Insulin is the hormone that is primarily responsible for lowering glucose levels in the blood, also called blood sugar. Insulin is produced by specialized cells in the pancreas called beta cells. When we eat, our bodies begin to break food down into glucose and other nutrients. In response, the pancreas produces insulin, which allows cells to absorb glucose from the blood.

C-peptide is a byproduct created when insulin is produced. Measuring the amount of C-peptide in blood indicates how much insulin is being produced. Generally, high C-peptide production indicates high insulin production, and vice versa.

The insulin C-peptide test is used to monitor insulin production in the body. The test can give doctors a lot of information about what is happening in your body. It can be used to:

  • determine the cause of hypoglycemia, also called low blood sugar
  • reveal how much insulin the pancreas is producing in a person newly diagnosed with type 1 diabetes
  • distinguish between type 1 and type 2 diabetes, if the doctor is not sure which type of diabetes is present
  • provide information about how well the beta cells in the pancreas are working

The test may also be performed on patients who experience symptoms related to hypoglycemia in the absence of type 1 or type 2 diabetes. In this case, the body may be producing too much insulin. Symptoms of hypoglycemia include:

  • sweating
  • heart palpitations
  • excessive hunger
  • nervousness or irritability
  • confusion
  • blurred vision
  • fainting
  • seizures and/or loss of consciousness

The preparation needed for the insulin C-peptide test depends on a person’s age and the reason for the test. In some instances, you may be required to fast for up to 12 hours before the test. Fasting requires that you to not eat or drink anything but water before the test. You may also need to stop taking certain medications. Your doctor will provide you with specific instructions based on your particular medical needs.

The insulin C-peptide test requires a blood sample be collected by a qualified doctor or nurse. The blood is drawn from a vein, typically in the arm or on the back of the hand. The procedure can cause minor discomfort, but the discomfort is temporary. The blood will be collected in a tube and sent to a lab for analysis.

Results are typically available within a few days. In general, normal results for C-peptide in the bloodstream are between 0.5 and 2.0 ng/mL (nanograms per milliliter). However, results for the insulin C-peptide test may vary based on the lab. Your doctor will be able to provide you with more information about the results and what they mean.

The insulin C-peptide test may cause some discomfort when the blood sample is drawn. Common side effects include temporary pain or throbbing at the needle site. Less common side effects include:

  • difficulty obtaining a sample, resulting in multiple needle sticks
  • excessive bleeding at the needle site
  • fainting as a reaction to the sight of blood
  • accumulation of blood under the skin, known as a hematoma (bruise)
  • infection where the skin is broken by the needle