Which lab tests are useful in the evaluation of suspected vasculitis?
- • Tests suggesting systemic inflammation:
Complete blood count: look for anemia of chronic disease and thrombocytosis. White blood cell count and differential to look for neutrophilia or eosinophilia. Pearl: Note that primary systemic vasculitis never causes pancytopenia (must rule out SLE, B-cell lymphoma, myeloma, leukemia).
Westergren erythrocyte sedimentation rate >100 mm/hour and C-reactive protein >10 mg/dL in the absence of bacterial infection and widespread cancer should suggest vasculitis.
Low albumin: this is a negative acute-phase reactant and decreases with systemic inflammation.
- • Tests suggesting organ involvement:
Creatinine and urinalysis.
Liver-associated enzymes: if extremely elevated, consider hepatitis B/C.
Creatine kinase.
Lumbar puncture with cerebrospinal fluid analysis: if CNS symptoms.
Stool for occult blood.
Chest radiograph.
Electromyography (EMG) and nerve conduction velocity (NCV): if neuropathy symptoms.
Brain magnetic resonance imaging or an abdominal computed tomography (CT) scan if symptoms suggest involvement.
- • Tests suggesting immune complex formation and/or deposition:
RF and ANA. Pearl: RF and ANA should not be positive in primary systemic vasculitis. If RF is positive, consider cryoglobulinemia and SBE. If ANA is positive, consider SLE or Sjögren’s syndrome.
Anti-C1q: if history of urticarial vasculitis.
Cryoglobulins: if positive, rule out hepatitis C.
Complement levels—C3/C4 are low in cryoglobulinemia, hypocomplementemic urticarial vasculitis, and SLE. Other vasculitides usually have normal values except PAN, where they are low in up to 25% of cases, and in some cases of hypersensitivity vasculitides.
- • Tests suggesting ANCA-related vasculitis:
c-ANCA: if against serine proteinase 3, usually GPA; sometimes MPA.
p-ANCA: if against myeloperoxidase, consider MPA and EGPA; sometimes GPA.
Eosinophil count/IgE level if suspect EGPA.
Pearl: Cocaine-associated vasculitis can be c-ANCA, p-ANCA, and/or atypical ANCA (antihuman neutrophil elastase)-positive.
- • Tests suggesting etiology:
Blood cultures: rule out SBE.
Infectious serologies: Hep BsAg (PAN), hepatitis C (cryoglobulinemia), parvovirus IgM (GPA, PAN), herpes (IgM and PCR), cytomegalovirus (IgM and PCR), Epstein–Barr virus (IgM and PCR), HIV (any vasculitis).
Antiglomerular basement membrane antibody: any patient with pulmonary–renal syndrome.
Serum protein electrophoresis: rule out myeloma.
Cerebrospinal fluid studies: herpes, varicella-zoster virus (VZV DNA and anti-VZV antibody).
Urinary toxicology screen: rule out cocaine use.
Not all of these tests are ordered for all patients. The physician must choose which test to order according to the clinical situation.
