C-Peptide Test: Purpose, Procedure, Normal Range, Results and Uses

Understanding the C-Peptide Test and What It Reveals About Insulin Production

The C-peptide test is a commonly performed diagnostic test used to evaluate C-peptide levels in the blood or urine, determine the type of diabetes, monitor and diagnose other medical conditions.

Doctors primarily recommend the C-peptide test to assess insulin production, differentiate types of diabetes (Type 1 vs Type 2), evaluate hypoglycaemia, and monitor pancreatic beta-cell function.

What is a C-peptide Test?

​The C-peptide blood test measures or evaluates the amount of connecting peptide or C-peptide in the body. The pancreas releases it while producing insulin. That is why the test is sometimes called Insulin C-peptide, Connecting peptide insulin or Proinsulin C-peptide.

The pancreas typically secretes insulin and C-peptide in roughly the same quantities. ​This build-up can generate severe health issues, making the C-peptide test report a vital tool for doctors. ​It helps doctors accurately diagnose or monitor health conditions such as:

• Diabetes (even helps determine the type of diabetes)
• ​Pancreatic cancer
• Hypoglycemia (when your blood glucose level is lower than normal)
• ​Kidney failure
• ​Cushing syndrome
• Addison disease

​Why is the C-Peptide Test Done?

Knowing the C-peptide level serves two purposes: diagnosing diseases and monitoring how well the diabetes treatment plan is working. If the report shows a high level of C-peptide, it might mean the body is producing too much insulin, which can lead to several conditions:

​Doctors may strongly recommend the C-peptide test for several key reasons, including:

• Determining the type of diabetes (such as Type 1 or Type 2 diabetes) and evaluating how much insulin the pancreas is producing.
• Monitoring insulin production in people receiving insulin treatment for diabetes.
• Investigating the cause of hypoglycaemia. There are types of hypoglycemia that are associated with high insulin levels. However, checking the C-peptide level is actually helpful, especially when the cause is unclear.
• Diagnosing or monitoring treatment for insulinoma, a rare pancreatic tumour that produces excessive insulin and reduces blood glucose levels.
• Assessing the functionalities of the pancreas and checking whether the body is making enough natural insulin.

​What Symptoms or Conditions Need a C-Peptide Test?

Doctors may ask you to show the C-peptide test report if you have recently been diagnosed with a pancreatic tumour or hypoglycemia and they need to determine the underlying cause.

Your doctor may advise this test if you experience symptoms of Hypoglycemia, which commonly include:

1. Headache
2. Shakiness
3. Sweating
4. Hunger or nausea
5. Fast or irregular heartbeat (called arrhythmia)
6. Light-headedness or Dizziness
7. Fatigue

In extreme cases, patients living with hypoglycemia can experience confusion and seizures.

Doctors may also order a C-peptide level test if a person has diabetes and:

1. Use insulin by syringe and the doctor wants to adjust the treatment plan.
2. Has been diagnosed with type 2 diabetes and the doctor wants to determine whether insulin therapy is necessary.
3. Experience hypoglycaemia, and the medical professional wants to know whether diabetes medicines are the main contributing factor.

​How is the C-Peptide Test Performed?

Doctors usually measure C-peptide level by taking a sample of your blood. However, they may suggest taking a sample of all your urine collected over 24 hours in some cases.

The steps for performing a C-peptide blood test are as follows:

1. ​A trained healthcare professional will take your blood sample from a vein in your arm.
2. They will select a vein first. Then they insert a sterile, small needle into the vein to draw the blood.
3. ​The medical professional collects the blood into a test tube or vial for laboratory analysis.

The steps you need to follow for a 24-hour urine sample test:

1. The laboratory will give you a special container to collect your urine.
2. Release your pee as usual. You need not collect this urine. Only note down the time you urinated.
3. After this, you will have to collect all your urine in that container for the next 24 hours.
4. Keep the urine container in a fridge or in front of a cooler with ice during the whole period.
5. Deposit the container with your urine sample in the laboratory.

​Preparation Before the C-Peptide Blood or Urine Test

Preparation ensures the accuracy of the test results. Although there are no specific preparations for this blood test, your doctor may ask you to:

• ​Fasting: Your doctor may ask you to fast (not eat or drink) for 8 to 12 hours prior to the test.
• ​Inform About Medications: Some medicines can alter the C-peptide level. So it is better to inform your doctor about all the medications that you need to consume.
• ​Stay Hydrated: Drinking adequate water is necessary before taking the test. It makes the blood drawing process smooth.
• ​Avoiding Alcohol or Smoking: Do not consume alcohol or tobacco in any form before taking the test.

