c-ANCA (anti-proteinase 3; coarse diffuse cytoplasmic pattern) is highly specific (>90%) for active Wegener granulomatosis. Sensitivity is >90% in systemic vasculitic phase, approximately 65% in predominantly granulomatous disease of respiratory tract, and approximately 30% during complete remission.
ELISA titer does not correlate with disease activity; a high titer may persist during remission for years. c-ANCA is also occasionally found in other vasculitides (polyarteritis nodosa, microscopic polyangiitis [e.g., lung, idiopathic crescentic and pauci-immune GN], Churg-Strauss vasculitis).
p-ANCA (against various proteins [e.g., myeloperoxidase, elastase, lysozyme; perinuclear pattern]) occurs only with fixation in alcohol, not formalin. A positive result should be confirmed by ELISA. The test has poor specificity and 20–60% sensitivity in a variety of autoimmune diseases (microscopic polyangiitis, Churg-Strauss vasculitis, SLE, inflammatory bowel disease, Goodpasture syndrome, Sjögren syndrome, idiopathic GN, chronic infection). However, pulmonary small vessel vasculitis is strongly linked to myeloperoxidase antibodies.
Both p-ANCA and c-ANCA may be found in non–immune-mediated polyarteritis and other vasculitides.
Atypical pattern (neither c-ANCA nor p-ANCA; unknown target antigens) has poor specificity and unknown sensitivity in various conditions (e.g., HIV infection, endocarditis, CF, Felty syndrome, Kawasaki disease, ulcerative colitis, Crohn disease).
ANTIPHOSPHOLIPID ANTIBODY SYNDROME
See Chapter
9
, Hematologic Disorders.
HENOCH-SCHÖNLEIN PURPURA
Definition
Henoch-Schönlein purpura is a self-limited hypersensitivity systemic vasculitis of the small vessels. It involves the skin and to variable degrees joints, kidneys, and GI tract. The small vessel and renal involvement is caused by IgA deposition.
Who Should Be Suspected?
This condition is seen more commonly in children (90% of cases), but it may affect adults as well.
In adults, renal disease is common. The renal picture may vary, with minimal urinary abnormalities occurring for years. Patients may present with palpable purpura without thrombocytopenia or a coagulopathy and acute abdominal pain, or with purpura and joint symptoms.
Laboratory Findings
Diagnosis is made clinically; there are no pathognomonic laboratory findings.
Histology: renal or skin biopsy supports the diagnosis; it shows focal segmental necrotizing GN that becomes more diffuse and crescentic with IgA and C3 deposition.
