What Is a C-Peptide Test?

Medically Reviewed by Michael Dansinger, MD on March 04, 2023
3 min read

The C-peptide test is a tool your doctor uses to test whether you have type 1 diabetes, when the immune system attacks and destroys cells in the pancreas, or type 2, when your body doesn't use insulin as well it should. It shows how well your body makes insulin, which moves sugar (or “glucose”) from your blood into your cells.

The test can help your doctor decide whether you need to take insulin to control your condition.

Beta cells in your pancreas make insulin. During that process, these cells also release C-peptide.

This substance doesn't actually affect your blood sugar. But your doctor can measure the level of it to help them figure out how much insulin you’re making.

Doctors don't use it to actually diagnose diabetes, but it can give them a reading to help treat it.

It can tell the difference between insulin your body has made and insulin that you took because only insulin your pancreas is making is associated with C-peptide.

You might get the C-peptide test:

  • To find out whether you have type 1 or type 2 diabetes
  • When you have type 1 and your doctor needs to know how much insulin your pancreas still makes
  • To find out if it is time to start insulin in a patient with type 2 diabetes.
  • To find out why you have low blood sugar (hypoglycemia)
  • To diagnose a tumor of the pancreas that releases insulin, called an insulinoma
  • If you've had your pancreas removed

You might need to stop eating 8 to 10 hours before the test. Sometimes, it’s done after you eat.

Let your doctor know about any medicines you are on. Include medicine you take by prescription and those you buy over the counter, such as herbal supplements or vitamins.

The C-peptide test typically uses a sample of your blood.

To take a blood test, someone in your doctor’s office or a lab places a needle into a vein, usually in your forearm. You may feel a slight prick. The blood will collect into a vial or syringe.

For a urine test, you will pee into a cup as instructed. For a 24-hour sample, you'll need to collect all that you produce in a day.

You should have your results in a few days. A normal C-peptide range is 0.5 to 2.0 nanograms per milliliter.

These levels can be high when your body makes more insulin than usual. Levels are low when your body makes less than it normally should.

A high level can mean that you:

  • Have insulin resistance -- meaning your body doesn't use it as well as it should; if you have type 2 diabetes, your insulin levels may be high.
  • Have a tumor, called an insulinoma
  • Have kidney disease

A high level may also show you take too much of a certain class of medicine to treat type 2 diabetes. They’re called sulfonylureas, and they include:

A low C-peptide level may mean that:

  • You have type 1 or type 2 diabetes  with a tired pancreas--or more advanced disease
  • You are on insulin shots and it is suppressing the release of insulin from your pancreas
  • You have low blood sugar so your pancreas stops releasing insulin or you are taking insulin so your pancreas isn't releasing any
  • Treatment has shrunk your insulinoma

Ask your doctor for more details about what your test results mean. And, find out what treatment steps you'll need to take.