C-peptide levels are used to assess insulin production and pancreatic beta-cell function, aiding in the diagnosis and management of various conditions, particularly diabetes and insulin-related disorders. Here’s a concise overview of their diagnostic utility:

Key Uses of C-Peptide Levels

1. Differentiating Type 1 vs. Type 2 Diabetes:
   • Low or undetectable C-peptide: Suggests type 1 diabetes (autoimmune destruction of beta cells, minimal/no insulin production).
   • Normal or elevated C-peptide: Indicates type 2 diabetes (insulin resistance with preserved or increased insulin production, at least initially).
   • Borderline cases: Can help identify latent autoimmune diabetes in adults (LADA), where C-peptide is low but not as absent as in type 1.
2. Evaluating Hypoglycemia:
   • High C-peptide with hypoglycemia: Suggests insulinoma (insulin-producing tumor) or sulfonylurea use (stimulates insulin release).
   • Low C-peptide with hypoglycemia: Points to exogenous insulin administration (e.g., factitious hypoglycemia), as injected insulin lacks C-peptide.
   • Normal C-peptide: May indicate non-insulin-mediated causes of hypoglycemia (e.g., liver disease, alcohol).
3. Assessing Residual Beta-Cell Function:
   • In type 1 diabetes, low C-peptide indicates progression to insulin dependency.
   • In type 2 diabetes, declining C-peptide over time may signal the need for insulin therapy.
4. Monitoring Post-Pancreatic Surgery or Transplant:
   • C-peptide levels help evaluate beta-cell function after pancreatectomy or islet cell transplantation.
5. Diagnosing Insulin Resistance:
   • Elevated C-peptide with high insulin levels can indicate insulin resistance (e.g., in metabolic syndrome or PCOS).

Interpretation of C-Peptide Levels

• Normal range: ~0.5–2.0 ng/mL (fasting); varies by lab.
• Low C-peptide: Reduced endogenous insulin production (e.g., type 1 diabetes, advanced type 2 diabetes, or pancreatectomy).
• High C-peptide: Increased insulin production (e.g., type 2 diabetes, insulinoma, or sulfonylurea use).
• Context matters: Levels are interpreted alongside glucose, insulin, and clinical history. Fasting or stimulated (e.g., glucagon challenge) tests may be used.

Clinical Considerations

• Fasting C-peptide: Most reliable when glucose is normal or low to avoid confounding by hyperglycemia.
• Stimulated C-peptide: Used to assess beta-cell reserve (e.g., after glucagon or mixed-meal test).
• Limitations: Renal impairment can elevate C-peptide (cleared by kidneys), and obesity may increase levels due to insulin resistance.

Example Scenarios

• Young patient with hyperglycemia, low C-peptide, and positive autoantibodies: Likely type 1 diabetes.
• Older patient with hyperglycemia, high C-peptide, and no autoantibodies: Likely type 2 diabetes.
• Hypoglycemia with high C-peptide and no sulfonylurea use: Suspect insulinoma.
• Hypoglycemia with low C-peptide and high insulin: Suspect exogenous insulin administration.

C-peptide is a valuable marker because it is co-secreted with insulin in equimolar amounts but not affected by exogenous insulin, making it a direct measure of endogenous insulin production.

Disclaimer: owerl is not a doctor; please consult one.