What does a C-peptide (Connecting peptide) level of 1.9 ng/mL indicate?

Interpretation of C-peptide Level of 1.9 ng/mL

A C-peptide level of 1.9 ng/mL (>600 pmol/L) strongly indicates a Type 2 diabetes pattern with preserved beta-cell function, suggesting that non-insulin therapies may be appropriate for treatment. 1, 2

Clinical Significance of C-peptide Level 1.9 ng/mL

C-peptide is produced in equimolar amounts to endogenous insulin and serves as the best measure of beta-cell function in patients with diabetes. According to the American Diabetes Association's 2024 Standards of Care, C-peptide values can be interpreted as follows:

• <0.6 ng/mL (<200 pmol/L): Type 1 diabetes pattern
• 0.6-1.8 ng/mL (200-600 pmol/L): Indeterminate status

• 1.8 ng/mL (>600 pmol/L): Type 2 diabetes pattern 1, 2

At 1.9 ng/mL, this C-peptide level exceeds the 1.8 ng/mL threshold, indicating preserved beta-cell function consistent with Type 2 diabetes.

Diagnostic Algorithm Based on C-peptide Level

When evaluating a patient with a C-peptide of 1.9 ng/mL:

1. Confirm diabetes diagnosis using standard criteria (fasting glucose ≥126 mg/dL, 2-hour OGTT ≥200 mg/dL, random glucose ≥200 mg/dL with symptoms, or HbA1c ≥6.5%)

2. Evaluate for Type 2 diabetes features:

   • BMI ≥25 kg/m²
   • Absence of significant weight loss
   • No history of ketoacidosis
   • Less marked hyperglycemia at presentation
   • Features of metabolic syndrome 1

3. Consider autoantibody testing if:

   • Age <35 years
   • BMI <25 kg/m²
   • Poor response to oral hypoglycemic agents
   • Personal or family history of autoimmune disease 2

Clinical Implications for Treatment

With a C-peptide of 1.9 ng/mL:

• Initial therapy can focus on non-insulin approaches:

  • Lifestyle modifications (diet, exercise)
  • Oral hypoglycemic agents (metformin typically first-line)
  • GLP-1 receptor agonists or other newer agents as appropriate 2

• Insulin therapy is generally not immediately required but may be needed later if beta-cell function declines 2

Important Considerations and Pitfalls

• Timing of measurement matters: C-peptide should not be measured within 2 weeks of a hyperglycemic emergency 1, 2

• Concurrent glucose levels affect interpretation: If the C-peptide was measured when glucose was <70 mg/dL, consider repeating the test 1

• Fasting vs. random sample: A random C-peptide within 5 hours of eating can replace formal stimulation tests for classification purposes 1

• Potential for LADA (Latent Autoimmune Diabetes in Adults): Despite high C-peptide, if the patient has autoantibodies, they may have LADA, which represents 5-10% of adults initially diagnosed with Type 2 diabetes 2, 3

• C-peptide levels decline over time: Regular reassessment may be warranted, especially if treatment response is suboptimal 2, 4

• Insulin-treated patients: In patients already on insulin therapy, C-peptide should be measured prior to insulin discontinuation to avoid severe insulin deficiency 1

In summary, a C-peptide level of 1.9 ng/mL indicates preserved beta-cell function consistent with Type 2 diabetes, suggesting that non-insulin therapies are likely appropriate as initial treatment, though regular monitoring for disease progression remains important.