What lab tests are used for arthritis diagnosis?

Laboratory Tests for Arthritis Diagnosis

For suspected inflammatory arthritis, order a core panel including ESR or CRP (CRP preferred), rheumatoid factor (RF), anti-citrullinated protein antibodies (ACPA/anti-CCP), complete blood count, comprehensive metabolic panel (liver and renal function), and urinalysis at the initial evaluation. 1, 2

Essential First-Line Laboratory Tests

Inflammatory Markers

• C-reactive protein (CRP) is preferred over ESR because it is more reliable, not age-dependent, and provides better measurement of the acute phase response 1, 3, 4
• Erythrocyte sedimentation rate (ESR) should be measured at baseline for both diagnostic and prognostic purposes, though it can be affected by non-inflammatory factors like immunoglobulins and anemia 1, 2, 3
• Both CRP and ESR provide diagnostic and prognostic information, but 40% of patients with rheumatoid arthritis have normal values, so normal results do not exclude inflammatory arthritis 5, 6

Autoantibody Testing

• Rheumatoid factor (RF) has 70% specificity and moderate sensitivity for rheumatoid arthritis and provides important prognostic information 1, 7, 6
• Anti-CCP antibodies have higher specificity (90%) than RF with 60% sensitivity and are essential for diagnosis and prognosis 1, 2, 7, 6
• When both RF and ACPA are positive, the specificity for rheumatoid arthritis diagnosis increases significantly 1
• High-positive RF or ACPA carries more diagnostic weight than low-positive results in classification criteria 1

Baseline Laboratory Panel

• Complete blood count (CBC) with differential to assess for anemia of chronic disease, cytopenias, and rule out other conditions 1, 2
• Comprehensive metabolic panel including liver function tests, renal function, glucose, and urate levels for baseline assessment 1, 2
• Urinalysis to evaluate for kidney involvement in systemic diseases 1, 2

Conditional Testing Based on Clinical Presentation

When to Add Additional Autoantibodies

• Antinuclear antibodies (ANA) should be tested if connective tissue disease is suspected or the diagnosis remains uncertain after initial workup 8, 1, 2
• Antibodies to extractable nuclear antigens and anti-double-stranded DNA if systemic lupus erythematosus is suspected 1
• Thyroid function tests to rule out thyroid disease presenting with joint symptoms 1

HLA-B27 Testing

• Order HLA-B27 if symptoms suggest reactive arthritis, affect the spine, or spondyloarthropathy is suspected 8, 1, 2
• Consider HLA-B27 testing when inflammatory arthritis involves axial skeleton or has asymmetric large joint involvement 8, 1

Imaging Studies as Part of Initial Workup

• Plain radiographs of affected joints should be performed at baseline to assess for erosions and joint damage 8, 1, 2
• X-rays of hands, wrists, and feet should be obtained even if not clinically involved, as erosions predict rheumatoid arthritis diagnosis and disease persistence 1, 7
• Follow-up X-rays should be repeated within 1 year if symptoms persist 1
• Ultrasound with Power Doppler is superior to clinical examination for detecting subclinical synovitis when diagnosis is uncertain 1, 7

Monitoring After Diagnosis

• Repeat ESR and CRP every 4-6 weeks after treatment initiation to monitor disease activity and treatment response 8, 1, 2
• Serial rheumatologic examinations including inflammatory markers guide treatment decisions 8, 1
• CBC and biochemistry should be monitored regularly to assess for medication side effects 1

Critical Pitfalls to Avoid

• Do not dismiss inflammatory arthritis based solely on normal ESR/CRP, as 40% of rheumatoid arthritis patients have normal acute phase reactants even with active disease 7, 5, 6
• Do not delay treatment waiting for positive serology, as seronegative rheumatoid arthritis accounts for 20-30% of cases and has similar prognosis 7, 5
• Do not rely solely on laboratory tests without thorough clinical examination, as clinical synovitis is the cornerstone of diagnosis 1, 2, 7
• Peripheral leukocyte counts are not elevated in most patients with infected prostheses, so normal WBC does not exclude infection in post-arthroplasty patients 8
• Morning stiffness lasting >30 minutes to 1 hour is more specific for inflammatory arthritis than laboratory values alone 8, 9, 6

Pre-Treatment Screening

• Before starting biologic agents, screen for hepatitis B, hepatitis C, and tuberculosis 7, 6
• This screening is mandatory before initiating any biologic therapy 7, 6