What laboratory tests are used for the diagnosis and management of vasculitis?

Laboratory Tests for Diagnosis and Management of Vasculitis

The diagnosis of vasculitis requires a comprehensive panel of laboratory tests including complete blood count with differential, comprehensive metabolic panel, inflammatory markers (ESR, CRP), urinalysis with microscopic examination, ANCA testing (both MPO-ANCA and PR3-ANCA using high-quality antigen-specific assays), and tissue biopsy of affected organs when possible. 1

Initial Diagnostic Workup

Core Laboratory Tests

• ANCA Testing:

  • PR3-ANCA and MPO-ANCA using high-quality antigen-specific assays (ELISA) 1
  • Both indirect immunofluorescence (IIF) and ELISA testing are recommended 1
  • A positive c-ANCA test and proteinase-3 (PR3) will confirm GPA in up to 95% of patients with active systemic disease 2
  • p-ANCA is typically associated with MPO and EGPA, though ~5% of GPA patients have a positive p-ANCA/MPO 2

• Inflammatory Markers:

  • Erythrocyte sedimentation rate (ESR)
  • C-reactive protein (CRP)
  • These are elevated in active vasculitis but are nonspecific 1, 3

• Complete Blood Count:

  • With differential to assess for anemia, leukocytosis, eosinophilia (particularly in EGPA)
  • Neutrophil-to-lymphocyte ratio (NLR) may be elevated in vasculitis 4

• Renal Function Tests:

  • Serum creatinine
  • Blood urea nitrogen (BUN)
  • Urinalysis with microscopic examination for proteinuria, hematuria, and red cell casts 2, 1

• Complement Studies:

  • C3 and C4 levels (particularly useful in immune complex vasculitis) 5

• Immunoglobulin Levels:

  • Particularly important in IgA vasculitis (Henoch-Schönlein purpura) 6

Additional Specific Tests

• Hepatitis Serology:

  • Hepatitis B and C testing (particularly important in polyarteritis nodosa and cryoglobulinemic vasculitis) 5, 3

• Cryoglobulins:

  • Important for diagnosing cryoglobulinemic vasculitis 3

• Anti-GBM Antibodies:

  • For anti-GBM antibody disease (Goodpasture's syndrome) 3

• Anti-C1q Antibodies:

  • Useful in immune complex-associated small-vessel vasculitis 3

Tissue Diagnosis

• Biopsy:
  • Strongly recommended to establish a new diagnosis of vasculitis 1
  • Diagnostic yield exceeds 70% when properly targeted 1
  • Should extend to the subcutis from the most tender, reddish, or purpuric lesional skin 6
  • Direct immunofluorescence helps distinguish IgA-associated vasculitis from IgG/IgM-associated forms 6

Disease Assessment and Monitoring

• Disease Activity Measurement:

  • Birmingham Vasculitis Activity Score (BVAS) 2, 1
  • Disease Extent Index 1
  • Vasculitis Damage Index (VDI) 2, 1
  • ELK-classification for GPA (ENT, lung, kidney involvement) 2

• Functional Assessment:

  • Short Form 36 (SF-36) for quality of life evaluation 2, 1

Imaging Studies

Select imaging based on suspected vessel size:

• Large Vessel Vasculitis: CT/CTA, MRI/MRA, or FDG-PET/CT
• Medium Vessel Vasculitis: CTA of affected regions (renal, mesenteric, coronary)
• Small Vessel Vasculitis: MRI/MRA or FDG-PET/CT 1

Monitoring During Treatment

• Regular monitoring should include:
  • Renal function tests
  • Urinalysis with microscopic examination
  • Inflammatory markers (ESR, CRP)
  • ANCA levels (can be useful for predicting relapse in some patients) 2, 1
  • Complete blood count with differential 1

Important Clinical Considerations

• ANCA testing should be considered in any patient with suspicious clinical manifestations, particularly nasal crusting and bleeding, especially if they feel disproportionally unwell 2
• Treatment should not be delayed while waiting for biopsy results in ANCA-positive patients with compatible clinical presentation, especially in rapidly deteriorating patients 1
• A negative ANCA does not exclude vasculitis, particularly in limited forms of disease or following corticosteroid therapy 2
• Serial measurements of acute phase reactants are useful but cannot distinguish between disease activity and concomitant infection 3

Pitfalls to Avoid

• Relying solely on ANCA testing without clinical correlation or tissue diagnosis when possible
• Delaying treatment in rapidly deteriorating patients while waiting for biopsy results
• Failing to distinguish cocaine-induced midline destruction from GPA (both can have positive c-ANCA and PR3) 2
• Overlooking the need for comprehensive monitoring during treatment and follow-up
• Using ESR and CRP alone to guide treatment decisions, as they are nonspecific markers of inflammation 3