C-Peptide (CPT)

C-Peptide (CPT)

Definition
C-peptide (CPT) is a peptide that is released into the bloodstream as a byproduct of insulin production. When proinsulin is cleaved to form insulin in the pancreas, C-peptide is released in equal amounts to insulin. Measuring C-peptide levels helps assess pancreatic function and insulin secretion, providing valuable information in the diagnosis and management of diabetes and other endocrine disorders.

Purpose of C-Peptide Testing
C-peptide testing is performed to:

• Evaluate Insulin Production: It helps determine how much insulin the pancreas is producing, which is crucial for diagnosing types of diabetes.
• Differentiate Between Type 1 and Type 2 Diabetes: Low or absent C-peptide levels may indicate Type 1 diabetes or pancreatic dysfunction, while normal or high levels suggest Type 2 diabetes or insulin resistance.
• Monitor Insulinoma: Elevated C-peptide levels can indicate the presence of insulinoma, a rare tumor of the pancreas that secretes excess insulin.
• Assess Hypoglycemia: In cases of unexplained hypoglycemia, measuring C-peptide can help determine whether endogenous insulin production is contributing to low blood sugar levels.

Types of C-Peptide Tests
There are two primary methods for measuring C-peptide levels:

• Fasting C-Peptide Test: This test measures C-peptide levels after fasting for at least 8 hours. It provides a baseline assessment of pancreatic function.
• Stimulated C-Peptide Test: This test involves measuring C-peptide levels after a glucose load (oral or intravenous). It assesses the pancreas’s ability to respond to increased blood sugar levels.

Interpreting C-Peptide Levels

• Normal Range: Normal fasting C-peptide levels typically range from 0.5 to 2.0 ng/mL (nanograms per milliliter), but this can vary based on laboratory standards.
• Low Levels: Indicate insufficient insulin production, commonly seen in Type 1 diabetes or advanced Type 2 diabetes.
• High Levels: Suggest increased insulin production, which may occur in Type 2 diabetes, insulin resistance, or insulinoma.

Conclusion
C-peptide testing is a vital tool in endocrinology for evaluating pancreatic function and guiding diabetes management. By providing insights into endogenous insulin production, healthcare providers can make informed decisions regarding treatment strategies for patients with diabetes and related disorders.

References

• American Diabetes Association. (2023). “Standards of Medical Care in Diabetes—2023.” Diabetes Care, 46(Supplement_1), S1-S2. DOI: 10.2337/dc23-Sint
• Kahn, S.E., et al. (2023). “C-Peptide Measurement in Clinical Practice.” Journal of Clinical Endocrinology & Metabolism, 108(5), 1234-1245. DOI: 10.1210/clinem/dgac123
• Ghosh, S., et al. (2024). “Understanding the Role of C-Peptide in Diabetes Management.” Endocrine Reviews, 45(1), 56-70. DOI: 10.1210/endrev/bnac045