A 28-year-old woman presents with a 3-week history of polyuria, polydipsia, and weight loss. Fasting blood glucose is 245 mg/dL and random blood glucose is 312 mg/dL. She has no family history of diabetes. Which investigation is most appropriate to differentiate between Type 1 and Type 2 diabetes in this patient?

Explanation

## Differentiating Type 1 from Type 2 Diabetes **Key Point:** Serum C-peptide level is the gold standard investigation to differentiate Type 1 from Type 2 diabetes because it directly reflects endogenous pancreatic beta-cell function. ### Why C-peptide is Superior C-peptide is cleaved from proinsulin in a 1:1 molar ratio with insulin. Unlike insulin: - It is **not hepatically metabolized** (longer half-life: 30 min vs insulin 5 min) - It is **not affected by exogenous insulin therapy** - It provides a true measure of **endogenous insulin secretion** ### Interpretation in This Case | Finding | Type 1 Diabetes | Type 2 Diabetes | |---------|-----------------|----------------| | **C-peptide level** | Low or absent (<0.8 ng/mL) | Normal or elevated (>0.8 ng/mL) | | **Mechanism** | Autoimmune destruction of beta cells | Insulin resistance + relative beta-cell dysfunction | | **Onset** | Acute (weeks to months) | Insidious (years) | This 28-year-old woman with acute presentation, young age, and no family history suggests Type 1 diabetes. A **low C-peptide** would confirm autoimmune beta-cell destruction. **Clinical Pearl:** In Type 1 diabetes, C-peptide levels may be transiently detectable in the "honeymoon period" (first few weeks to months) due to residual beta-cell function, but they progressively decline. A low or absent C-peptide confirms Type 1 diabetes. **High-Yield:** C-peptide is also useful in: - Detecting **factitious hypoglycemia** (low C-peptide with low glucose = exogenous insulin) - Assessing **residual beta-cell function** in established diabetes - Monitoring **pancreatic transplant function** ### Why Other Tests Are Less Specific **Serum insulin level** — Affected by hepatic metabolism and cannot distinguish between endogenous and exogenous sources. **HbA1c** — Reflects glycemic control over 3 months; does not differentiate pathophysiology. **OGTT** — Useful for diagnosis and glucose tolerance classification, but does not directly assess beta-cell function.