What is an Insulin C-Peptide Test?

    Insulin is the hormone that is primarily responsible for regulating glucose (sugar) levels in the blood. It is produced by beta cells (specialized cells in the pancreas). When we eat, our bodies begin to break food down into glucose and other nutrients.

    The pancreas produces insulin, which allows cells to absorb glucose from the blood. When insulin is released, the pancreas also releases C-peptide. Measuring the amount of C-peptide in blood tells how much insulin is being produced. Generally, high C-peptide production indicates high insulin production, and vice versa.

    Why is the Test Ordered?

    The insulin C-peptide test (or simply the C-peptide test) is used to monitor insulin production in the body and determine the cause of hypoglycemia (low blood sugar). The test is often ordered to diagnose diabetes or to determine how much insulin is being produced in recently diagnosed diabetics. It can also provide information as to how well the beta cells in the pancreas are working.

    The test may also be performed for patients that 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
    • confusion
    • blurred vision
    • fainting
    • seizures and/or loss of consciousness

    Preparing for the Test

    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 six 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.

    How the Test is Administered

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

    Results are typically available within a few days. Your doctor will be able to provide you with more information about the results and what they mean. 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.

    Risks Associated with the Test

    Patients undergoing the insulin C-peptide test may experience some discomfort when the blood sample is drawn. Common side effects include pain or throbbing at the injection site. However, symptoms are temporary.

    Less common side effects include:

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