p-ANCA is sensitive and specific for-
Antineutrophil cytoplasmic antibodies (ANCA)

ANCA are a heterogeneous group of autoantibodies directed against antigens which are found within the primary granules of neutrophil and in the lysosomes of monocytes and in EC's.
The description of these autoantibodies is based on the immunofluorescence pattern of staining of ethanol-fixed neutrophils. With immunofluorescence two principal patterns are recognized:

1. Cytoplasmic (c-ANCA):

This shows cytoplasmic localization of the staining and the target antigen for this type of autoantibody is — proteinase-3 (PR3), a neutrophil granule constituent.

2. Perinuclear staining (p-ANCA) —

This shows perinuclear staining and the target antigen here is myeloperoxide (MPO). Remember that either of these antibodies may occur in a patient with ANCA associated small vessel vasculitis but

(i) cANCA Typically found in

Wegener's granulomatosis

(ii) pANCA Typically found in

Microscopic polyangiitis
Churg-Strauss syndrome
Idiopathic crescentic glomerulonephritis
Goodpasture's syndrome.
pANCA's are also associated with certain non-vasculitic entities such as certain rheumatic and nonrheumatic autoimmune diseases, Inflammatory bowel diseases, certain drugs,  infections such as endocarditis and bacterial airway infection in patients with cystic fibrosis.
Post streptococcal glomerulonephritis
Idiopathic crescentic glomerulonephritis
Diffuse glomerulosclerosis
Wegener's granulomatosis