A C-peptide test is a blood test that measures levels of the protein fragment C-peptide in a person’s bloodstream. Because the body releases C-peptide in equal amounts to insulin in most cases, blood C-peptide levels can indicate how much insulin is circulating in the body.

The amount of C-peptide in a person’s bloodstream can indicate the presence or absence of a number of diseases and disorders. It may be used to establish the type of diabetes patients have after they have been diagnosed. The test may also be used to monitor a patient’s diabetes or other disorder.

The test usually requires a blood sample be taken from a vein (venipuncture) on the inside of the elbow or the back of the hand. Results are then compared to normal values. A blood glucose test may also be performed at the time of the C-peptide test and both test results may be compared.

When a disease or disorder is detected by the blood test, treatment for the condition usually follows.

Although the function of C-peptide is unknown, researchers are exploring the possibility that C-peptide therapy may decrease a patient’s risk of certain diabetic complications.

A C-peptide test is a blood test that measures the amount of C-peptide in the blood. Peptides are protein fragments that contain two or more amino acids. By measuring the level of C-peptide in a person’s blood, a physician can determine the amount of insulin the body is producing.

C-peptide, or connecting peptide, is a byproduct of insulin production. The pancreas releases C-peptide into the bloodstream in equal amounts to insulin. When the beta cells of the pancreas manufacture insulin, it is produced as a large molecule known as proinsulin. The molecule is then split into two parts. The larger molecule is insulin and the smaller is C-peptide.

In most cases, for every molecule of insulin the body produces, one molecule of C-peptide is also produced. Although they are produced at the same time, C-peptide molecules are detectable in the body longer than insulin. As a result, there is normally about five times more C-peptide in the bloodstream than insulin.

A physician may order a C-peptide test to:

• Distinguish type 2 diabetes from type 1 diabetes. After patients have been diagnosed with diabetes, a C-peptide test may be ordered to determine the amount of insulin their pancreas is producing. People with type 2 diabetes will have C-peptide, whereas people with type 1 diabetes will have no C-peptide because their pancreas is unable to produce insulin.
  
  
• Evaluate hyperinsulinemia and hypoglycemia. A C-peptide test may be ordered to determine if a person’s body is producing too much insulin and to identify the cause of low blood glucose (sugar). Excessive insulin production can be the result of kidney disorders, tumors and other conditions.
  
  
• Monitor a patient’s recovery after the removal of an insulinoma. An insulinoma is a tumor that causes the pancreas to release excessive amounts of insulin. A physician can monitor the effectiveness of the procedure by measuring the C-peptide in the patient’s blood.
  
  
• Determine the effectiveness of a pancreas transplant or an islet cell transplant. These transplants are performed to promote the production of insulin. By measuring the C-peptide in the blood, a physician can monitor the success of the procedure.
  
  
• Qualify for insurance coverage. Some insurance companies may require a C-peptide test as part of their qualification process for an insulin pump.

• Monitor insulin production. A physician may order C-peptide tests to monitor insulin production over time and to determine if less insulin is being produced. In this case, a change in treatment may be recommended. 

The function of C-peptide is unknown. However, researchers are exploring the possibility that C-peptide therapy may decrease a patient’s risk of certain diabetes complications by improving blood flow and nerve function.

The preparation for a C-peptide test may vary depending on the reason for the test. A physician may recommend fasting and/or withholding medications that may interfere with test results.

On the day of the test, the procedure will be explained and patients will have the opportunity to ask questions. The medical professional will also ask questions about the patient’s medical history before the test, to determine if the patient is taking any medications that will interfere with the test’s accuracy. Patients will also be asked about any medical problems that they may have in order to identify any conditions that may interfere with the test’s accuracy.

C-peptide test results may be affected by:

• Certain medications. C-peptide tests may be affected by a number of medications, including:
  
• • Insulin
  • Antidiabetic agents
  • Corticosteroids
  • Birth control pills
  • Deferoxamine (used to treat anemia)
  • Rifampin (used to treat tuberculosis)
  • Terbutaline (used to treat breathing problems)
  • Propranolol (a type of beta blocker)
    
    
• Certain medical conditions. Insulinoma, liver disorders, kidney disorders and other medical conditions can cause inaccurate results.

• Obesity. The increased production of insulin associated with insulin resistance can result in increased C-peptide levels.

C-peptide tests are normally performed in a physician’s office for laboratory analysis. For this blood test, a sample of blood is usually drawn from a vein (venipuncture) from the inside of the elbow or the back of the hand. First the puncture site is sterilized with antiseptic and an elastic band is wrapped around the patient’s upper arm. As a result, the pressure restricts blood flow through the vein and causes the veins below the band to fill with blood.

