p-ANCA, or MPO-ANCA, or Perinuclear Anti-Neutrophil Cytoplasmic Antibodies, show a perinuclear staining pattern.
This pattern occurs because the vast majority of the antigens targeted by ANCAs are highly cationic (Positively charged) at pH 7.00. During ethanol (pH ~7.0 in water) fixation, antigens which are more cationic migrate and localize around the nucleus, attracted by its negatively charged DNA content. Antibody staining therefore results in fluorescence of the region around the nucleus.
p-ANCA is associated with several medical conditions:
- It is fairly specific, but not sensitive for ulcerative colitis, so is not useful as a sole diagnostic test. When measured together with Anti-saccharomyces cerevisiae antibodies (ASCA), p-ANCA has been estimated to have a specificity of 97% and a sensitivity of 48% in differentiating patients with ulcerative colitis from normal controls.
- about 50% of cases of Churg-Strauss syndrome
- A majority of primary sclerosing cholangitis
- sometimes polyarteritis nodosa (only in the variant microscopic polyangiitis, usually negative in typical polyarteritis nodosa)
- microscopic polyangiitis 
- Focal necrotising and crescentic glomerulonephritis
- Rheumatoid arthritis
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