What blood tests should be ordered to screen for inflammatory and arthritic conditions?

Blood Tests for Screening Inflammatory and Arthritic Conditions

The recommended blood tests for screening inflammatory and arthritic conditions should include erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), rheumatoid factor (RF), anti-citrullinated protein antibodies (ACPA), complete blood count (CBC), and antinuclear antibodies (ANA). 1, 2

Core Laboratory Tests

First-Line Tests

• Inflammatory Markers

  • ESR (Erythrocyte Sedimentation Rate) - Useful for monitoring chronic inflammatory conditions due to longer half-life 3
  • CRP (C-reactive Protein) - More responsive to acute changes in inflammation; better for diagnosis and monitoring response to therapy 1, 3

• Autoantibodies

  • Rheumatoid Factor (RF) - Predictive of RA diagnosis and prognosis 1
  • Anti-Citrullinated Protein Antibodies (ACPA) - Higher specificity for RA than RF 1, 2
  • Antinuclear Antibodies (ANA) - Screening for systemic autoimmune disorders 1

• Complete Blood Count (CBC)

  • Provides information on anemia of chronic disease, leukocytosis/leukopenia, and thrombocytosis/thrombocytopenia 4
  • CBC-derived inflammatory markers like neutrophil-to-lymphocyte ratio (NLR) can assess disease activity in RA 4

Second-Line Tests (Based on Clinical Suspicion)

• Metabolic Panel

  • Liver function tests, glucose, urate, and renal function 1
  • Particularly important before starting DMARDs 1

• Additional Autoantibodies

  • Antibodies to Extractable Nuclear Antigens (ENA) - If ANA is positive 1
  • Anti-double-stranded DNA (anti-dsDNA) - For suspected SLE 1

• HLA Typing

  • HLA-B27 - For suspected axial spondyloarthritis or reactive arthritis 1, 2
  • HLA-DR - For certain genetic associations with inflammatory arthritis 1

Test Interpretation and Clinical Application

Inflammatory Markers

• CRP levels should be categorized as:
  • Low risk: <1.0 mg/L
  • Average risk: 1.0-3.0 mg/L
  • High risk: >3.0 mg/L 1
• If CRP >10 mg/L, repeat test and examine for sources of infection or inflammation 1
• ESR and CRP should be performed at baseline and repeated when clinically relevant 1
• Normal ESR and CRP do not exclude inflammatory arthritis, as they are elevated in only about half of patients with psoriatic arthritis 5

Autoantibodies

• Negative RF and ACPA do not exclude progression to RA 1
• RF may be positive in 5-13% of patients with psoriatic arthritis 5
• Consider both sensitivity and specificity of tests - for example, HLA-B27 has 90% sensitivity for axial spondyloarthritis 2

Clinical Scenarios and Test Selection

For Undifferentiated Peripheral Inflammatory Arthritis

• Start with ESR, CRP, RF, ACPA, CBC, and ANA 1
• If symptoms persist, consider additional tests including HLA-B27 for spine involvement or suspected reactive arthritis 1

For Suspected Chronic Non-Bacterial Osteitis

• Include full blood count with differential, inflammatory markers, renal function, alkaline phosphatase, calcium, 25-hydroxy-vitamin D, parathyroid hormone, and phosphate 1
• Consider bone turnover markers, anti-CCP, RF, and HLA-B27 on a case-by-case basis 1

For Hand Osteoarthritis

• Blood tests are not required for diagnosis but may be needed to exclude coexistent inflammatory arthritis 1
• If marked inflammatory symptoms are present, especially at atypical sites, screen for additional inflammatory arthritides 1

Common Pitfalls to Avoid

• Overreliance on a single test - No single test is diagnostic; interpret in clinical context 1
• Ignoring discordance between ESR and CRP - Discrepancies are common and may be clinically significant 6
• Delayed testing - Early diagnosis and treatment improve outcomes 2
• Failure to repeat abnormal results - Confirm elevated inflammatory markers before making treatment decisions 1
• Misinterpreting normal inflammatory markers - Normal ESR/CRP doesn't exclude inflammatory arthritis 5

Monitoring Considerations

• For patients with inflammatory arthritis, monitor with serial examinations including inflammatory markers every 4-6 weeks after treatment initiation 1
• For patients on DMARDs, test for viral hepatitis B, C, and TB before treatment and repeat screening annually for patients on biologics 1

Remember that laboratory tests should always be interpreted in conjunction with clinical findings and imaging studies when appropriate for a comprehensive assessment of inflammatory and arthritic conditions.