Inflammatory Arthritis Differential: RA vs PsA vs SpA Clinical Features
Clinical Pattern Recognition
Rheumatoid arthritis (RA) classically presents with symmetric polyarthritis affecting the MCP, PIP, and wrist joints, with morning stiffness exceeding 60 minutes. Psoriatic arthritis (PsA) demonstrates five clinical patterns: asymmetric oligoarthritis (most common, 60-70%), symmetric polyarthritis (mimicking RA, 15-20%), distal interphalangeal predominant (5%), axial disease (5%), and arthritis mutilans (less than 5%). Key PsA features that distinguish it from RA include DIP joint involvement, dactylitis (present in 40-50% of PsA), enthesitis (Achilles, plantar fascia, lateral epicondyle), and nail dystrophy (pitting, onycholysis, oil drop sign), which are present in 80-90% of PsA patients with concomitant nail disease. Axial spondyloarthritis (axSpA) is characterized by inflammatory back pain (insidious onset before age 45, improvement with exercise, no relief with rest, nocturnal pain) present in over 75% of cases.
Serologic and Laboratory Differentiation
Rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPA/anti-CCP) are positive in 70-80% of RA patients, with anti-CCP having 95% specificity. In PsA, RF is negative in 85-90% and anti-CCP in 90-95%, though 5-10% positivity can cause diagnostic confusion. Elevated CRP and ESR are present in 40-60% of RA and SpA flares but may be normal in 50% of PsA patients with active disease. HLA-B27 positivity occurs in 6-8% of the general population, 50-70% of PsA patients with axial involvement, and 80-90% of ankylosing spondylitis. Ferritin, complement levels, and ANA help exclude other mimics such as SLE and adult-onset Still disease.
Imaging Distinctions
Radiographic features differ characteristically: RA shows periarticular osteopenia, uniform joint space narrowing, and marginal erosions at bare areas. PsA demonstrates a pathognomonic pattern of concurrent erosion and new bone formation (pencil-in-cup deformity), periostitis, asymmetric erosions, and DIP joint predilection. SpA imaging shows sacroiliitis (bilateral and symmetric in AS, often asymmetric in PsA) and syndesmophytes (marginal and flowing in AS versus paramarginal/nonmarginal in PsA). MRI with STIR sequences detects bone marrow edema at sacroiliac joints with sensitivity of 90% for active sacroiliitis, critical for diagnosing non-radiographic axSpA (nr-axSpA) before structural changes appear on conventional radiographs.
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Classification Criteria Comparison
The 2010 ACR/EULAR RA classification criteria use a scoring system (score of 6 or above out of 10) incorporating joint distribution, serology (RF and anti-CCP), acute-phase reactants, and symptom duration. The CASPAR criteria for PsA require inflammatory articular disease plus 3 or more points from: current psoriasis (2 points), personal history of psoriasis, family history of psoriasis, dactylitis, juxta-articular new bone formation, RF negativity, and nail dystrophy (1 point each). Sensitivity is 91.4% and specificity 98.7%. The ASAS criteria for axSpA require sacroiliitis on imaging (radiographic or MRI) plus one SpA feature, or HLA-B27 positivity plus two SpA features, with sensitivity of 82.9% and specificity of 84.4%.
Treatment Implications of Accurate Diagnosis
Accurate differentiation drives treatment selection. RA responds to methotrexate (15-25 mg weekly, first-line DMARD), while methotrexate has limited efficacy in axial SpA and enthesitis-predominant PsA. TNF inhibitors are effective across all three diseases. IL-17A inhibitors (secukinumab 150-300 mg, ixekizumab 80 mg monthly) are first-line biologic options for SpA and PsA but are ineffective in RA. IL-23 inhibitors (guselkumab, risankizumab) show efficacy in PsA but not SpA axial disease. JAK inhibitors (tofacitinib, upadacitinib) are approved across all three conditions with varying efficacy profiles. Misdiagnosis of PsA as RA leads to suboptimal treatment of entheseal and axial components, while misdiagnosis of SpA delays appropriate biologic initiation.
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