A C-peptide test measures the level of this
peptide in the blood. It is generally found in amounts equal to
insulin because insulin and C-peptide are linked when first made by the
pancreas. Insulin helps the body use and control the amount of sugar (glucose)
in the blood. Insulin allows glucose to enter body cells where it is used for
energy. The level of C-peptide in the blood can show how much insulin is being
made by the
pancreas . C-peptide does not affect the blood sugar
level in the body.
A C-peptide test can be done
diabetes has just been found and it is not clear
type 1 diabetes or
type 2 diabetes is present. A person whose pancreas
does not make any insulin (type 1 diabetes) has a low level of insulin and
C-peptide. A person with type 2 diabetes can have a normal or high level of
A C-peptide test can also help find the cause of low
blood sugar (hypoglycemia), such as excessive use of medicine to
treat diabetes or a noncancerous growth (tumor) in the pancreas (insulinoma).
Because man-made (synthetic) insulin does not have C-peptide, a person with a
low blood sugar level from taking too much insulin will have a low C-peptide
level but a high level of insulin. An insulinoma causes the pancreas to release too much insulin, which
causes blood sugar levels to drop (hypoglycemia). A person with an insulinoma
will have a high level of C-peptide in the blood when they have a high level of insulin.
Why It Is Done
A C-peptide test is done to:
- Help tell the difference between type 1
diabetes and type 2 diabetes.
- Find the cause of low blood sugar
- Check to see whether a tumor of the pancreas
(insulinoma) was completely removed.
How To Prepare
You may be asked to stop eating and
drinking for 8 hours before having this blood test.
some oral medicines used to treat type 2 diabetes can change the test results.
Your doctor may ask you to stop these medicines before your blood test.
Talk to your doctor about any concerns you have regarding the need for
the test, its risks, how it will be done, or what the results will mean. To
help you understand the importance of this test, fill out the
medical test information form(What is a PDF document?).
How It Is Done
The health professional drawing your
- Wrap an elastic band around your upper arm to
stop the flow of blood. This makes the veins below the band larger so it is
easier to put a needle into the vein.
- Clean the needle site with
- Put the needle into the vein. More than one needle stick
may be needed.
- Attach a tube to the needle to fill it with
- Remove the band from your arm when enough blood is
- Put a gauze pad or cotton ball over the needle site as
the needle is removed.
- Put pressure on the site and then put on a