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Diabetes Health Center

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What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • Taking medicines, such as insulin, or sulfonylurea medicines for type 2 diabetes.
  • Having kidney failure. Both insulin and C-peptide are removed from the body by the kidneys. C-peptide levels may be high in a person with kidney failure.
  • Being obese. More insulin is made in obese people and can cause high levels of C-peptide.

What To Think About

  • A C-peptide test must be done at the same time as a blood glucose test. To learn more, see the topic Blood Glucose.
  • A person with new type 2 diabetes often has a normal or high level of C-peptide in the blood. Over time, a person with type 2 diabetes may develop a low level of C-peptide.
  • To help tell the difference between type 1 and type 2 diabetes and to help guide treatment, most doctors look at a person's age, weight, and how long symptoms have been present. In rare cases, a C-peptide stimulation test may be done to help tell the difference between the two types of diabetes. During a C-peptide stimulation test, a blood sample is taken to measure C-peptide. Then a shot of a hormone to increase blood sugar (glucagon) is given into a vein in the arm. Another blood sample is taken. In people with type 1 diabetes, C-peptide levels will be low because the pancreas cannot make any insulin in response to the glucagon. In people with type 2 diabetes, C-peptide levels will be higher than the first blood test because the pancreas is making more insulin in response to the glucagon.


  1. Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.

  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.

WebMD Medical Reference from Healthwise

Last Updated: November 14, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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