Classification of Adverse Drug Reactions
This clinical presentation — fever, rash, eosinophilia, and hepatitis appearing 2 weeks after drug initiation — is a classic Type B (Bizarre/Idiosyncratic) reaction, specifically a Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) or Drug-Induced Hypersensitivity Syndrome (DIHS).
Type B Reactions: Key Features
Key Point
Type B reactions are unpredictable, dose-independent, and unrelated to the drug's pharmacological action. They occur in genetically predisposed individuals and involve immune mechanisms.
| Feature | Type A | Type B |
|---|
| Dose-dependent | Yes | No |
| Predictable | Yes | No |
| Frequency | Common (80% of ADRs) | Uncommon (15–20%) |
| Mechanism | Exaggerated pharmacology | Immune/genetic |
| Onset | Early (dose-related) | Delayed (2–8 weeks) |
| Examples | Bleeding on warfarin, hypoglycemia on insulin | DRESS, Stevens-Johnson syndrome, anaphylaxis |
Why This Is Type B
- 1.
Unpredictable onset — occurred 2 weeks after initiation, not immediately
- 2.
Immune-mediated features — fever, rash, eosinophilia, hepatitis (systemic involvement)
- 3.
Dose-independent — the reaction severity is not proportional to drug dose
- 4.
Genetic predisposition — certain HLA alleles increase risk (e.g., HLA-A*3101 with carbamazepine)
High-YieldNEET PG
DRESS syndrome is a Type B reaction characterized by the triad of fever + rash + internal organ involvement (liver, kidney, lymph nodes), often with eosinophilia and atypical lymphocytosis. It is a medical emergency requiring immediate drug withdrawal and often systemic corticosteroids.
Clinical Pearl
Type B reactions include anaphylaxis, Stevens-Johnson syndrome (SJS), Toxic Epidermal Necrolysis (TEN), DRESS, and serum sickness-like reactions. These are the most serious ADRs and require immediate cessation of the offending drug.
