What tests do you order for diagnosing rheumatoid arthritis?

Diagnostic Tests for Rheumatoid Arthritis

For diagnosing rheumatoid arthritis, you should order rheumatoid factor (RF), anti-citrullinated protein antibodies (ACPA), C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), complete blood count, and baseline radiographs of affected joints. 1

Core Laboratory Tests

The American College of Rheumatology (ACR) and European League Against Rheumatism (EULAR) recommend the following essential laboratory tests:

1. Serological markers:

   • Rheumatoid factor (RF) - sensitivity 91.0%, specificity 74.4% 2
   • Anti-citrullinated protein antibodies (ACPA) - sensitivity 88.0%, specificity 90.4% 2
   • Combined RF and ACPA testing improves diagnostic accuracy to 89.5% 2

2. Inflammatory markers:

   • C-reactive protein (CRP) - more responsive to short-term changes 1
   • Erythrocyte sedimentation rate (ESR) - may be normal in up to 40% of RA patients 1, 3

3. Additional baseline tests:

   • Complete blood count with differential
   • Liver and kidney function tests
   • Urinalysis 1

Imaging Studies

• Baseline radiographs of affected joints (especially hands, wrists, and feet) are recommended even if not clinically affected 1
• Ultrasound or MRI should be considered when clinical examination is inconclusive 1

Diagnostic Algorithm

1. Step 1: Order the core laboratory panel (RF, ACPA, CRP, ESR, CBC, liver/kidney function)
2. Step 2: Order baseline radiographs of affected joints plus hands, wrists, and feet
3. Step 3: Apply the ACR/EULAR 2010 classification criteria which requires a score ≥6/10 based on:
   • Joint involvement (0-5 points)
   • Serology (0-3 points)
   • Acute phase reactants (0-1 point)
   • Duration of symptoms (0-1 point) 1

Special Considerations

• Before starting DMARD therapy, additional tests should include:

  • Hepatitis B and C screening
  • Tuberculosis testing 1

• For differential diagnosis consider:

  • HLA-B27 testing if symptoms suggest reactive arthritis or spinal involvement 1

Important Caveats

• Despite their utility, more than 30% of RA patients may have negative RF or ACPA, and 40% may have normal ESR or CRP 3
• ACPA has higher specificity than RF for RA diagnosis 1, 2
• Early diagnosis allows for earlier treatment with disease-modifying agents, improving outcomes 4
• Patients consider CRP most helpful in understanding their disease activity 5

Remember that diagnosis requires at least one joint with definite swelling not explained by another disease, with likelihood increasing with the number of small joints involved 4.