Indications, Testing, Interpretation, and Management of ANCA in Suspected Small‑Vessel Vasculitis

Indications for ANCA Testing

• Order an ANCA panel using high‑quality PR3‑ANCA and MPO‑ANCA antigen‑specific immunoassays as the primary screening method when clinical features suggest small‑vessel vasculitis, and plan to confirm positive results with tissue biopsy while initiating therapy based on clinical presentation, ANCA serotype, and organ involvement. 1
• Test for ANCA in patients with rapidly progressive glomerulonephritis manifested by red‑blood‑cell casts in urine. 1
• Test for ANCA in patients with mononeuritis multiplex confirmed by electrophysiological studies. 1

Testing Methodology

• Use antigen‑specific immunoassays for both PR3‑ANCA and MPO‑ANCA as the first‑line screening test rather than indirect immunofluorescence (IIF). 1
• If the immunoassay is negative but clinical suspicion remains high, perform a second test using an alternative immunoassay or IIF. 1
• Limit ANCA testing to defined clinical situations indicative of AAV to avoid false‑positive results that can occur in other inflammatory diseases, infections, drug‑induced conditions, or in patients without vasculitis. 1

Interpretation of Positive ANCA Results

• ANCA serotype (PR3 vs. MPO) is now considered a more important classification criterion than traditional clinical subtypes because each serotype is linked to distinct genetic alleles, clinical features, and prognoses. 1
• The 2022 ACR/EULAR classification criteria assign heavy weight to ANCA serotype in the diagnostic algorithm. 1

Confirmation with Tissue Biopsy

• Obtain a tissue biopsy whenever feasible to confirm the diagnosis before initiating treatment. 1
• Biopsy helps differentiate active disease from chronic damage or alternative diagnoses. 1
• Preferred biopsy sites based on organ involvement:
  
  • Kidney biopsy for suspected glomerulonephritis.
  • Thoracoscopic or open lung biopsy for pulmonary lesions that cannot be clearly attributed to AAV.
  • Upper‑airway biopsies (≈30 % sensitivity) and transbronchial biopsies (≈12 % sensitivity) should be avoided because of low diagnostic yield. 1

Management When Biopsy Is Not Feasible

• Use surrogate markers together with positive PR3‑ANCA or MPO‑ANCA and a typical clinical picture:
  
  • Clinical: mononeuritis multiplex confirmed by electrophysiology.
  • Laboratory: red‑blood‑cell casts in urine indicating glomerulonephritis.
  • Imaging: chest CT for pulmonary involvement; MRI for CNS, pachymeningitis, retro‑orbital, or cardiac disease. 1
• Do not delay treatment while awaiting biopsy if the patient has life‑threatening or organ‑threatening disease. 1
• Test for anti‑glomerular basement membrane (anti‑GBM) antibodies in patients presenting with pulmonary‑renal syndrome; anti‑GBM/AAV overlap patients have lower renal survival and may benefit from plasma exchange. 1

Treatment Initiation Based on Disease Severity

• For new‑onset or relapsing GPA or MPA with organ‑threatening or life‑threatening disease, start combination therapy with glucocorticoids plus either rituximab or cyclophosphamide; rituximab is preferred in relapsing disease. 1

ANCA‑Negative Cases

• A negative ANCA does not exclude AAV, especially when disease is limited to the respiratory tract or is renal‑limited. 1
• In ANCA‑negative patients with high clinical suspicion, tissue biopsy becomes even more critical for establishing the diagnosis. 1

Common Pitfalls to Avoid

• Do not diagnose AAV based solely on ANCA serology without supporting clinical features or tissue confirmation. 1
• Do not use traditional IIF screening as the primary method; antigen‑specific immunoassays provide superior diagnostic accuracy. 1

ANCA‑Associated Vasculitides: Definitions, Clinical Features, Serology, and Epidemiology

Disease Definitions

• Granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis (EGPA) are the three distinct entities that comprise ANCA‑associated vasculitis (AAV). [2][3][4][5]

Granulomatosis with Polyangiitis (GPA)

Histopathology

• GPA is characterized by necrotizing granulomatous inflammation together with small‑vessel vasculitis. [2][3]

Typical Clinical Manifestations

• Patients commonly present with destructive sinonasal lesions, pulmonary nodules, and pauci‑immune glomerulonephritis. [2][3]

Serological Profile

• The disease is most frequently associated with cytoplasmic ANCA (c‑ANCA) and antibodies directed against proteinase‑3 (PR3). [2][3]

Epidemiology

• Reported prevalence in European populations ranges from 24 to 157 cases per million, with the highest rates observed in Sweden and the United Kingdom. [2][3]

Microscopic Polyangiitis (MPA)

Histopathology

• MPA shows small‑vessel vasculitis without granulomatous inflammation. [2][3]

Typical Clinical Manifestations

• The disease commonly manifests as rapidly progressive pauci‑immune glomerulonephritis and alveolar hemorrhage. [2][3]

Serological Profile

• MPA is most often associated with perinuclear ANCA (p‑ANCA) and antibodies against myeloperoxidase (MPO). [2][3]

Epidemiology

• Prevalence varies from 0 to 66 cases per million in European countries and reaches 86 cases per million in Japan. [2][3]

Eosinophilic Granulomatosis with Polyangiitis (EGPA)

Histopathology

• EGPA features eosinophilic tissue infiltration in addition to small‑vessel vasculitis. [2][3]

Typical Clinical Manifestations

• Characteristic findings include asthma, peripheral eosinophilia, and peripheral neuropathy. [2][3]

Serological Profile

• Only about 40 % of patients have a detectable ANCA, most often MPO‑ANCA. [2][3]

Epidemiology

• Prevalence in European populations is reported as 2 to 38 cases per million. [2][3]

Shared Features of ANCA‑Associated Vasculitides

• All AAV entities involve small‑ and medium‑sized vessels. [2][3]
• Multi‑organ involvement is a hallmark, frequently affecting kidneys, lungs, upper respiratory tract, skin, eyes, and peripheral nerves. [2][3]

Historical Prognosis

• Prior to the introduction of alkylating agents, median survival for patients with GPA was approximately five months, reflecting a very poor prognosis. [2][3]
• Contemporary treatment regimens have markedly improved survival, although they remain associated with notable toxicity. [2][3]