diabetes mellitus. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 210.

Kahn CR, Ferris HA, O'Neill BT. Pathophysiology of type 2 diabetes mellitus. In: Melmed S, Auchus RJ, Goldfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 34.

Sacks DB. Diabetes mellitus. In: Rifai N, Chiu RWK, Young I, Burnham Carey-Ann D, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. St Louis, MO: Elsevier; 2023:chap 47.

Updated by: Sandeep K. Dhaliwal, MD, board-certified in Diabetes, Endocrinology, and Metabolism, Springfield, VA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.