What are Anti neutrophil cytoplasm antibodies (ANCA)?
Kidney biopsies from patients with type III RPGN have no immune deposits, but type III RPGN usually results from small-vessel vasculitides associated with circulating Anti neutrophil cytoplasm antibodies.
The Anti neutrophil cytoplasm antibodies -associated small vessel vasculitides are:
• Granulomatosis with polyangiitis (GPA, formerly Wegener’s granulomatosis)
• Microscopic polyangiitis (MPA; systemic and kidney-limited)
• Eosinophilic granulomatosis with polyangiitis (EGPA, formerly known as Churg-Strauss syndrome)
These are usually systemic diseases but can be limited to the kidney. ANCA are directed against neutrophil proteinase 3 (PR3) or myeloperoxidase (MPO). Screening for Anti neutrophil cytoplasm antibodies uses an indirect immunofluorescence examination of normal neutrophils, which, if positive, demonstrates a staining pattern characteristic of the target antigen (cytoplasmic for ANCA [C-ANCA] against PR3 and perinuclear for anti-MPO ANCA [P-ANCA]).
The P-ANCA screening test has low specificity because other anti-neutrophil antibodies give a similar pattern by indirect immunofluorescence. A screening test that is positive for either C-ANCA or P-ANCA needs confirmation with an antigen-specific technique. C-ANCA positivity is associated with active GPA, and P-ANCA positivity is associated with active MPA and less so EGPA.
However, ANCA testing results have to be interpreted in the context of the clinical presentation, and the diagnosis of a vasculitis or glomerulonephritis often requires a tissue biopsy.
Anti neutrophil cytoplasm antibodies antibodies can be present in patients with non-vasculitic rheumatologic diseases, infections, and autoimmune gastrointestinal diseases, and can co-occur with other types of acute glomerulonephritis (e.g., anti-GBM disease).