What are the initial blood tests to order when suspecting vasculitis?

Initial Blood Workup for Suspected Vasculitis

When suspecting vasculitis, the essential initial blood tests should include complete blood count, renal function tests, inflammatory markers (ESR, CRP), urinalysis with microscopic examination, and ANCA testing (both immunofluorescence and ELISA for PR3 and MPO antibodies). 1

Core Laboratory Tests for Initial Evaluation

First-Line Tests

• Complete blood count with differential

  • Identifies anemia, leukocytosis, eosinophilia (particularly important in EGPA)
  • Eosinophil count >1500 cells/μl or >10% is characteristic of active EGPA 2

• Inflammatory markers

  • Erythrocyte sedimentation rate (ESR)
  • C-reactive protein (CRP)
  • These correlate strongly with clinical activity of vasculitis 3

• ANCA testing

  • Indirect immunofluorescence (IIF) for c-ANCA and p-ANCA patterns
  • ELISA for specific antibodies:
    • Anti-proteinase 3 (PR3)
    • Anti-myeloperoxidase (MPO)
  • Combined IIF and ELISA techniques provide 99% specificity with 73% sensitivity for GPA 2

• Renal function assessment

  • Serum creatinine
  • Blood urea nitrogen (BUN)
  • Estimated glomerular filtration rate (eGFR)

• Urinalysis with microscopic examination

  • Check for proteinuria, hematuria, and red cell casts
  • Microscopic hematuria strongly correlates with renal involvement in vasculitis 3

Second-Line Tests

• Liver function tests
• Serum albumin (correlates with disease activity) 3
• Coagulation studies (fibrinogen, d-dimer)
• Complement levels (C3 and C4)
• 24-hour urine protein collection (if urinalysis shows proteinuria)

Disease-Specific Testing

For ANCA-Associated Vasculitis

• PR3-ANCA (c-ANCA) - highly specific for GPA (Wegener's)
• MPO-ANCA (p-ANCA) - associated with MPA and EGPA
• An ANCA test should be considered in any patient with suspicious clinical manifestations, particularly nasal crusting and bleeding, especially if they feel disproportionally unwell 2

For Other Vasculitis Types

• Hepatitis B and C serology - for secondary vasculitis
• Cryoglobulins - for cryoglobulinemic vasculitis
• Antinuclear antibody (ANA) - for lupus-associated vasculitis
• Anti-GBM antibodies - to rule out anti-GBM disease

Diagnostic Algorithm

1. Initial presentation with vasculitis symptoms

   • Order core laboratory tests (CBC, renal function, inflammatory markers, ANCA, urinalysis)

2. If ANCA positive with compatible clinical presentation

   • Do not delay treatment while waiting for biopsy results, especially in rapidly deteriorating patients 2
   • Proceed with treatment while arranging for confirmatory biopsy

3. If ANCA negative with compatible clinical presentation

   • Proceed with biopsy for definitive diagnosis
   • Consider additional serological testing based on clinical presentation

4. If rapidly progressive glomerulonephritis suspected

   • Urgent ANCA, anti-GBM antibodies, and ANA testing
   • Evaluate for extrarenal manifestations
   • Exclude infection before immunosuppression 2

Important Considerations

• The combination of immunofluorescence and ELISA testing for ANCA provides better diagnostic accuracy than either test alone 4

• ANCA levels may not correlate with disease severity but can be useful for monitoring disease activity and predicting relapse 3, 2

• Approximately 10% of patients with clinical features of small-vessel vasculitis are ANCA-negative, highlighting the importance of clinical assessment and biopsy in diagnosis 1

• Procalcitonin may be slightly elevated in active vasculitis without infection but high levels strongly suggest infectious complications 3

• Delays in diagnosis are common, especially with non-renal manifestations, and can negatively impact outcomes 5

• Patients with suspected vasculitis should be referred to centers with experience in vasculitis management for optimal care 2

Remember that while laboratory tests are crucial, they must be interpreted in the context of clinical presentation. A definitive diagnosis often requires tissue biopsy, but treatment should not be delayed in patients with high clinical suspicion and positive ANCA serology who are rapidly deteriorating.