Young Onset Diabetes

Source: SGH endocrine update 2026

C-peptide

• no fasting required
• within 5 hours after a meal
• paired with blood glucose
• do not test within 2/52 of hyperglycemic emergency
• best measured after glycemic control has improved

C-peptide Range	Equivalent (ng/mL)	Interpretation	Clinical Guidance / Action
< 80 pmol/L	< 0.24 ng/mL	Very low / Near-absolute insulin deficiency	Repeat testing generally not required
< 200 pmol/L	< 0.6 ng/mL	Severe insulin deficiency	If blood glucose < 4 mmol/L or patient was fasting, repeat the test
200 – 600 pmol/L	0.6 – 1.8 ng/mL	Indeterminate	If blood glucose < 4 mmol/L or patient was fasting, repeat the test
> 600 pmol/L	> 1.8 ng/mL	Substantial endogenous insulin production	Circumstances of testing generally do not matter

Pancreatic islet autoantibodies

Pancreatic islet autoantibodies are positive in up to 85% of Caucasian T1D, but only up to 40% in Asian populations

• Anti-GAD
• Anti-IA2
• Anti-ZnT8

GCK monogenic diabetes

• Second most common form of monogenic diabetes
• Mutation in the glucokinase (GCK) gene
• Defect in glucose sensing → higher glucose set point
• Monogenic diabetes is rare, around 1% of all DM
• No treatment required, unless pregnant