C Peptide Test

The C-peptide test measures the amount of C-peptide in a sample of blood. The test is also called the insulin C-peptide test, the connecting peptide insulin test, and the proinsulin C-peptide test.

C-peptide is a short chain of amino acids (the substances that make up proteins) produced by the pancreas when it produces the insulin responsible for maintaining blood sugar levels in the body. Both insulin and C-peptide are released into the blood at the same time. Insulin is metabolised at a variable rate by the liver, while C-peptide is metabolised at a steady rate by the kidneys. Hence, C-peptide is considered a more reliable measure of insulin levels than insulin itself. C-peptide may also have pro-inflammatory or anti-inflammatory effects on the body. Doctors use this test to find the underlying cause of hypoglycaemia (low blood sugar), manage diabetes treatment, and help distinguish between different types of diabetes when the diagnosis is uncertain.

The C-peptide test is advised if you are experiencing the following symptoms of hypoglycaemia:

• Tiredness
• Headache
• Excess hunger and thirst
• Nausea
• Blurry vision
• Excessive sweating
• A fast or irregular heartbeat
• Shakiness
• Coma and seizures in more severe cases

It is also highly advised for the following categories of people:

• Individuals with hypoglycaemia: C-peptide levels can be measured to check whether the low blood sugar is due to high insulin levels.
• Individuals with diabetes: C-peptide levels can be measured to see how much insulin the body is producing, which helps doctors to decide on the best treatment.
• Individuals with insulinoma (pancreatic tumour): The C-peptide test can help identify this tumour, which produces too much insulin and causes low blood sugar levels.

The C-peptide test cannot be used to diagnose any conditions on its own, but it can be used to monitor and make treatment decisions for a few conditions:

• Hypoglycaemia: This refers to low blood glucose, often caused by your body not making enough insulin.
• Type 1 diabetes: Your pancreas does not make enough insulin, which is reflected in the C-peptide levels, causing high levels of glucose in the blood.
• Type 2 diabetes: Your cells do not normally respond to insulin, leading to excess glucose in the blood.
• Insulinoma: This refers to a pancreatic tumour that produces too much insulin.

What to Expect Before the C-Peptide Test

You must fast for 10 to 12 hours before the test. During the fasting period, drink plain water freely. Avoid all food, milk, tea, coffee, juices, and beverages other than water. Avoid smoking and alcohol for at least 24 hours, as both can falsely elevate triglyceride levels. Avoid strenuous exercise in the 24 hours before the test. Continue your regular medications unless your doctor advises otherwise. Morning appointments are strongly recommended.

What to Expect During the Blood Collection

A phlebotomist cleans the skin with an antiseptic, then places an elastic band around your upper arm to make the veins easier to access. A small needle is inserted into the vein; you may feel a brief pinch. Blood is collected into a labelled tube in under a minute.

What to Expect After the C-Peptide Test

A small cotton swab and bandage are placed over the site to stop any minor bleeding. You can eat and drink immediately after the test - have a light meal as soon as possible after the fast. If dizzy, sit and relax for a few minutes.

In the C-peptide test, the primary parameter being measured is:

• The amount of C-peptide in a sample of blood.

The C-peptide normal range is indicated in the table below.

Note: The normal range may vary slightly depending on the laboratory and individual health conditions.

The table below provides a general interpretation of the C-peptide test results based on whether the levels are low or high.

Note:

If you have been treated for an insulin-producing tumour in the pancreas, a drop in C-peptide levels may indicate that the treatment has been successful. On the other hand, rising C-peptide levels after treatment can suggest that the tumour has returned.

• The C-peptide test must be interpreted along with clinical symptoms, medical conditions, and other blood test results.

Like any other medical test, this test also has certain risks and limitations.

Risks:

• The C-peptide test is generally safe and involves minimal risk. You may experience slight pain when the needle is inserted or mild bruising afterwards, which usually resolves spontaneously.

Limitations:

• A C-peptide testalonecannot diagnose diabetes; other tests, such as blood glucose or urine glucose tests, are required.
• You may need to fast for around 8–12 hours before the test. You may also need to check your C-peptide levels after you eat or drink something. Ask your doctor for more specific instructions on this.

Depending on your symptoms and results, your doctor might recommend certain additional tests to better understand your condition:

• Blood glucose test
• Glucagon blood test
• Insulin in the blood
• Ketones in blood and urine
• Adrenocorticotropic hormone (ACTH) test
• Cortisol test
• Aldosterone test
• Autoantibody testing
• Renin test