A C-peptide test measures the level of C-peptide in your blood or urine to evaluate how much insulin your pancreas is producing. Because C-peptide and insulin are released into the bloodstream at the same time and in equal amounts, this test provides a reliable picture of your body’s natural insulin production.
Unlike insulin levels, C-peptide is not affected by injected or inhaled insulin used for diabetes treatment. For this reason, healthcare providers often rely on a C-peptide test to distinguish between insulin made by the body and insulin taken as medication.
Understanding C-Peptide and Insulin
C-peptide is a short chain of amino acids created when the pancreas produces insulin. Insulin is the hormone responsible for helping glucose enter the body’s cells, where it is used for energy. Proper insulin function is essential for maintaining healthy blood sugar levels.
In people with Type 1 diabetes, the pancreas produces little or no insulin. In Type 2 diabetes, insulin is produced, but the body may not use it effectively, or insulin production may decline over time. Measuring C-peptide helps clarify which situation is present.
Why Is a C-Peptide Test Performed?
Healthcare providers may order a C-peptide test to better understand blood sugar disorders and guide treatment decisions. Common reasons include:
• Determining whether diabetes is Type 1 or Type 2 when the diagnosis is unclear
• Evaluating how well current diabetes treatment is working
• Deciding whether insulin therapy is necessary
• Investigating unexplained low blood sugar (hypoglycemia)
• Differentiating between insulin produced by the body and insulin taken as medication
• Monitoring or diagnosing rare insulin-producing pancreatic tumors (insulinomas)
How Is the Test Done?
A C-peptide test is usually performed using a blood sample taken from a vein in the arm. In some cases, especially when more detailed information is needed, the test may be done using urine collected over a 24-hour period. Blood testing is more common and quicker.
Depending on the clinical situation, your provider may ask you to fast for 8–12 hours or may measure C-peptide after eating to see how your pancreas responds to food. The test is often performed alongside a blood glucose measurement.
Understanding the Results
C-peptide results must always be interpreted together with blood sugar levels and clinical findings. In general:
•
Low C-peptide levels
suggest reduced insulin production, which may occur in Type 1 diabetes, advanced Type 2 diabetes, after insulin injections, or with certain conditions such as Addison disease or severe liver disease.
•
High C-peptide levels
indicate excessive insulin production, which may be associated with insulin resistance, Type 2 diabetes, Cushing syndrome, kidney disease, low potassium levels, or insulin-producing tumors.
•
If no C-peptide is detected
, insulin replacement therapy is usually required.
Important Notes
A C-peptide test does not diagnose diabetes on its own. Blood glucose tests and hemoglobin A1c (HbA1c) remain the primary tools for diagnosing diabetes. Instead, the C-peptide test plays a supportive role by showing how well the pancreas is functioning and helping tailor long-term treatment strategies.