When should a C‑peptide test be ordered in adults and how are fasting and stimulated C‑peptide results interpreted for diabetes type differentiation, hypoglycemia evaluation, and assessment of residual β‑cell function?

C-Peptide Testing in Clinical Practice

When to Order C-Peptide Testing

C-peptide testing should be ordered primarily in insulin-treated patients when diabetes type is ambiguous—particularly in adults presenting with ketoacidosis but having a type 2 phenotype, or in antibody-negative patients where distinguishing type 1 from type 2 diabetes will fundamentally alter management. 1

Primary Clinical Indications

• Diabetes type differentiation: Order C-peptide when clinical presentation makes it difficult to distinguish between type 1 and type 2 diabetes, especially in adults who present with ketoacidosis but have phenotypic features of type 2 diabetes 1, 2

• Antibody-negative patients: For patients under 35 years with suspected type 1 diabetes who test negative for islet autoantibodies and lack clinical features of type 2 or monogenic diabetes, C-peptide testing helps confirm the diagnosis 2

• Insurance requirements: Measure fasting C-peptide when payers require documentation for insulin pump therapy coverage 1, 2

• Hypoglycemia evaluation: C-peptide measurement is essential in investigating nondiabetic hypoglycemia to rule out surreptitious insulin administration 1

• Residual β-cell function assessment: After at least 3 years of diabetes duration in antibody-negative patients, C-peptide can help confirm classification and guide treatment decisions 2, 3

When NOT to Order C-Peptide

• Do not test within 2 weeks of a hyperglycemic emergency (diabetic ketoacidosis or hyperosmolar state), as results will be artificially suppressed 2, 4

• Routine testing is not recommended in most people with diabetes or those at risk for diabetes—these assays are primarily useful for research purposes 1

• Not indicated for polycystic ovary syndrome evaluation, as measuring insulin resistance through C-peptide has no advantage over clinical assessment (BMI, acanthosis nigricans) 1

Testing Methodology: Fasting vs. Stimulated

Fasting C-Peptide Testing

For insurance coverage of insulin pump therapy, measure fasting C-peptide when simultaneous fasting plasma glucose is ≤220 mg/dL (12.5 mmol/L). 1, 2

• An 8-hour fast is the standard recommendation for C-peptide testing 5

• Fasting C-peptide is most practical for routine clinical use and insurance documentation 2

Random (Non-Fasting) C-Peptide Testing

A random C-peptide sample collected within 5 hours of eating can replace a formal C-peptide stimulation test for diabetes classification purposes. 2, 4, 5

• This approach significantly simplifies testing while maintaining diagnostic accuracy 2

• Random testing is particularly useful when fasting is impractical or when immediate classification is needed 2

Stimulated C-Peptide Testing

• Glucagon stimulation testing offers the best balance of sensitivity and practicality when stimulated testing is required 6

• Stimulated C-peptide (in response to intravenous glucagon) can aid in differentiating type 1 from type 2 diabetes in difficult cases 1

• However, given that random C-peptide within 5 hours of eating performs comparably, formal stimulation testing is rarely necessary in routine practice 2

Interpretation of C-Peptide Results

For Diabetes Type Differentiation

C-peptide <200 pmol/L (<0.6 ng/mL) is consistent with type 1 diabetes and indicates significant β-cell loss requiring insulin therapy. 2, 4

• Very low C-peptide <80 pmol/L (<0.24 ng/mL) indicates absolute insulin deficiency and severe β-cell loss—this definitively confirms type 1 diabetes and does not require repeat testing 2, 4

• C-peptide 200-600 pmol/L (0.6-1.8 ng/mL) represents an indeterminate zone that may indicate type 1 diabetes, latent autoimmune diabetes in adults (LADA), maturity-onset diabetes of the young (MODY), or long-standing insulin-treated type 2 diabetes 2, 4

• C-peptide >600 pmol/L (>1.8 ng/mL) suggests type 2 diabetes with preserved β-cell function 2, 4

