How to Differentiate Between Types of Arthritis
Begin with a systematic clinical assessment focusing on six key factors: disease chronology (acute vs. chronic), presence of inflammation, joint distribution pattern, extra-articular manifestations, disease course, and patient demographics—this structured approach will guide you toward the correct diagnosis. 1, 2
Initial Clinical Assessment
Disease Chronology
- Acute onset (<6 weeks) suggests viral infection, crystal-induced arthritis (gout, pseudogout), or serum sickness 2, 3
- Chronic presentation (≥6 weeks) indicates rheumatoid arthritis, systemic lupus erythematosus, or other chronic inflammatory arthritides 2, 3
Inflammatory vs. Non-Inflammatory Features
- Inflammatory arthritis presents with prolonged morning stiffness (>30-60 minutes), palpable synovitis (joint warmth, redness, swelling), and improvement with activity 1, 2, 3
- Non-inflammatory arthritis (osteoarthritis) shows brief morning stiffness (<30 minutes), bony hypertrophy, crepitus on examination, and worsening with activity 4, 3
Critical pitfall: Do not rely solely on inflammatory markers (ESR/CRP) to distinguish inflammatory from non-inflammatory arthritis—up to 20-30% of rheumatoid arthritis patients have normal acute phase reactants even with active disease 5
Joint Distribution Pattern
- Rheumatoid arthritis: Symmetric involvement of small joints (MCPs, PIPs, wrists, MTPs); typically spares DIPs, first CMC joints, and first MTP joints 5, 6
- Osteoarthritis: DIPs, PIPs, first CMC joints, knees, hips; asymmetric pattern 4
- Psoriatic arthritis: DIPs commonly involved, asymmetric oligoarthritis, dactylitis ("sausage digits") 5
- Gout: First MTP joint (podagra) most common, but can affect any joint; often monoarticular initially 3
Targeted Physical Examination
Essential Joint Assessment
- Perform 28-joint count examination assessing PIPs, MCPs, wrists, elbows, shoulders, and knees for tenderness and swelling 5
- Squeeze test of MCPs and MTPs to detect clinical synovitis 5
- Palpate for synovial thickening (boggy, warm swelling) versus bony hypertrophy (hard, cool enlargement) 3
Extra-Articular Manifestations
- Skin examination: Look for psoriatic plaques, nail pitting (psoriatic arthritis), rheumatoid nodules (RA), tophi (gout), malar rash (SLE) 1, 5
- Axial/entheseal involvement: Assess for inflammatory back pain, heel pain, dactylitis (spondyloarthropathies) 1
- Systemic features: Fever, weight loss, fatigue, malaise may indicate systemic inflammatory disease or malignancy 2, 6
Laboratory Investigations
First-Line Testing
- ESR and CRP should be performed at baseline for diagnostic and prognostic purposes, but normal values do not exclude inflammatory arthritis 1, 5
- RF and/or ACPA testing is predictive of RA diagnosis and prognosis; however, negative tests do not exclude RA (20-30% are seronegative) 1, 5
- Complete blood count with differential to assess for cytopenias 5
- Comprehensive metabolic panel including liver function, renal function, glucose, and urate levels 5
- Urinalysis as part of standard workup 5
Additional Testing Based on Clinical Suspicion
- ANA and extractable nuclear antigens if connective tissue disease/systemic inflammatory disorder is suspected 1, 5
- HLA-B27 typing if spondyloarthropathy is being considered 1, 5
- Arthrocentesis with synovial fluid analysis when septic arthritis or crystal-induced arthritis is suspected, particularly with acute monoarticular or oligoarticular presentation 2
Critical pitfall: RF positivity occurs in ~15% of first-degree relatives of RA patients and in other conditions (hepatitis C, cryoglobulinemia, Sjögren's syndrome)—always interpret in clinical context 5
Imaging Studies
Initial Imaging
- Plain radiographs of affected joints should be performed at baseline 1, 2
- Bilateral hand, wrist, and foot X-rays should be obtained when evaluating for RA, as erosions predict RA diagnosis and disease persistence 1, 5
- Repeat X-rays within 1 year to monitor for radiographic progression 1
Advanced Imaging (When Indicated)
- Ultrasound is superior to clinical examination for detecting synovitis and structural damage; useful when patients do not meet classification criteria but clinical suspicion remains high 5
- MRI with IV contrast is more sensitive than ultrasound in early stages, detecting bone marrow edema (osteitis) which predicts future disease progression 5
- There is insufficient evidence to recommend routine use of MRI and US for diagnosis in undifferentiated arthritis, but consider in specific clinical settings 1
Applying the 2010 ACR/EULAR Classification Criteria for RA
Use this scoring system when at least one joint has definite clinical synovitis not better explained by another disease: 5, 7
Joint Involvement (0-5 points)
- 1 large joint = 0 points
- 2-10 large joints = 1 point
- 1-3 small joints = 2 points
- 4-10 small joints = 3 points
10 joints (at least 1 small joint) = 5 points 5
Serology (0-3 points)
- Negative RF and ACPA = 0 points
- Low positive RF or ACPA = 2 points
- High positive RF or ACPA = 3 points 5
Acute Phase Reactants (0-1 point)
- Normal CRP and ESR = 0 points
- Abnormal CRP or ESR = 1 point 5
Duration of Symptoms (0-1 point)
- <6 weeks = 0 points
- ≥6 weeks = 1 point 5
A score of ≥6/10 points indicates definite RA classification 5, 7
Predictors of Persistent Inflammatory Arthritis
Document these features as they predict chronicity: 1
- Disease duration ≥6 weeks
- Morning stiffness >30 minutes
- Functional impairment
- Involvement of small joints and/or knee
- Involvement of ≥3 joints
- ACPA and/or RF positivity
- Presence of radiographic erosion
Common Pitfalls to Avoid
Do not delay treatment waiting for positive serology—seronegative RA is common (20-30% of cases) and has similar prognosis to seropositive disease 5
Do not dismiss inflammatory arthritis based on normal ESR/CRP—acute phase reactants are poor predictors and can be normal even in active disease 5
Do not rely on a single clinical feature—use the systematic six-factor approach (chronology, inflammation, distribution, extra-articular features, course, demographics) to avoid misdiagnosis 1, 2, 3
Do not forget to exclude infectious causes—fever with joint pain requires urgent evaluation for septic arthritis until proven otherwise 2
Do not overlook the need for baseline imaging—erosions on initial X-rays significantly alter prognosis and treatment decisions 1, 5