Laboratory Tests for Diagnosing Polyarthralgia

Initial Laboratory Evaluation

• The European League Against Rheumatism recommends Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP) testing at baseline for diagnosis and prognosis of undifferentiated peripheral inflammatory arthritis, with repeat testing when clinically indicated 1, 2
• The European League Against Rheumatism suggests Rheumatoid Factor (RF) and Anti-Citrullinated Protein Antibodies (ACPA) testing as these factors are predictive of rheumatoid arthritis diagnosis and prognosis, though negative tests do not exclude progression to RA 1, 2

Additional Testing Based on Clinical Suspicion

• Additional autoantibody tests, such as Antinuclear antibody (ANA) for systemic lupus erythematosus and other connective tissue diseases, should be considered if a connective tissue disease or systemic inflammatory disorder is suspected 1, 2, 3
• HLA-B27 testing may be helpful in specific clinical settings, particularly when spondyloarthropathies are suspected 1, 2, 3

Synovial Fluid Analysis

• Joint aspiration and synovial fluid analysis should be performed when there is joint effusion, particularly to rule out septic arthritis (cell count, Gram stain, culture) 3

Imaging to Complement Laboratory Testing

• X-rays of affected joints should be performed at baseline, with particular attention to hands, wrists, and feet as the presence of erosions is predictive for the development of RA and persistence of disease 1, 2, 4
• MRI of hands and wrists could be considered for diagnosis if there is suspicion of RA, as it can detect early inflammatory changes not visible on conventional radiographs 2, 5

Clinical Predictors to Guide Laboratory Testing

• Disease duration ≥6 weeks is a predictor of persistent inflammatory arthritis 3
• Morning stiffness >30 minutes is a predictor of persistent inflammatory arthritis 3
• Involvement of small joints and/or knee is a predictor of persistent inflammatory arthritis 1
• Involvement of ≥3 joints is a predictor of persistent inflammatory arthritis 1
• Functional impairment is a predictor of persistent inflammatory arthritis 1, 2, 3

Pitfalls to Avoid

• Overreliance on negative RF or ACPA results should be avoided, as these can be negative in early RA 1, 2
• Neglecting to repeat laboratory tests when clinically indicated should be avoided, as serological markers may evolve over time 1, 2