When should an anti‑neutrophil cytoplasmic antibody (ANCA) panel be ordered and how should positive c‑ANCA/proteinase‑3 (PR3) or p‑ANCA/myeloperoxidase (MPO) results be managed in an adult with suspected small‑vessel vasculitis?

When to Order ANCA Testing and How to Manage Positive Results

Order an ANCA panel using high-quality PR3 and MPO antigen-specific immunoassays as the primary screening method when clinical features suggest small-vessel vasculitis, and manage positive results by confirming the diagnosis with tissue biopsy whenever feasible while initiating treatment based on the combination of clinical presentation, ANCA serotype, and organ involvement. 1

Indications for ANCA Testing

Clinical Scenarios Warranting ANCA Testing

Order ANCA testing when patients present with clinical features suggestive of ANCA-associated vasculitis (AAV), including: 1

• Renal manifestations: Rapidly progressive glomerulonephritis with red blood cell casts in urine 1
• Pulmonary involvement: Alveolar hemorrhage, pulmonary nodules, or destructive lung lesions 2
• Upper respiratory tract disease: Destructive sinonasal lesions, subglottic stenosis 2
• Neurological findings: Mononeuritis multiplex confirmed by electrophysiological studies 1
• Multi-organ involvement: Particularly combinations of kidney, lung, upper respiratory tract, skin, eyes, and peripheral nerves 2

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, 3
• If the immunoassay is negative but clinical suspicion remains high, perform a second test using either another immunoassay or IIF. 1
• This approach provides the highest diagnostic accuracy compared to traditional IIF screening followed by confirmation. 3

Critical Caveat About Testing Strategy

Limit ANCA testing to defined clinical situations indicative of AAV to avoid false-positive results. 4 ANCA can be detected in other inflammatory diseases, infections, drug-induced conditions, and even in patients without vasculitis. 1, 5

Interpreting Positive ANCA Results

Understanding ANCA Serotypes and Disease Associations

c-ANCA/PR3-ANCA positive results:

• Most strongly associated with granulomatosis with polyangiitis (GPA) 2
• Typical features include destructive sinonasal lesions, pulmonary nodules, necrotizing granulomatous inflammation, and pauci-immune glomerulonephritis 2

p-ANCA/MPO-ANCA positive results:

• Most commonly associated with microscopic polyangiitis (MPA) 2
• Characteristic presentations include rapidly progressive glomerulonephritis and alveolar hemorrhage without granulomatous inflammation 2
• Also found in approximately 40% of eosinophilic granulomatosis with polyangiitis (EGPA) cases 2

The Importance of ANCA Serotype Over Clinical Diagnosis

ANCA serotype (PR3 vs. MPO) is emerging as a more important classification criterion than traditional clinical subtypes because PR3-ANCA and MPO-ANCA diseases are associated with distinct genetic alleles, different clinical features, and varying prognoses. 1, 6 The 2022 ACR/EULAR classification criteria weight ANCA serotype heavily in diagnosis. 1

Management of Positive ANCA Results

Step 1: Confirm Diagnosis with Tissue Biopsy

Obtain tissue biopsy whenever feasible to confirm the diagnosis before initiating treatment. 1 This is critical because:

• ANCA positivity alone should not establish the diagnosis, as false positives occur in infections, other autoimmune diseases, malignancies, and drug-induced conditions 5, 7
• Biopsy helps distinguish current disease activity from damage or alternative diagnoses 1
• In one study, 9.7% of c-ANCA/PR3-ANCA positive patients did not have AAV 5

Preferred biopsy sites based on organ involvement:

• Kidney biopsy for suspected glomerulonephritis 1
• Thoracoscopic or open lung biopsy for pulmonary lesions that cannot be clearly attributed to AAV 1
• Avoid upper airway biopsies (30% sensitivity) and transbronchial biopsies (12% sensitivity) due to low diagnostic yield 1

Step 2: When Biopsy Is Not Feasible

If biopsy cannot be obtained or is delayed, use surrogate markers to support the diagnosis in combination with positive PR3-ANCA or MPO-ANCA serology and typical clinical presentation: 1

• Clinical surrogates: Mononeuritis multiplex confirmed by electrophysiology 1
• Laboratory surrogates: Red blood cell casts in urine indicating glomerulonephritis 1
• Imaging findings: CT chest for pulmonary involvement, MRI for CNS lesions, pachymeningitis, retro-orbital lesions, or cardiac disease 1

Do not delay treatment while awaiting biopsy if the patient has life-threatening or organ-threatening disease. 1

Step 3: Additional Testing Considerations

• Test for anti-glomerular basement membrane (anti-GBM) antibodies in patients presenting with pulmonary-renal syndrome, as anti-GBM/AAV overlap patients have lower renal survival and may benefit from plasma exchange 1
• Exclude underlying infections, other autoimmune disorders, and malignancies that can cause false-positive ANCA results 5

Step 4: Treatment Initiation Based on Disease Severity

For new-onset or relapsing GPA or MPA with organ-threatening or life-threatening disease, initiate combination therapy with glucocorticoids plus either rituximab or cyclophosphamide, with rituximab preferred in relapsing disease. 1

Special Considerations for ANCA-Negative Cases

• A negative ANCA does not exclude AAV, particularly in patients with disease limited to the respiratory tract or renal-limited vasculitis 1
• In ANCA-negative cases with high clinical suspicion, tissue biopsy becomes even more critical for diagnosis 1

Follow-Up of Isolated Positive ANCA Without Vasculitis

Patients with positive c-ANCA/PR3-ANCA or p-ANCA/MPO-ANCA but insufficient evidence for vasculitis require close monitoring. 7 While the risk of developing vasculitis appears small (none developed disease over mean 6.8 years in one cohort), at least one case report documents progression to AAV 6 months after initial positive testing. 5, 7

Common Pitfalls to Avoid

• Do not diagnose AAV based on ANCA serology alone without supporting clinical features or tissue confirmation 1, 4
• Do not use traditional IIF screening as the primary method; antigen-specific immunoassays have superior diagnostic accuracy 1, 3
• Do not overlook alternative diagnoses such as infection, drug-induced ANCA, or other inflammatory conditions in ANCA-positive patients 5
• Do not assume all ANCA-positive patients have the same prognosis; PR3-ANCA and MPO-ANCA define distinct disease subsets with different outcomes 6