What labs should be checked to diagnose rheumatoid arthritis?

Laboratory Tests for Diagnosing Rheumatoid Arthritis

For diagnosing rheumatoid arthritis, essential laboratory tests include rheumatoid factor (RF), anti-citrullinated protein antibodies (ACPA), erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP), along with complete blood count and basic metabolic panels. 1

Core Laboratory Tests

Serological Tests

• Rheumatoid Factor (RF):

  • Essential for diagnosis and prognosis
  • Positive in approximately 70-80% of RA patients
  • Can be negative in early disease or in "seronegative" RA 1, 2

• Anti-Citrullinated Protein Antibodies (ACPA):

  • Also known as anti-CCP (cyclic citrullinated peptide)
  • Higher specificity than RF
  • Strong predictor of RA diagnosis and worse prognosis
  • Presence increases probability of developing persistent synovitis 3, 1

Inflammatory Markers

• Erythrocyte Sedimentation Rate (ESR):

  • Measures inflammation level
  • Should be performed at baseline and repeated according to clinical need
  • May be normal in up to 40% of RA patients 3, 2

• C-Reactive Protein (CRP):

  • Acute phase reactant that rises with inflammation
  • More responsive to short-term changes than ESR
  • Important for monitoring disease activity 3, 1

Additional Basic Tests

• Complete Blood Count (CBC):

  • Evaluates for anemia (common in chronic RA)
  • Checks white blood cell and platelet counts
  • May reveal thrombocytosis in active disease 1, 4

• Liver and Kidney Function Tests:

  • Important baseline tests before starting medications
  • Helps monitor for medication toxicity during treatment 1

• Urinalysis:

  • Screens for renal involvement
  • Helps differentiate between types of inflammatory arthritis 1

Diagnostic Significance

According to the ACR/EULAR 2010 classification criteria, serology contributes significantly to diagnosis with points assigned as follows:

• Negative RF and negative ACPA: 0 points
• Low-positive RF or low-positive ACPA: 2 points
• High-positive RF or high-positive ACPA: 3 points 1

A score of ≥6/10 (considering joint involvement, serology, acute phase reactants, and symptom duration) is required for RA diagnosis.

Important Considerations

• Test Limitations: More than 30% of RA patients may have negative RF or ACPA, and 40% may have normal ESR or CRP 2
• Early Testing: Early diagnosis allows for earlier treatment with disease-modifying agents, improving outcomes 4, 5
• Pre-Treatment Testing: Patients who will be starting biologic agents should be tested for hepatitis B, hepatitis C, and tuberculosis 4
• Monitoring: After diagnosis, ESR and CRP should be repeated every 1-3 months until remission is achieved 1

Additional Tests When Indicated

• HLA-B27 Testing: Consider if symptoms are suggestive of reactive arthritis or affect the spine 3
• Autoimmune Panel: Consider additional autoantibody tests if non-RA systemic inflammatory disorders are suspected 3
• Viral Hepatitis Screening: Test for hepatitis B and C before starting disease-modifying antirheumatic drug (DMARD) treatment 3
• Tuberculosis Testing: Screen for latent or active TB before DMARD treatment 3

Remember that laboratory tests should be interpreted in conjunction with clinical findings and imaging studies for a comprehensive diagnosis of rheumatoid arthritis.