C-Peptide Test

This test measures the level of C-peptide in a sample of your blood or urine (pee). C-peptide is a chain of amino acids (the building blocks of proteins) that your pancreas makes in the process of making insulin. Insulin is a hormone that your pancreas makes to help blood glucose (blood sugar) get into your cells, where it can be used for energy.

This helps keep your blood glucose at healthy levels. C-peptide and insulin enter your bloodstream at the same time and in equal amounts, but C-peptide stays in your blood longer. It is also unaffected by outside sources of insulin such as the medicines involved in diabetes treatment.

In other words, C-peptide can give an accurate picture of how much insulin your body is making naturally. Because of this, health care providers may rely on a C-peptide test to measure insulin levels rather than testing insulin levels directly.

Why this test is done

You may need a C-peptide test if you have been diagnosed with a pancreatic tumor (insulinoma) or hypoglycemia (low blood glucose) without a clear cause.

Hypoglycemia is a condition in which your blood glucose is lower than what is healthy for you. If you have hypoglycemia, you may experience:

• Shakiness
• Sweating
• Headache
• Hunger or nausea
• An irregular or fast heartbeat (called arrhythmia)
• Dizziness
• Fatigue

In more extreme cases, hypoglycemia can cause confusion, coma, and seizures.

You may also need the test if you have diabetes and:

• Take insulin, and your provider is considering changing your treatment.
• Have type 2 diabetes, and your provider wants to see if you need to start taking insulin.
• Have hypoglycemia, which could be caused by taking too much diabetes medicines.

How to prepare

For a C-peptide 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've eaten something. This way, they can see what your C-peptide levels are when your pancreas is actively working.

Ask your provider if there are any specific instructions you need to follow before either a blood or a urine test.

Sample type

Blood Urine

Other names for this test

insulin C-peptide, connecting peptide insulin, proinsulin C-peptide