What are the criteria for diagnosing Rheumatoid Arthritis (RA)?

Diagnosis of Rheumatoid Arthritis

The diagnosis of RA requires at least one joint with definite clinical synovitis not better explained by another disease, followed by application of the 2010 ACR/EULAR Classification Criteria, which uses a score-based algorithm where ≥6/10 points confirms the diagnosis. 1

Clinical Presentation

Joint Involvement Pattern:

• Symmetric involvement of small joints is characteristic, particularly metacarpophalangeal (MCP) joints, proximal interphalangeal (PIP) joints, wrists, and metatarsophalangeal (MTP) joints 1, 2
• Large joints (shoulders, elbows, hips, knees, ankles) may also be affected 1
• Distal interphalangeal (DIP) joints, first carpometacarpal joints, and first metatarsophalangeal joints are typically spared 1
• Morning stiffness lasting ≥1 hour before maximal improvement is a typical sign 3

The 2010 ACR/EULAR Classification Criteria

Scoring System (Total Score ≥6/10 = Definite RA): 1

Joint Involvement:

• 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 1

Serology:

• Negative RF and ACPA = 0 points
• Low positive RF or ACPA = 2 points
• High positive RF or ACPA = 3 points 1

Acute Phase Reactants:

• Normal CRP and ESR = 0 points
• Abnormal CRP or ESR = 1 point 1

Duration of Symptoms:

• <6 weeks = 0 points
• ≥6 weeks = 1 point 1

Laboratory Assessment

Serologic Testing:

• Anti-citrullinated protein antibody (ACPA) has high specificity (90%) with moderate sensitivity (60%) 1
• Rheumatoid factor (RF) has lower specificity (70%) with similar sensitivity to ACPA 1
• C-reactive protein (CRP) is preferred over ESR as it is more reliable and not age-dependent 1

Important Caveat: No single laboratory test is diagnostic of RA; RF and ACPA can be found in other pathologies and healthy donors 2, 4

Imaging Studies

Initial Imaging:

• Plain radiographs are the initial imaging method, showing periarticular osteopenia, uniform joint space narrowing, and osseous erosions 1

Advanced Imaging (when diagnosis uncertain):

• Ultrasound is superior to clinical examination for detecting inflammation and structural damage, and can detect synovitis that predicts disease progression 5, 1
• MRI with IV contrast is more sensitive than ultrasound in early stages, detecting bone marrow edema (osteitis) which is the best single predictor of future disease progression and functional deterioration 5, 1
• Power Doppler ultrasound is useful when patients with early arthritis do not meet the 2010 ACR/EULAR criteria 5

Diagnostic Algorithm

1. Identify definite clinical synovitis in at least one joint not explained by another condition 1, 2
2. Apply the 2010 ACR/EULAR scoring system using joint involvement, serology, acute phase reactants, and symptom duration 1
3. If score ≥6/10, diagnose as definite RA 1
4. If score <6/10, consider advanced imaging (ultrasound or MRI) to detect subclinical synovitis 1
5. Follow-up evaluation as patients may develop additional features over time 1

Critical Differential Diagnoses

Must exclude: 5

• Psoriatic arthritis (may target DIP joints or single ray)
• Gout (may superimpose on pre-existing OA)
• Hemochromatosis (mainly targets MCP joints and wrists)
• Septic arthritis must always be ruled out in cases of joint effusion with pain, warmth, erythema, and fever through joint aspiration 6