What are the initial lab investigations for diagnosing vasculitis?

Initial Laboratory Investigations for Diagnosing Vasculitis

The initial laboratory investigations for diagnosing vasculitis should include complete blood count with differential, comprehensive metabolic panel, inflammatory markers (ESR, CRP), urinalysis with microscopic examination, ANCA testing (both MPO-ANCA and PR3-ANCA), and tissue biopsy of affected organs. 1

Core Laboratory Tests

First-Line Investigations:

• Complete blood count with differential
  • Evaluates for anemia, leukocytosis, eosinophilia (particularly in Churg-Strauss syndrome)
• Comprehensive metabolic panel
  • Assesses renal function (creatinine, BUN) and liver function
• Inflammatory markers
  • Erythrocyte sedimentation rate (ESR)
  • C-reactive protein (CRP)
  • Note: While elevated inflammatory markers are highly sensitive for vasculitis, they are nonspecific 2, 1
• Urinalysis with microscopic examination
  • Look for red cell casts, dysmorphic erythrocytes, proteinuria 2, 1
• ANCA testing
  • Both MPO-ANCA and PR3-ANCA should be ordered using high-quality antigen-specific assays 1
  • Important for diagnosing ANCA-associated vasculitis (Wegener's granulomatosis, microscopic polyangiitis, Churg-Strauss syndrome)
  • Note: A negative ANCA does not exclude vasculitis diagnosis 1

Additional Important Tests:

• Complement levels (C3, C4)
  • Particularly important for immune complex-associated vasculitis 1, 3
• Hepatitis B and C serology
  • Essential to rule out secondary causes of vasculitis 1, 4, 3
• Anti-GBM antibodies
  • Consider in pulmonary-renal syndrome 1, 3
• Cryoglobulins
  • Important in suspected cryoglobulinemic vasculitis 1
• ANA testing on HEp-2 cells
  • For suspected SLE-associated vasculitis 1
  • Follow with Crithidia luciliae test for anti-dsDNA if positive

Diagnostic Procedures

Biopsy:

• Tissue biopsy is the gold standard for diagnosis with a diagnostic yield of over 70% 1
• Target clinically affected tissue for highest diagnostic yield
• In giant cell arteritis, temporal artery biopsy should be performed (minimum 1 cm length) 2
• Note: Treatment should not be delayed while waiting for biopsy results, especially in rapidly deteriorating patients 1

Imaging Based on Vessel Size:

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

Special Considerations

• ANCA status is valuable but should not be the sole determinant for treatment decisions 1
• Renal function tests should be regularly monitored for kidney involvement 1
• Serial ANCA testing can be useful for monitoring disease activity and predicting relapse 1
• Aldolase levels may be elevated even when CK is normal in vasculitic myopathy 5

Common Pitfalls to Avoid

1. Delaying treatment while waiting for biopsy results in rapidly deteriorating patients
2. Excluding vasculitis diagnosis based solely on negative ANCA
3. Relying only on ESR and CRP which are nonspecific markers
4. Failing to consider secondary causes of vasculitis (infections, drugs, malignancies)
5. Not obtaining adequate biopsy sample - ensure sufficient length (at least 1 cm for temporal artery) 2

Remember that patients with suspected primary small and medium vessel vasculitis should be managed in collaboration with, or at centers of expertise, due to the complexity of diagnosis and management 1.