Reviewed Mike Fuller, MD
C-peptide tests have been around since at least the 1970’s but have not been regularly used for the diagnosis of diabetes until fairly recently. It is currently the only accurate measure of insulin production, especially with actively injecting insulin dependent diabetics. Insulin level tests measure all available insulin (including injected), not just naturally made insulin. Current tests are unable to distinguish between the two.
What is C-peptide?
C-peptide is an amino acid released in direct proportion to the amount of natural insulin.
What is C-peptide test? What Does It Measure?
A C-peptide test is a blood test that measures the amount of C-peptide present in the blood stream. Since there is the same amount of C-peptide as there is natural insulin, it is considered a good measure of insulin production.
Other names for a C-peptide test are Connecting Peptide, Insulin C-peptide test or proinsulin C-peptide test.
What is it the Test Used to Determine Relative to Diabetes?
Physicians and other caregivers use a C-peptide test to make or confirm a diagnosis of diabetes. It can measure the levels of residual beta cell function in type 1 diabetics. It can measure normal insulin production and thus, indicate insulin resistance in type 2 diabetics. It can also indicate beta cell destruction and reduced insulin production in type 2 diabetics.
What are Normal Levels?
Normal C-peptide levels while fasting are considered to be from 0.5 nanograms (ng) per millileter (ml) to 3 ng/ml.
What Do Out of Range C-Peptide Levels Mean?
•Low C-peptide Levels – indicate beta cell destruction and a subsequent reduction in insulin production. It may indicate new onset type 1 diabetes or advanced type 2 diabetes.
•Normal or High C-peptide Levels – indicate “normal” insulin production. If a patient has high blood sugar and high C-peptide, levels it may indicate insulin resistance (e.g. type 2 diabetes or PCOS).
What Affects the Test Results?
Many things can affect C-peptide test results including certain medications and medical conditions can. Things that may affect the test include:
•Insulin injections containing C-peptide
•Sulfonylurea medications (type 2 diabetes medications)
•Birth control pills
•Deferoxamine (used to remove excess iron)
•Diuretics (used for hypertension)
•Propranolol (beta blocker used for hypertension)
•Rifampin (an antibiotic)
•Terbutaline (asthma medication)
•Recent radioactive scans
What is a C-peptide Stimulation Test?
Physicians may also use A C-peptide Stimulation test to further distinguish between type 1 and type 2 diabetes. The test involves an initial blood draw to measure C-peptide levels, an injection of glucagon to raise blood glucose levels, and a second blood draw to re-measure C-peptide levels.
A type 1 diabetic would be unable to produce insulin (or C-peptide) in response to the rise in blood sugar levels. A type 2 diabetic’s C-peptide levels rise in response to the increase in blood sugar if the pancreas still producing insulin.
Last updated: 20-Sep-09