C-Peptide (C-Terminal Insulin)

The C-Peptide test measures the level of C-peptide in the blood. C-peptide is a byproduct released when the pancreas produces insulin. It is used to evaluate how much insulin the body is producing and can help differentiate between type 1 and type 2 diabetes or assess insulin resistance.

1. What Is C-Peptide?

• C-peptide (connecting peptide) is released in equal amounts to insulin when proinsulin splits into insulin and C-peptide in the pancreas.

• Unlike insulin, C-peptide is not affected by insulin injections, making it a useful marker for natural insulin production.

• It helps assess pancreatic beta-cell function.

2. Why Is the C-Peptide Test Performed?

This test is used to:

• Determine whether the body is producing insulin.

• Differentiate between type 1 diabetes (low or absent C-peptide) and type 2 diabetes (normal or high C-peptide).

• Evaluate the cause of low blood sugar (hypoglycemia).

• Monitor insulinoma (insulin-producing tumor).

• Assess beta-cell function in diabetes management or after pancreas transplant.

3. C-Peptide Reference Range

• Fasting: 0.5 – 2.0 ng/mL

• Levels can vary depending on lab methods and whether the test is done fasting or post-glucose intake.

4. How to Interpret C-Peptide Results

🔹 Low C-Peptide:

• Suggests reduced insulin production, as seen in type 1 diabetes or advanced type 2 diabetes.

• May also occur after removal of the pancreas or due to autoimmune destruction of beta cells.

🔹 High C-Peptide:

• Suggests high insulin production, which may occur in type 2 diabetes, insulin resistance, or insulinoma.

• May also be elevated in people taking sulfonylureas or with certain kidney disorders.

5. What Can Affect C-Peptide Levels?

• Type of diabetes and treatment (insulin vs. oral medications)

• Glucose levels at the time of testing

• Kidney function (C-peptide is cleared by the kidneys)

• Medications affecting insulin production (e.g., sulfonylureas)

6. How Is the Test Performed?

• A blood sample is taken, usually after fasting or sometimes after a glucose challenge.

• It may be ordered alongside blood glucose and insulin tests for a more complete assessment.

7. When to See a Doctor

🚨 Contact your healthcare provider if you:

• Have unexplained low or high blood sugar.

• Are newly diagnosed with diabetes and need to determine type.

• Are being evaluated for pancreatic function or insulin-producing tumors.

🔹 Key Takeaways

✅ C-peptide is a reliable marker of your body’s natural insulin production.

✅ It helps distinguish between different types of diabetes and assess beta-cell function.

✅ Testing is useful for diagnosis, management, and treatment planning in diabetes care.