C-Peptide Levels in Diabetes Diagnosis

Introduction to C-Peptide Thresholds

The American Diabetes Association provides clear cutoffs for C-peptide interpretation, where levels >600 pmol/L (>1.8 ng/mL) indicate type 2 diabetes, regardless of testing circumstances 1, 2
C-peptide levels between 200-600 pmol/L (0.6-1.8 ng/mL) are usually consistent with type 1 diabetes or MODY, but may occur in insulin-treated type 2 diabetes 1, 2
Levels <200 pmol/L (<0.6 ng/mL) are consistent with type 1 diabetes 1, 2

The American Diabetes Association recommends checking islet autoantibodies (GAD, IA-2, ZnT8) in patients with ambiguous presentation 4
If antibody-positive, the diagnosis is type 1 diabetes regardless of C-peptide level, though C-peptide should still be low 1, 2
If antibody-negative with C-peptide >600 pmol/L, it strongly suggests type 2 diabetes, and features of type 2 diabetes should be looked for, such as BMI ≥25 kg/m², absence of weight loss, absence of ketoacidosis, and features of metabolic syndrome 1, 2, 5

MODY typically presents with C-peptide 200-600 pmol/L, age <35 years, HbA1c <7.5% at diagnosis, and one parent with diabetes 1, 3
Ketosis-prone type 2 diabetes can present with DKA but maintains higher C-peptide levels 1
Insulin-treated type 2 diabetes of long duration may have C-peptide in the intermediate range (200-600 pmol/L) 1

Approximately 5-10% of adults with type 1 diabetes are antibody-negative, making C-peptide measurement essential in this subset 1, 2
Misdiagnosis occurs in 40% of adults with new type 1 diabetes, often misclassified as type 2 diabetes 1
C-peptide testing is only indicated in insulin-treated patients for classification purposes, and in non-insulin-treated patients, clinical features and autoantibodies should guide diagnosis 1, 2, 4
Very low C-peptide levels (<80 pmol/L or <0.24 ng/mL) do not need repeat testing as they definitively indicate severe insulin deficiency 1, 2, 4