Critical Testing Considerations

• If concurrent glucose is <70 mg/dL (<4 mmol/L), consider repeating the test, as hypoglycemia can suppress C-peptide secretion 2, 4

• For insulin-treated patients, C-peptide must be measured prior to insulin discontinuation to exclude severe insulin deficiency 2, 4

• Results showing very low levels (<80 pmol/L) do not need to be repeated, as they definitively indicate severe insulin deficiency 2

Integration with Autoantibody Testing

Always test for islet autoantibodies first in patients with ambiguous presentation—if antibody-positive, the diagnosis is type 1 diabetes regardless of C-peptide level. 2, 4

• Test for glutamic acid decarboxylase (GAD65) antibodies, islet tyrosine phosphatase 2 (IA-2) antibodies, and zinc transporter 8 (ZnT8) antibodies 4

• Approximately 5-10% of adults with type 1 diabetes are antibody-negative, making C-peptide measurement essential in this subset 4

• If antibody-positive with low C-peptide, the diagnosis is definitively type 1 diabetes or LADA, and lifelong insulin therapy is required 4

Assessment of Residual β-Cell Function

Clinical Significance

Persistence of substantial insulin secretion (C-peptide >600 pmol/L) after 3-5 years from diagnosis suggests type 2 or monogenic diabetes rather than type 1 diabetes. 3

• Low C-peptide levels (<0.4 nmol/L or 400 pmol/L) indicate absolute insulin deficiency and confirm the need for insulin therapy 2

• Patients with robust C-peptide levels may respond to oral agents, while those with low levels require insulin therapy 2

• C-peptide levels may correlate with microvascular and macrovascular complications and predict future insulin requirements 6

Prognostic Value

• The rate of β-cell destruction in type 1 diabetes is variable—rapid in children and slower in adults 4

• In late-stage type 1 diabetes, there is little or no insulin secretion, manifested by low or undetectable plasma C-peptide levels 4, 5

• With C-peptide in the indeterminate range (200-600 pmol/L), progressive β-cell loss is likely, with eventual complete insulin dependence 4

Hypoglycemia Evaluation

C-peptide measurement is essential in investigating nondiabetic hypoglycemia to rule out surreptitious insulin administration. 1

• The diagnosis of an islet cell tumor is based on inappropriately increased plasma insulin concentrations in the face of low glucose 1

• An increased ratio of fasting proinsulin to insulin in an individual with hypoglycemia strongly suggests an islet cell tumor 1

• The absence of elevated insulin and proinsulin with fasting hypoglycemia makes islet cell tumor unlikely, and alternative explanations should be sought 1

Common Pitfalls and Caveats

Timing and Clinical Context

• Misdiagnosis occurs in 40% of adults with new type 1 diabetes, often misclassified as type 2 diabetes—C-peptide testing helps prevent this error 4

• Never test C-peptide within 2 weeks of diabetic ketoacidosis or hyperosmolar hyperglycemic state, as acute illness suppresses β-cell function 2, 4

• C-peptide testing is only indicated in insulin-treated patients for classification purposes 4

Interpretation Errors

• Do not interpret C-peptide in isolation—always consider concurrent glucose levels, diabetes duration, and autoantibody status 2, 4

• In insulin-treated type 2 diabetes of long duration, C-peptide may fall into the indeterminate range (200-600 pmol/L), mimicking type 1 diabetes 2, 4

• Normal C-peptide levels exclude absolute insulin deficiency states like type 1 diabetes 2

Treatment Implications

• For patients with C-peptide <200 pmol/L, initiate or continue intensive insulin therapy with basal-bolus regimen or insulin pump 4

• Consider automated insulin delivery systems for optimal glycemic control in patients with very low C-peptide 4

• Monitor for diabetic ketoacidosis symptoms (nausea, vomiting, abdominal pain, rapid breathing) in patients with low C-peptide, and check ketones when glucose >250 mg/dL 4