RA vs lupus
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"rheumatoid arthritis" AND "systemic lupus erythematosus" AND differential
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Rheumatoid Arthritis vs. Systemic Lupus Erythematosus
Both are systemic autoimmune diseases that predominantly affect women of reproductive age, but they differ significantly in pathophysiology, organ targets, serology, and management.
Epidemiology
| RA | SLE | |
|---|---|---|
| Prevalence | 1-2% of adults | ~1 in 1,000-2,500 (higher in Black/Latina women: up to 1 in 250) |
| Female:Male | 3:1 | 2:1 (pre-puberty) → 4:1 (post-puberty) |
| Peak onset | 20-50 years | 15-40 years (reproductive age) |
| Genetics | HLA-DR4 strongly linked | HLA-DR2, DR3, DR4, DR5 |
Pathophysiology
RA: Primarily a disease of the synovium. A genetic trigger (HLA-DR) initiates synovial lining cell and subsynovial vessel proliferation, forming a pannus. Leukocyte invasion follows, with a cascade of proteases and cytokines leading to joint destruction. Rheumatoid factor (RF) produces autoantibodies to the IgG Fc fragment, influenced by HLA-DR polymorphism.
SLE: A generalized autoimmune disease driven by antibodies against cell nucleus components (ANAs) - binding DNA, RNA, nuclear proteins, and protein-DNA/RNA complexes. Immune complex deposition causes inflammation and vasculitis across multiple organ systems. B-cell, T-cell, and monocyte abnormalities all contribute.
Clinical Features
Joints
| RA | SLE | |
|---|---|---|
| Pattern | Symmetric polyarthritis | Polyarthritis (often migratory, less symmetric) |
| Hallmark | Symmetric synovitis, morning stiffness >1 hour | Arthritis present, but typically non-erosive |
| Joint damage | Erosive - pannus destroys cartilage and bone | Non-erosive (Jaccoud arthropathy - reducible deformities) |
| Classic joints | PIPs, MCPs, wrists, MTPs ("spare DIPs") | Similar distribution but milder structural damage |
| Deformities | Swan-neck, Boutonniere, ulnar deviation, subluxation | Usually reducible, not fixed |
Skin
- RA: Rheumatoid nodules (subcutaneous, over pressure points); rarely skin vasculitis
- SLE: Malar (butterfly) rash across cheeks/nose (sparing nasolabial folds); discoid lupus (scarring plaques); subacute cutaneous LE; photosensitivity (>90% eventually have mucocutaneous involvement); oral ulcers
Systemic/Organ Involvement
| System | RA | SLE |
|---|---|---|
| Kidney | Rare (drug side effects more common) | Lupus nephritis - immune complex deposition; classified by WHO/ISN biopsy |
| CNS | Cervical spine instability (atlantoaxial subluxation), peripheral neuropathy | Neuropsychiatric SLE (seizures, psychosis, stroke, cognitive dysfunction) |
| Heart | Pericarditis, myocarditis (less common) | Pericarditis (most common cardiac manifestation), Libman-Sacks endocarditis, accelerated atherosclerosis |
| Lungs | ILD, pleuritis, nodules | Pleuritis/pleural effusion, ILD, pulmonary hypertension, "shrinking lung" |
| Blood | Anemia of chronic disease; Felty syndrome (splenomegaly + neutropenia) | Cytopenias (hemolytic anemia, leukopenia, lymphopenia, thrombocytopenia) - classic SLE criteria |
| Eyes | Keratoconjunctivitis sicca, scleritis, episcleritis | Sicca symptoms |
| Serositis | Occasional | Pleuritis + pericarditis very common (ACR criterion) |
| Antiphospholipid syndrome | Rare | Frequent - venous/arterial thrombosis, recurrent miscarriage |
Serology (Key Differentiator)
| Antibody | RA | SLE |
|---|---|---|
| ANA | ~30-40% positive | >95% positive (sensitive, not specific) |
| Anti-dsDNA | Negative | Highly specific for SLE; correlates with disease activity and nephritis |
| Anti-Smith (Sm) | Negative | Highly specific for SLE (less sensitive ~25%) |
| RF (IgM anti-IgG) | ~80% positive | ~30-40% positive (not specific) |
| Anti-CCP | Highly specific (~95%); correlates with erosive disease | Usually negative |
| Anti-Ro/SSA, Anti-La/SSB | Occasional (with secondary Sjögren) | Positive (~70% with photosensitivity) |
| Anti-histone | Negative | Positive in drug-induced lupus |
| Complement (C3/C4) | Normal or elevated | Low during flares (consumed by immune complexes) |
Diagnostic Criteria
RA - 2010 ACR/EULAR Criteria (score ≥6/10 = definite RA)
The traditional 1987 criteria require 4 of 7:
- Morning stiffness ≥1 hour
- Arthritis of ≥3 joint areas
- Arthritis of hand joints
- Symmetric arthritis
- Rheumatoid nodules
- Serum RF
- Radiographic erosions
Symptoms must be present ≥6 weeks.
