What labs should be ordered for the diagnosis and management of vasculitis?

Laboratory Testing for Vasculitis Diagnosis and Management

For the diagnosis and management of vasculitis, a structured panel of laboratory tests should include complete blood count with differential, comprehensive metabolic panel, inflammatory markers (ESR, CRP), urinalysis with microscopic examination, and ANCA testing (both MPO-ANCA and PR3-ANCA) using high-quality antigen-specific assays as the primary diagnostic method. 1

Essential Laboratory Tests for Vasculitis

Initial Diagnostic Workup:

• ANCA Testing:

  • Perform both indirect immunofluorescence and ELISA testing 2
  • Test for both PR3-ANCA and MPO-ANCA using high-quality antigen-specific assays 1
  • ANCA testing has level of evidence 1A, grade of recommendation A 2

• Basic Laboratory Tests:

  • Complete blood count with differential
  • Comprehensive metabolic panel including renal function tests
  • Inflammatory markers: ESR and CRP
  • Urinalysis with microscopic examination (look for red cell casts, hematuria) 1
  • Complement levels (C3, C4)
  • Immunoglobulin levels 3

• Additional Serological Tests:

  • Hepatitis B and C serology (particularly important with liver involvement) 4, 3
  • Cryoglobulin testing 3
  • Anti-glomerular basement membrane (GBM) antibodies (for anti-GBM disease) 3
  • Anti-C1q antibodies (for immune complex-associated small-vessel vasculitis) 3

Tissue Biopsy

Biopsy remains the gold standard for diagnosis of vasculitis:

• Strongly recommended to assist in establishing a new diagnosis 2
• Diagnostic yield is over 70% when properly targeted 2, 1
• Target clinically affected tissue for highest diagnostic yield 1
• Biopsy should be performed whenever there is access to clinically affected tissue 4

Monitoring Disease Activity

Regular monitoring should include:

• Renal function tests
• Urinalysis with microscopic examination
• Inflammatory markers (ESR, CRP)
• ANCA levels (useful for monitoring disease activity in ANCA-associated vasculitis)
• Complete blood count with differential 1, 4

Imaging Studies

Select imaging based on suspected 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

Disease Assessment Tools

For comprehensive disease assessment:

• Birmingham Vasculitis Activity Score
• Disease Extent Index
• Vasculitis Damage Index
• Functional assessment (Short Form 36) 1

Common Pitfalls and Caveats

1. Do not delay treatment while waiting for biopsy results in ANCA-positive patients with compatible clinical presentation, especially in rapidly deteriorating patients 1

2. Elevated ESR and CRP are nonspecific and cannot distinguish between vasculitis disease activity and concomitant infection or another source of inflammation 3

3. Risk of bleeding following percutaneous renal biopsy is higher in patients treated with plasma exchange, in older patients, those with increased systolic blood pressure, and worse renal function 2

4. False negative biopsies can occur due to the segmental nature of vasculitic lesions; consider multiple biopsies if clinical suspicion is high 2

5. ANCA testing interpretation: While highly specific for certain vasculitides, ANCA results must be interpreted in the appropriate clinical context 2, 1

By following this structured approach to laboratory testing for vasculitis, clinicians can efficiently diagnose and monitor these complex conditions, leading to improved patient outcomes through timely and appropriate management.