Note: ​Your healthcare provider may ask you to test your C-peptide level after you've eaten something. This approach helps them assess your C-peptide levels while your pancreas is actively producing insulin.

​Normal Range / Expected Results of C-Peptide Test

​A normal result for a C-peptide test can vary depending on the specific laboratory, the testing methods used, and the individual’s health condition.

​Generally, C-peptide is measured in nanograms per millilitre (ng/mL), and insulin is measured in micro-units per millilitre (µU/mL). Below is a standard reference table for the C-peptide test ranges:

​Results Interpretation of C-Peptide Test

When you show the report to your healthcare provider, they will interpret it based on factors such as your age, overall health, medications, and the findings of other tests, including blood glucose levels.

1. Higher-than-normal C-peptide levels

It may indicate that your body is producing excess insulin. It can occur in health conditions such as:

• Type 2 diabetes
• Low blood potassium levels (hypokalaemia)
• Insulinoma (an insulin-producing tumour of the pancreas)
• Kidney failure
• Cushing’s syndrome

2. Lower-than-normal C-peptide levels

It may suggest that your body is not producing enough insulin. Possible causes include:

• Type 1 diabetes
• Certain cases of advanced Type 2 diabetes
• Excessive insulin therapy can restrict the pancreas's ability to produce natural insulin production
• Addison’s disease
• Liver disorder
• Severe infections

For patients receiving treatment for an insulin-producing pancreatic tumour:

• A decrease in C-peptide levels is an indication that the treatment is effective.
• An increase in C-peptide levels may suggest that the tumour has returned.

Always discuss your test results with your healthcare provider. Only they can provide an accurate interpretation and appropriate medical advice.

​What are the Risks or Side Effects of Taking the C-Peptide Test?

​The C-peptide blood test is generally safe and associated with zero to minimal risks.

1. Mild Pain

You may experience mild pain or bruising at the spot where the healthcare professional inserts the needle. However, they usually fade within a few days.

2. Temporary Discomfort

Moreover, you may feel temporary discomfort in the arm for one or two hours following the test. Some individuals may get momentarily faint or dizzy during or immediately after the blood-drawing process.

3. Infection

Apart from that, there is a very rare risk of infection if the professional does not disinfect the skin before starting the process.

When considering a C-peptide urine test, there are no known risks to this.

​What are the Benefits of the C-Peptide Blood Test?

• Helps Identify: It helps to know the type of diabetes by showing how much insulin the pancreas is producing.
• Supports Accurate Diagnosis: It assists in the diagnosis of conditions associated with abnormal insulin production, such as insulinoma and hypoglycaemia.
• Assists in Treatment Planning: Not only does it help in diabetes treatment planning, but it also helps doctors monitor and assess whether insulin therapy is needed or working effectively.
• Helps Investigate the Cause: This test helps to know the cause of low blood sugar and distinguish between different underlying conditions.
• Monitors Treatment Outcomes: It is helpful for patients treated for insulin-producing pancreatic tumours through tracking changes in C-peptide levels.

​Who Should Get This Test?

This test may be recommended for:

• ​People with diabetes
• Individuals with newly diagnosed diabetes need to know the type
• People experiencing unexplained hypoglycaemia
• Individuals suspected of having an insulinoma
• Diabetic patients who need changes to their insulin therapy

​What is the C-Peptide Test Cost in India?

The cost of the C-peptide test can vary slightly depending on where you live, which diagnostic centre you choose, and whether you have chosen home or centre blood sample collection.  

​Below is an approximate city-wise C-peptide test pricing table:

​Does Health Insurance Cover C-Peptide Tests?

​Health insurance may cover the C-peptide blood test if it is medically necessary and a certified doctor prescribes it. ​Most medical insurance policies clearly differentiate between diagnostic tests for preventive measures and tests required to treat properly when you are in the hospital.

Suppose the test is a part of a pre-hospitalisation requirement or the doctor orders it during inpatient stay. You may get the insurance benefit. However, if you include this test for routine outpatient screening, coverage will depend on what type of OPD benefits you are offered.

The extent of coverage depends on hospitalisation status, your policy terms, OPD benefits, and the specific conditions of your insurer. So, carefully check the policy brochure to understand your coverage scope.

Reader information: This article is intended for informational purposes only. So, consult a medical professional before making any health-related decisions based on this content.