c-peptide-test
Reviewed
Also called: insulin C-peptide, connecting peptide insulin, proinsulin C-peptide
This test measures the level of C-peptide in your blood or urine. C-peptide is released by the pancreas at the same time and in equal amounts as insulin, but it stays in the blood longer and is not affected by insulin from diabetes medicines. Therefore, the test shows how much insulin your body is making naturally.
Specimen: blood or urine
Why this test may be ordered
- To find the cause of hypoglycemia (low blood glucose) that may be linked to high insulin levels.
- To help manage diabetes treatment by showing how much insulin your pancreas is making.
- To monitor treatment for a rare pancreatic tumor (insulinoma) that makes too much insulin.
- To help determine whether you have type 1 or type 2 diabetes when the diagnosis is uncertain.
Preparation and collection
For a blood test, you may need to fast (not eat or drink) for 8–12 hours before the test. Your provider may also want to test C-peptide after you have eaten. Ask your provider for specific instructions for either a blood or urine test.
A health care professional will take a blood sample from a vein in your arm. For a 24-hour urine sample, you will collect all your urine in a special container over 24 hours and store it in a refrigerator or cooler with ice.
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly. There are no known risks to a urine test.
Understanding results
The meaning of your results depends on your age, health, the medicines you take, and the results of other tests, such as a blood glucose test. Talk with your provider about what your results mean.
- High C-peptide level: Your body may be making too much insulin. Conditions that can cause this include type 2 diabetes, Cushing's syndrome, an insulinoma (pancreatic tumor), kidney failure, or low potassium in your blood. Limitation: This result alone does not diagnose a specific condition. Your provider will interpret it along with other tests and your medical history. Next step: Your provider may recommend additional testing or evaluation to determine the cause.
- Low C-peptide level: Your body may not be making enough insulin. Conditions that can cause this include type 1 diabetes, some cases of type 2 diabetes, taking too much insulin for diabetes (which may block your pancreas from making its own insulin), a severe infection, Addison disease, or liver disease. Limitation: This result alone does not diagnose a specific condition. Your provider will interpret it along with other tests and your medical history. Next step: Your provider may recommend additional testing or evaluation to determine the cause.
- Decrease in C-peptide after treatment for an insulinoma: Your treatment for the insulin-producing tumor is working. Limitation: This result is meaningful only when compared with previous test results and in the context of your overall treatment plan. Next step: Your provider will continue to monitor your C-peptide levels and overall health.
- Increase in C-peptide after treatment for an insulinoma: The tumor may have returned. Limitation: This result is meaningful only when compared with previous test results and in the context of your overall treatment plan. Next step: Your provider will discuss further evaluation and possible treatment options.
- A C-peptide test is not used to diagnose diabetes. Other tests, such as blood glucose and urine glucose, are used to screen for and diagnose diabetes.
- Results must be interpreted by a healthcare professional based on your age, health, medicines, and other test results.
What to do next
If you have questions about your results, talk with your provider.
- What do my C-peptide test results mean for my health?
- Do I need any other tests based on these results?
- How will these results affect my diabetes treatment plan?