SLE - ACR 11 Criteria (≥4 required, serially or simultaneously)
- Malar rash
- Discoid rash
- Photosensitivity
- Oral ulcers
- Non-scarring alopecia (SLICC adds this)
- Arthritis (≥2 joints)
- Serositis (pleuritis or pericarditis)
- Renal disorder (proteinuria >500 mg/day, or cellular casts)
- Neurologic disorder (seizures or psychosis)
- Hematologic disorder (hemolytic anemia, leukopenia <4,000, lymphopenia <1,000, thrombocytopenia <100,000)
- Immunologic disorder (anti-dsDNA, anti-Sm, antiphospholipid antibodies, or low complement)
- Positive ANA
5-year survival: 90%; >90% of treated patients survive 15+ years.
Overlapping Features (can cause diagnostic difficulty)
- Both cause symmetric polyarthritis with morning stiffness
- Both can be RF-positive
- Both can cause serositis, ILD, and secondary Sjögren syndrome
- Both are treated with hydroxychloroquine
- "Rhupus" - a genuine overlap syndrome exists (~10% of SLE patients have features meeting RA criteria)
- SLE arthritis is often confused for RA early in the course
Key clues favoring SLE over RA:
- Malar rash, photosensitivity, oral ulcers
- Renal involvement (proteinuria, casts)
- CNS symptoms (seizures, psychosis)
- Cytopenias (especially hemolytic anemia)
- Low complement during flares
- Positive anti-dsDNA or anti-Sm
- Non-erosive arthritis on X-ray
Key clues favoring RA over SLE:
- Erosive, destructive joint disease on X-ray
- High-titer RF + anti-CCP
- Rheumatoid nodules
- No multi-organ criteria for SLE met
Treatment Comparison
| RA | SLE | |
|---|---|---|
| First-line DMARD | Methotrexate (anchor drug) | Hydroxychloroquine (all patients) |
| Mild disease | HCQ, sulfasalazine, methotrexate | HCQ + NSAIDs/low-dose steroids |
| Moderate-severe | Methotrexate + biologics (anti-TNF, anti-IL-6, abatacept, rituximab) | Mycophenolate, azathioprine, belimumab (anti-BLyS) |
| Nephritis / severe | Not applicable (RA rarely causes nephritis) | High-dose steroids + cyclophosphamide or mycophenolate |
| Role of steroids | Short-term bridging; avoid long-term | More integral - 0.5-1 mg/kg/day for flares |
| Biologics | Anti-TNF agents dominant | Belimumab (anti-BLyS), voclosporin (for nephritis) |
Hydroxychloroquine is used in both diseases - it reduces flares, is cardioprotective in SLE, and is the least toxic DMARD for mild RA.
Sources: Textbook of Family Medicine 9e, pp. 879-902; Goldman-Cecil Medicine; Andrews' Diseases of the Skin.
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