## Distinguishing Type 1 from Type 2 Diabetes **Key Point:** Autoimmune markers (anti-GAD, anti-IA2, anti-ZnT8, anti-insulin antibodies) are the gold-standard discriminators between Type 1 and Type 2 diabetes. Their presence confirms autoimmune beta-cell destruction, pathognomonic for Type 1 DM. ### Comparative Features | Feature | Type 1 DM | Type 2 DM | |---------|-----------|----------| | **Autoimmune markers** | Present (anti-GAD, anti-IA2, anti-ZnT8) | Absent | | **Pathophysiology** | Autoimmune beta-cell destruction | Insulin resistance + relative insulin deficiency | | **Age of onset** | Usually <30 years | Usually >40 years (but increasing in youth) | | **BMI** | Normal or low | Elevated (>25 kg/m²) | | **Fasting glucose** | Often >200 mg/dL at diagnosis | Variable, may be <126 mg/dL initially | | **C-peptide** | Low/absent | Normal or elevated | | **Ketosis risk** | High (DKA common) | Low (HHS more common) | **High-Yield:** Anti-GAD antibodies are present in ~80% of Type 1 DM at diagnosis and persist lifelong. They are virtually never present in Type 2 DM, making them the single best discriminator. **Clinical Pearl:** In India, Type 1 DM often presents with acute symptoms (polyuria, polydipsia, weight loss, DKA) within weeks, whereas Type 2 DM is often discovered incidentally on screening or after months of asymptomatic hyperglycemia. **Warning:** Do NOT confuse elevated fasting glucose or insulin resistance with Type 1 DM — both occur in both types. Autoimmune markers are the only pathognomonic feature. ### Why Antibody Testing Matters Autoimmune markers allow: 1. Early identification of Type 1 DM before significant beta-cell loss 2. Prediction of Type 1 DM in at-risk relatives (screening in siblings) 3. Distinction from MODY (monogenic) or secondary diabetes 4. Prognostication: multiple antibodies = faster progression to insulin dependence [cite:Harrison 21e Ch 417]
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