Bit · Rheum/Immuno
SLE vs Drug-induced lupus vs MCTD
Three overlapping autoimmune syndromes that share rash, arthralgias, and fatigue. The pivot is the specific autoantibody.
Mechanism
All three are systemic autoimmune diseases on a connective-tissue-disease spectrum. They share many features but have distinct antibody fingerprints:
- SLE — multisystem autoimmunity (malar rash, photosensitivity, arthritis, serositis, renal disease, neuropsychiatric, hematologic cytopenias). ANA nearly always positive (sensitive, not specific). The specific antibodies are anti-dsDNA (correlates with disease activity and lupus nephritis) and anti-Smith (anti-Sm) — both highly specific. Low C3/C4 during flares.
- Drug-induced lupus (DIL) — lupus-like syndrome triggered by certain drugs: procainamide, hydralazine, isoniazid, minocycline, anti-TNF agents. Arthritis and serositis predominate; rash and renal involvement uncommon. Resolves on drug withdrawal. Antibody: anti-histone antibodies (>95%); anti-dsDNA usually negative.
- Mixed connective tissue disease (MCTD) — overlap of SLE + scleroderma + polymyositis features. Raynaud phenomenon, puffy hands, arthralgias, myositis, pulmonary hypertension. Antibody: high-titer anti-U1 RNP (defining feature); anti-Sm and anti-dsDNA usually negative.
Differentiator Table
| SLE | Drug-induced lupus | MCTD | |
| Most specific antibody | Anti-dsDNA, anti-Smith | Anti-histone | Anti-U1 RNP (high titer) |
| ANA | Almost always positive | Positive | Positive (high titer) |
| Renal involvement | Common — lupus nephritis | Rare | Less common |
| CNS involvement | Can occur | Rare | Rare |
| Rash | Malar, photosensitive, discoid | Less prominent | Variable |
| Raynaud | Sometimes | Rare | PROMINENT — often presenting |
| Pulmonary HTN | Sometimes | Rare | Common — major cause of mortality |
| Sclerodactyly / puffy hands | Less common | No | Yes |
| Complement (C3, C4) | ↓ in active disease | Normal | Normal |
| Trigger / setting | Spontaneous, young women, African ancestry overrepresented | Specific drug exposure (procainamide, hydralazine, INH, minocycline, anti-TNF) | Spontaneous |
| Resolution | Chronic, lifelong management | Resolves on stopping the drug | Chronic |
The Pivot
The antibody almost always answers it:
- Anti-dsDNA or anti-Smith positive? → SLE.
- Anti-histone positive, on a known offending drug? → Drug-induced lupus. (Trial off-drug → symptoms resolve.)
- High-titer anti-U1 RNP with Raynaud, puffy hands, pulmonary HTN? → MCTD.
Common drugs that cause DIL — mnemonic SHIPP-M: Sulfa drugs, Hydralazine, INH, Procainamide, Phenytoin, Minocycline (also anti-TNF agents).
NBME-Style Stem
A 64-year-old man on long-term procainamide for atrial arrhythmia develops fevers, polyarthritis of the hands and wrists, and a pleural effusion. ANA is positive at 1:1280. Anti-dsDNA is negative. Anti-Smith is negative. Anti-histone antibody is strongly positive. Which of the following is the most likely diagnosis?
Concept Anchor
All three diseases ring the ANA bell. The specific antibody decides which: dsDNA/Smith for SLE, histone for drug-induced, U1 RNP for MCTD — and the drug history or Raynaud/sclerodactyly tilts the clinical context.