In a 27‑year‑old male with borderline‑elevated glycated hemoglobin (HbA1c) and mildly elevated fasting glucose, not on insulin or oral hypoglycemic agents, and presenting with three weeks of vomiting, loss of appetite, and emaciation, should a C‑peptide level be obtained to assess his condition?

Yes, C-peptide testing is strongly indicated in this patient

This 27-year-old male with borderline glycemia (HbA1c 6.6%, FBS 6.9 mmol/L) presenting with severe catabolic symptoms—3 weeks of vomiting, anorexia, and emaciation—requires urgent C-peptide measurement to distinguish between type 1 diabetes with severe insulin deficiency versus other causes of his presentation. 1

Why C-peptide is Critical Here

This clinical picture is highly concerning for undiagnosed type 1 diabetes presenting atypically:

• Age 27 years falls within the adult-onset type 1 diabetes range
• Severe catabolic state (emaciation, prolonged vomiting, weight loss) suggests profound insulin deficiency
• Borderline HbA1c may reflect recent onset—HbA1c integrates glucose over 2-3 months and can be misleadingly low in rapidly progressive diabetes 2
• Not yet on treatment, so C-peptide interpretation is straightforward

The Diagnostic Algorithm

According to current diabetes classification guidelines 1:

1. Measure C-peptide with concurrent glucose (random sample within 5 hours of eating is acceptable)

   • <200 pmol/L (<0.6 ng/mL): Indicates severe insulin deficiency → Type 1 diabetes
   • 200-600 pmol/L (0.6-1.8 ng/mL): Indeterminate, usually consistent with type 1 or monogenic diabetes
   • >600 pmol/L (>1.8 ng/mL): Suggests preserved beta-cell function → Type 2 diabetes unlikely given clinical picture

2. Check islet autoantibodies (GAD, IA-2, ZnT8) to confirm autoimmune etiology 1

3. If C-peptide is very low (<0.6 ng/mL), this patient needs immediate insulin therapy regardless of antibody results

Critical Clinical Context

The Endocrine Society guidelines emphasize that in seemingly well individuals with unexplained symptoms, you should measure plasma glucose, insulin, C-peptide, proinsulin, and β-hydroxybutyrate during symptomatic episodes 3. However, this patient is not well—he's emaciated with 3 weeks of vomiting.

Key considerations:

• Rule out diabetic ketoacidosis (DKA): Check beta-hydroxybutyrate or serum ketones immediately. His symptoms could represent ongoing ketosis 4, 5
• Concurrent glucose measurement is mandatory when interpreting C-peptide—if glucose is <4 mmol/L (<70 mg/dL), the test may need repeating 1
• Do not delay insulin if DKA is present: C-peptide should not be measured within 2 weeks of a hyperglycemic emergency 1

What the C-peptide Will Tell You

• Low C-peptide (<0.6 ng/mL) confirms absolute insulin deficiency → diagnose type 1 diabetes, start insulin immediately, investigate other causes of vomiting
• Normal/high C-peptide → this is NOT typical diabetes presentation; investigate:
  • Gastrointestinal pathology causing the vomiting
  • Other endocrine disorders (adrenal insufficiency, thyroid disease)
  • Malignancy or chronic infection
  • Consider pancreatic imaging if indicated

Common Pitfalls to Avoid

1. Don't assume type 2 diabetes based solely on HbA1c 6.6%—this can be misleadingly low in rapid-onset type 1 diabetes 2, 5
2. Don't wait for antibody results to start insulin if C-peptide is low and patient is catabolic—5-10% of type 1 diabetes patients are antibody-negative 1
3. Don't measure C-peptide after starting insulin—it becomes uninterpretable 1
4. Don't ignore the vomiting—if ketones are elevated, this may be diabetic ketoacidosis requiring urgent treatment

Bottom line: Measure C-peptide NOW (with concurrent glucose and ketones) before any insulin is given. This single test will fundamentally change your diagnostic and therapeutic approach.