A 28-year-old woman with SLE presents with photosensitive malar rash, arthritis, and positive ANA. Her serum is tested for anti-double-stranded DNA (anti-dsDNA) and anti-histone antibodies. Which finding best distinguishes drug-induced lupus from idiopathic SLE?

## Distinguishing Drug-Induced Lupus from Idiopathic SLE ### Key Serological Differences **Key Point:** Anti-histone antibodies are the hallmark of drug-induced lupus (DIL), occurring in >95% of cases. Anti-dsDNA antibodies are rare in DIL but common in idiopathic SLE, making anti-dsDNA presence a strong discriminator toward SLE. ### Comparison Table | Feature | Drug-Induced Lupus | Idiopathic SLE | | --- | --- | --- | | **Anti-histone antibodies** | Present (>95%) | Present (60–70%) | | **Anti-dsDNA antibodies** | Absent or rare (<5%) | Present (60–70%) | | **Anti-Sm antibodies** | Absent | Present (20–30%) | | **Low C3/C4** | Rare | Common (75%) | | **Renal involvement** | Rare | Common (50%) | | **Photosensitivity** | Uncommon | Common | | **Onset** | Weeks to months after drug | Variable, often insidious | ### Clinical Pearl **Clinical Pearl:** The presence of anti-dsDNA antibodies without anti-histone antibodies strongly favors idiopathic SLE. Conversely, isolated anti-histone positivity (without anti-dsDNA) in the setting of drug exposure (e.g., procainamide, hydralazine, minocycline) points to drug-induced lupus. The combination of both antibodies is seen in SLE but not DIL. ### High-Yield Mnemonic **Mnemonic:** **DISH** — **D**rug-**I**nduced lupus = **S**elected **H**istone antibodies (anti-histone without anti-dsDNA). ### Why This Matters Drug-induced lupus is typically reversible upon drug withdrawal, whereas idiopathic SLE is a chronic autoimmune condition. The serological profile guides both diagnosis and prognosis.