## Investigation of Choice for DRESS Syndrome ### Clinical Context DRESS syndrome (also called anticonvulsant hypersensitivity syndrome) is a severe delayed hypersensitivity reaction to aromatic antiepileptics (phenytoin, carbamazepine, phenobarbital). The patient presents with the classic triad: rash, fever, and lymphadenopathy. ### Why CBC with Differential and LFTs? **Key Point:** DRESS syndrome diagnosis relies on demonstrating **systemic involvement** — specifically atypical lymphocytosis, eosinophilia, and hepatic dysfunction — alongside the clinical features. **High-Yield:** The RegiSCAR scoring system (used to diagnose DRESS) mandates: - Atypical lymphocytes or lymphocytosis - Eosinophilia (>1500/μL or >11% of WBC) - Elevated liver enzymes (ALT, AST, bilirubin) - Thrombocytopenia (may occur) These haematological and biochemical findings are **pathognomonic** for DRESS and distinguish it from simple drug rash or viral exanthem. ### Why Not the Other Investigations? | Investigation | Why Not Ideal | |---|---| | **Skin biopsy** | Histology shows non-specific dermatitis; does not confirm systemic disease or DRESS diagnosis | | **Chest X-ray** | May show pulmonary involvement (interstitial pneumonitis) in advanced DRESS, but is not confirmatory; not first-line | | **Serum phenytoin levels** | Measures drug exposure, not the immune reaction; does not confirm DRESS | **Clinical Pearl:** DRESS typically manifests 2–8 weeks after drug initiation (this patient at 3 weeks is in the window). Immediate action: **stop phenytoin**, perform CBC + LFTs, and consider systemic corticosteroids if RegiSCAR score is high. **Mnemonic — DRESS Features:** **D**elayed (2–8 weeks), **R**ash (maculopapular), **E**osinophilia, **S**ystemic (fever, lymphadenopathy, hepatosplenomegaly), **S**evere (can progress to multi-organ failure). [cite:KD Tripathi 8e Ch 11]
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