A needle is then inserted into a vein. Occasionally more than one puncture is necessary to locate a vein. Some people may feel a moderate level of pain when the needle is inserted, but most feel only a prick or stinging sensation. Once the needle is inserted into a vein, a tube is attached to collect the blood as it begins to flow out. The elastic band is then removed.

After the necessary amount of blood is collected, the needle is withdrawn and a small cotton ball or pad is applied with light pressure over the puncture site. After several minutes, the cotton will be discarded or replaced, and a small bandage will be placed on the puncture wound. The entire process takes less than ten minutes. The sample is sent to a laboratory for analysis and results are returned in several days.

Despite the precautions taken to avoid bruising and soreness, it does sometimes occur. Typically, this is not a cause for great concern. To minimize soreness, patients may immediately apply a warm compress to the puncture site and repeat the application every three hours until the discoloration or pain subsides.

Following the withdrawal of blood, patients may resume medications and food intake according to their physician’s orders. Immediately after the blood sample is taken, the test tubes are labeled with the date and the patient’s name, and they are sent to the laboratory for testing. There are two methods of conducting a C-peptide test: radioimmunoassay (RIA) and immunochemiluminometric assay (ICMA). The method used may vary from laboratory to laboratory.

Once the results are returned, they will be compared to the normal range for the test. C-peptide test results may also be compared to the results of a glucose test performed at the same time.

Normal values for a C-peptide test are usually between 0.5 and 2.0 ng/mL (nanograms per milliliter). However, normal values may vary from laboratory to laboratory. Results may also be expressed in different units (e.g., nanograms per milliliter and picomoles per liter) from lab to lab. As a result, patients should be aware of what the normal range is for the lab being used. People undergoing a series of C-peptide tests should have the tests performed at the same lab using the same method.

In patients with diagnosed diabetes, a glucose test may be performed at the same time as the C-peptide test to monitor insulin production.

C-peptide test results within the normal range and a high blood glucose level may indicate:

• Type 2 diabetes. Normal or high levels of C-peptide in combination with high blood glucose may indicate that the body is producing enough insulin but the cells are not responding to it properly.

C-peptide test results below the normal range and an abnormal glucose level may indicate:

• Type 1 diabetes. An undetectable level of C-peptide and high blood glucose (hyperglycemia) may indicate that the pancreas is not producing insulin.
  
  
• Type 2 diabetes. A low level of C-peptide and high blood glucose may indicate that the pancreas is producing too little insulin.
  
  
• Excessive use of diabetes medication. A low blood glucose level and low C-peptide level may indicate the use of too much insulin or antidiabetic agents.
  
  
• Medical disorder. Low levels of C-peptide and blood glucose may be associated with liver disease, a severe infection, hormonal disorders and other conditions.
  
  
• Successful pancreas transplant or islet cell transplant. The presence of C-peptide in the bloodstream may indicate that a pancreas or islet transplant was successful and the body has started to produce insulin.

C-peptide test results above the normal range and an abnormal glucose level may indicate:

• Type 2 diabetes. A high level of C-peptide in addition to a high level of blood glucose may indicate type 2 diabetes.
  
  
• Insulin resistance. A high level of C-peptide in addition to a high level of blood glucose may indicate insulin resistance and hyperinsulinemia.
  
  
• Insulinoma. A high level of C-peptide in addition to a low blood glucose level may indicate a tumor of the pancreas, called an insulinoma, that results in the production of an excessive amount of insulin.
  
  
• Unsuccessful treatment of insulinoma. An increasing level of C-peptide in the blood after the removal of an insulinoma may indicate that the tumor has either returned or spread to other parts of the body.

When the results of a C-peptide test indicate the presence of a disease or other medical disorder, a physician may initiate treatment for that condition. The type of treatment will vary based on the orientation of the condition. It may involve a change in lifestyle behavior, such as change in diet or exercise. Other disorders may require medication or surgery.

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about the C-peptide test:

1. Why do I need a C-peptide test?
   
   
2. Will I have a glucose test at the same time?
   
   
3. Do I need to fast, withhold medication or do anything else to prepare for the test?
   
   
4. When and where will I have the test?
   
   
5. What is the normal range according to this lab’s testing methods?
   
   
6. Who will explain my test results to me, and when?
   
   
7. What do my test results indicate?
   
   
8. Could anything be affecting my test results?
   
   
9. What follow-up tests might I need?
   
   
10. If treatment is needed, what are my options, and which do you recommend?
    
    
11. How often should I have a C-peptide test?