A 32-year-old woman with systemic lupus erythematosus is started on procainamide for atrial fibrillation. After 6 weeks of therapy, she develops fever, arthralgia, and a positive antinuclear antibody (ANA) titer that was previously negative. Her procainamide level is therapeutic. Which classification of adverse drug reaction best describes this presentation?

## Classification of Adverse Drug Reactions **Key Point:** This patient demonstrates a **Type B (Bizarre) reaction** — an idiosyncratic, unpredictable adverse effect that is not dose-dependent and occurs in genetically susceptible individuals. ### Why Type B? The clinical features point to **drug-induced lupus erythematosus (DILE)** from procainamide: 1. **Unpredictable occurrence** — not related to procainamide dose or serum level 2. **Genetic predisposition** — associated with slow acetylator phenotype (deficiency in N-acetyltransferase 2) 3. **Immunological mechanism** — autoimmune response (positive ANA conversion, fever, arthralgia) 4. **Idiosyncratic nature** — occurs in a minority of exposed individuals; most patients tolerate procainamide without issue ### Type B Reaction Characteristics | Feature | Type B | | --- | --- | | **Dose-dependent** | No | | **Predictable** | No | | **Incidence** | Rare (1–5% of exposed) | | **Mechanism** | Immunological, metabolic, genetic | | **Examples** | DILE, Stevens-Johnson syndrome, aplastic anemia from chloramphenicol | | **Onset** | Variable; may be weeks to months | **Clinical Pearl:** Procainamide-induced lupus is more common in **slow acetylators** (genetic polymorphism in NAT2 enzyme). The syndrome is reversible upon drug discontinuation, unlike idiopathic SLE. **High-Yield:** Type B reactions are NOT prevented by dose reduction — the only management is **drug withdrawal** and supportive care. ### Contrast with Other Types - **Type A (Augmented):** Dose-dependent exaggeration of the drug's known pharmacological effect (e.g., hypoglycemia from overdose of insulin). This patient's reaction is unrelated to procainamide's antiarrhythmic action. - **Type C (Chronic):** Long-term cumulative toxicity (e.g., pulmonary fibrosis from bleomycin). DILE onset is acute/subacute, not chronic cumulative. - **Type D (Delayed):** Teratogenicity or carcinogenicity with latency of years to decades. This patient's reaction occurs within weeks. [cite:KD Tripathi 8e Ch 7]