A 28-year-old woman with newly diagnosed type 1 diabetes mellitus presents with severe hyperglycemia (450 mg/dL), metabolic acidosis (pH 7.15, HCO₃⁻ 12 mEq/L), and ketonemia. Her serum insulin level is < 2 mIU/mL. To assess the degree of pancreatic beta-cell dysfunction and predict long-term insulin requirement, which investigation is most appropriate?

## Clinical Context: Type 1 Diabetes at Presentation The patient presents with **diabetic ketoacidosis (DKA)** — the hallmark presentation of type 1 diabetes. The question asks for an investigation to **assess pancreatic beta-cell dysfunction** and **predict long-term insulin requirement**. ## Why Fasting C-Peptide Is Correct **Key Point:** **C-peptide is the most accurate marker of endogenous insulin secretion** because it is produced in equimolar amounts with insulin from the proinsulin molecule and is not affected by exogenous insulin therapy. ### C-Peptide as a Biomarker of Beta-Cell Function 1. **Equimolar secretion with insulin** - Proinsulin → Insulin + C-peptide (1:1 ratio) - C-peptide reflects true endogenous beta-cell output 2. **Not degraded by first-pass hepatic metabolism** - Insulin is rapidly cleared by the liver - C-peptide has a longer half-life (~30 minutes) and reaches systemic circulation reliably - More stable and reproducible measurement 3. **Prognostic value in type 1 diabetes** - **Fasting C-peptide > 0.6 ng/mL** = preserved beta-cell function ("honeymoon phase") - **C-peptide < 0.2 ng/mL** = severe beta-cell destruction, complete insulin dependence - Predicts long-term insulin requirement and risk of complications **High-Yield:** In type 1 diabetes, **C-peptide levels at diagnosis correlate with residual beta-cell mass** and are used to stratify patients for clinical trials and prognostic counseling. ### Mnemonic: **C-PEPTIDE TRUTH** - **C** = Connects insulin to beta-cell function - **P** = Produced equimolar with insulin - **E** = Endogenous marker (not affected by exogenous insulin) - **P** = Prognostic (predicts long-term insulin need) - **T** = True reflection of beta-cell reserve - **I** = Independent of hepatic clearance - **D** = Diagnostic and predictive in type 1 DM - **E** = Essential for assessing residual function ## Investigation Comparison Table | Investigation | Measures | Diagnostic Value in Type 1 DM | |---|---|---| | **Fasting C-peptide** | Endogenous insulin secretion | ✓ Gold standard for beta-cell assessment; predicts long-term insulin requirement | | **HbA1c** | Average glycemic control over 3 months | Useful for monitoring, not for assessing beta-cell function at diagnosis | | **Serum glucagon** | Alpha-cell function | Elevated in DKA but does not assess beta-cell dysfunction | | **Urinary ketones** | Ketonuria (metabolic state) | Confirms DKA but does not assess beta-cell reserve | ## Clinical Significance of C-Peptide Levels ```mermaid flowchart TD A[Type 1 DM at Diagnosis]:::outcome --> B[Measure Fasting C-Peptide]:::action B --> C{C-Peptide Level?}:::decision C -->|> 0.6 ng/mL| D[Preserved beta-cell function<br/>Honeymoon phase<br/>May need less insulin initially]:::outcome C -->|0.2-0.6 ng/mL| E[Partial beta-cell dysfunction<br/>Moderate insulin requirement]:::outcome C -->|< 0.2 ng/mL| F[Severe beta-cell destruction<br/>Complete insulin dependence<br/>Higher complication risk]:::urgent ``` **Clinical Pearl:** Patients with **higher C-peptide levels at diagnosis** have better long-term metabolic control and lower rates of microvascular complications, even after adjusting for glycemic control. [cite:Harrison 21e Ch 417]