Differential Diagnosis of Psoriatic Arthritis

Diagnostic Considerations

The American College of Rheumatology suggests that rheumatoid factor (RF) and anti-CCP antibodies are mandatory for differentiating between psoriatic arthritis (PsA) and rheumatoid arthritis (RA), with their absence supporting PsA and high-titer positivity favoring RA 1, 2, 3, 4, 7
A thorough skin and nail examination is critical, as the presence of psoriatic plaques, nail pitting, or onycholysis establishes a diagnosis of PsA, while their absence with rheumatoid nodules confirms RA 7
The joint distribution pattern must be documented, with symmetric polyarticular involvement sparing DIP joints suggesting RA, and DIP involvement, dactylitis, or enthesitis being pathognomonic for PsA 1, 2, 7
Plain radiographs of hands and feet are essential, with proliferative marginal erosions and retained bone density indicating PsA, and non-proliferative marginal erosions with periarticular osteopenia indicating RA 1, 2, 8

Erosive hand osteoarthritis can present with elevated ESR and CRP, similar to inflammatory arthritis, and targets mainly IP joints with severe structural changes 1, 3
Gout may superimpose on pre-existing arthropathy and requires synovial fluid analysis for urate crystals 1, 3, 4
Hemochromatosis mainly targets MCPJs and wrists, and requires serum iron studies 1, 3
Calcium pyrophosphate deposition disease (CPPD) has radiographic changes similar to OA and may coexist with other arthropathies 1, 3

The European League Against Rheumatism recommends immediately checking RF and anti-CCP antibodies, performing a thorough skin and nail examination, documenting the joint pattern precisely, and obtaining plain radiographs of hands and feet 5, 6, 7
Synovial fluid analysis should be considered if monoarthritis or oligoarthritis is present to exclude crystal arthropathy or infection 5, 6

Early rheumatology referral is mandatory given the presence of significant inflammatory activity and diagnostic uncertainty 9, 7
Repeat inflammatory markers every 4-6 weeks to monitor disease activity regardless of final diagnosis 9
Screen for hepatitis B, C, and tuberculosis before initiating DMARDs if severe disease requires immunosuppression 9
Early DMARD therapy is critical for both RA and PsA to prevent irreversible joint damage within the first year 7