A C-peptide test is a measurement of C-peptide present in the blood or urine. When the pancreas produces and releases the hormone insulin, it creates C-peptide in roughly equal amounts. Therefore, this test can help measure insulin levels and differentiate between type 1 and type 2 diabetes.

Diabetes is a health condition that affects how the body processes blood glucose, or blood sugar. It occurs when the pancreas can no longer produce insulin or when tissues develop insulin resistance, and insulin is no longer effective in transporting glucose into the cells. Two of the most common types of diabetes include type 1 and type 2 diabetes.

As a person with type 1 can no longer produce insulin or C-peptide, and a person with type 2 produces more to compensate, this test can help monitor insulin production and discern between the two types of diabetes.

This article discusses the purpose of the C-peptide test, what it measures, and how to interpret the results.

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A C-peptide test, also known as an insulin C-peptide test, describes a test to measure the amount of a substance, known as C-peptide, in the blood or urine.

When the body produces insulin, it produces C-peptide in roughly equal amounts. As C-peptide tends to stay in the body longer than insulin, it can be a useful marker to help determine insulin production. High C-peptide levels suggest high insulin production and may indicate type 2 diabetes, while low levels suggest low insulin and can indicate type 1 diabetes.

The C-peptide test helps to assess the function of pancreatic beta cells by measuring the amount of C-peptide present in blood or urine. By doing so, it can help differentiate between type 1 and type 2 diabetes.

Insulin is a hormone that helps the body use glucose from food, providing cells with energy and helping regulate blood glucose. To turn food into energy, the pancreas produces proinsulin. In turn, the body then converts proinsulin into insulin and C-peptide in roughly equal measure.

This makes C-peptide a useful marker for insulin production, especially as it is more stable in blood than insulin. Additionally, it is preferable to measure C-peptide as it accounts for any insulin the body may produce and differentiates from any insulin a person may inject.

The test can also be useful for:

  • identifying possible causes of low blood sugar
  • checking the effectiveness of diabetes treatments
  • checking the status of a pancreatic tumor

A person may require a C-peptide test if their healthcare professional wants to monitor their insulin levels. Often, a doctor may recommend this test to evaluate a person’s risk for diabetes and to identify which type they may have.

In particular, a doctor may suggest this test if a person is experiencing symptoms of hypoglycemia. These symptoms can include:

A C-peptide test may also be advisable if an individual has recently had pancreatic surgery, such as a removal or a transplant. It can also help assess how a person is responding to weight loss after bariatric surgery and can help predict diabetes remission.

A C-peptide test usually requires blood samples. A medical professional takes blood from a vein in the arm or the back of the hand. Sometimes the test also uses a 24-hour urine sample.


People undergoing the C-peptide test may need to fast for 8–12 hours beforehand. Sometimes the test takes place after stimulation with a mixed meal, glucagon, glucose-like peptides, oral glucose, or intravenous glucose.

A doctor will provide appropriate instructions before the test based on individual requirements. This may include asking the person to fast or stop taking medications that may affect test results.


The C-peptide test has no significant risk. People may feel discomfort at the spot where the needle draws blood, but this typically goes away within a few hours. There are no known risks for the urine test.

Some of the less common risk factors of the blood test may include:

  • bleeding at the needle site
  • bruising and soreness at the needle site
  • multiple punctures to locate veins
  • fainting
  • infection

The typical C-peptide levels in a healthy individual may range from roughly 0.3–0.6 nanomoles per liter (nmol/l) in the fasting state and 1–3 nmol/l following a meal. However, these ranges may vary among different laboratories and may also use different measurements or test different samples. A person can consult their healthcare professional, who can then explain their specific results thoroughly.

As the presence of C-peptide suggests that the body is producing insulin, a low result or no C-peptide indicates that the pancreas is producing little or no insulin. This may suggest one of the following conditions:

High concentrations of C-peptide suggest high levels of insulin. This excessive insulin production can occur due to elevated blood sugars and insulin resistance, which both indicate type 2 diabetes. Aside from type 2 diabetes, high C-peptide results can also suggest:

While a C-peptide test can provide important information about diabetes, it is not a diagnostic test. Other tests, such as blood glucose, screen and diagnose diabetes.

C-peptide is a substance that the body produces in equal measure to insulin. As such, it is a useful indicator of beta-cell function and whether people are producing insulin. Testing blood or urine for this substance, can help a doctor determine if a person has type 1 diabetes, type 2 diabetes, or another condition that can affect insulin.

A low level of C-peptide indicates the body is not producing enough insulin and may suggest type 1 diabetes. A high level of C-peptide indicates that the body is producing more than the required levels of insulin and may suggest type 2 diabetes.