If anti-ribosomal P (anti-ribosomal protein) antibody is negative, is PR3 (Proteinase 3) and MPO (Myeloperoxidase) a separate antibody testing?

Are PR3 and MPO Separate Antibody Tests from Anti-Ribosomal P Antibody?

Yes, PR3-ANCA and MPO-ANCA are completely separate antibody tests from anti-ribosomal P antibody, targeting different antigens and associated with distinct disease entities. Anti-ribosomal P antibodies are associated with systemic lupus erythematosus (SLE), while PR3 and MPO are the primary target antigens in ANCA-associated vasculitis (AAV).

Understanding the Distinct Antibody Systems

Anti-Ribosomal P Antibody

• Anti-ribosomal P antibodies target ribosomal phosphoproteins and are primarily associated with SLE, particularly neuropsychiatric manifestations 1
• These antibodies are part of the anti-nuclear antibody (ANA) family and have no relationship to vasculitis diagnosis

PR3 and MPO-ANCA Testing

• PR3 (Proteinase 3) and MPO (Myeloperoxidase) are neutrophil granule proteins that serve as the primary target antigens in ANCA-associated vasculitis 1
• High-quality antigen-specific immunoassays (ELISA) for both PR3-ANCA and MPO-ANCA are the recommended screening method for suspected AAV 1
• Both PR3 and MPO should be tested simultaneously when AAV is suspected, as they represent different serotypes with distinct clinical characteristics 1

Clinical Significance of Each Antibody Type

PR3-ANCA Characteristics

• Present in 84-85% of patients with granulomatosis with polyangiitis (GPA) 1, 2
• Associated with c-ANCA pattern on immunofluorescence with 99% specificity and 73% sensitivity for GPA 2
• More commonly associated with upper respiratory tract involvement, eye manifestations (32%), and granuloma formation (45%) 3, 4
• Predominantly affects males (66%) with mean age 49 years 3

MPO-ANCA Characteristics

• Found in 75-97% of patients with microscopic polyangiitis (MPA) 1
• Present in only 30% of eosinophilic granulomatosis with polyangiitis (EGPA) 1, 5
• More frequently associated with pulmonary hemorrhage (40%), glomerulonephritis, and other autoimmune disorders 3, 4
• Predominantly affects females (62%) with mean age 57 years 3

Testing Algorithm for Suspected Vasculitis

When to Order ANCA Testing

• Order both PR3-ANCA and MPO-ANCA testing when clinical presentation suggests small-vessel vasculitis, including rapidly progressive glomerulonephritis, pulmonary-renal syndrome, or multi-system inflammatory disease 1
• Testing should include complete blood count, inflammatory markers (C-reactive protein), renal function tests, and urinalysis with microscopy for dysmorphic red blood cells and red cell casts 1, 6

Methodology

• Use high-quality antigen-specific immunoassays (ELISA) as the primary screening method rather than indirect immunofluorescence alone 1, 7
• If immunoassay is negative but clinical suspicion remains high, consider a second test (another immunoassay and/or indirect immunofluorescence) 1

Important Clinical Caveats

ANCA-Negative Disease

• Approximately 10% of patients with clinical features of AAV are persistently ANCA-negative, particularly those with disease limited to the respiratory tract or renal-limited vasculitis 1, 6
• ANCA negativity does not exclude AAV diagnosis—tissue biopsy becomes essential in these cases 1, 6

False Positives

• A significant proportion (60%) of patients with positive C-ANCA/PR3 or P-ANCA/MPO do not have evidence of vasculitis, particularly those with low-medium ELISA antibody titers 8
• Higher antibody titers correlate with higher likelihood of true vasculitis—specificity improves substantially in the highest tertile of ELISA values 8
• P-ANCA can be found in non-vasculitic conditions including autoimmune hepatitis, primary sclerosing cholangitis, inflammatory bowel disease, and drug-induced syndromes 1, 5

Overlap Syndromes

• Some patients may have both ANCA and anti-GBM antibodies, requiring consideration of plasma exchange 1
• In pulmonary-renal syndrome, always test for anti-GBM antibodies in addition to ANCA, as this affects treatment decisions 1

Treatment Implications Based on Antibody Results

When ANCA is Positive

• In patients with clinical presentation compatible with small-vessel vasculitis and positive MPO- or PR3-ANCA, do not delay immunosuppressive therapy while waiting for biopsy results, especially in rapidly deteriorating patients 1, 5
• Initial treatment consists of glucocorticoids (50-75 mg/day prednisolone equivalent) combined with rituximab or cyclophosphamide 2

Monitoring Considerations

• Sequential ANCA titer monitoring has limited utility—persistence of positivity or rising titers are only modestly predictive of relapse and should not solely guide treatment decisions 2, 5
• Disappearance of ANCA with treatment is always associated with absence of disease activity 3