The diagnosis is mainly based on clinical manifestations supported by laboratory abnormalities and pathology when biopsies are necessary and feasible. Anemia, leukocytosis, thrombocytosis, and high erythrocyte sedimentation rate and C-reactive protein may be present, but are neither diagnostic nor specific for WG. About 80% of all patients with WG are ANCA-positive and 80% of those patients produce ANCA with a diffuse cytoplasmic fluorescence pattern (C-ANCA) and antigen specificity for proteinase-3 (PR3-ANCA). About 10% to 20% of ANCA may have a perinuclear fluorescence pattern (P-ANCA) with specificity for myeloperoxidase (MPO-ANCA). The sensitivity of PR3-ANCA in WG is approximately 90% in patients with severe active disease and 55% to 75% in those with milder active disease (10). The combined use of indirect immunofluorescence (IIF) and antigen-specific enzyme-linked immunoassays results in diagnostic specificity of up to 98% (11). Urine sediment must be evaluated in all patients for the presence of microscopic hematuria and red blood cell casts, which are excellent surrogate markers of GN.

Differential diagnosis depends on the clinical presentation. In the setting of pulmonary-renal syndrome, WG needs to be distinguished from systemic lupus erythematosus (SLE), Goodpasture's syndrome, MPA, and CSS. MPA can closely mimic WG, except that the former lacks chronic, persistent ENT features (12). If WG presents in a classical manner and ANCA is positive, with specificity for PR3, the diagnosis is very likely. Diagnosis may be significantly delayed in patients with disease limited to the head and neck. The differential diagnosis of ENT disease includes chronic infections (e.g., tuberculous, fungal, and syphilitic), malignancy (e.g., lymphoma), sarcoidosis, CSS, and an unusual WG-mimic (13), a granulomatous syndrome that mimics WG and is associated with low surface expression of HLA class I molecules (14). Patients with class I HLA deficiency do not respond to the usual therapies for WG, which should suggest the possibility of this diagnosis. The differential diagnosis of septal perforation includes sarcoidosis, cocaine use, SLE, extranodal nasal lymphoma, lymphomatoid granulomatosis (LYG), and excessive use of intranasal corticosteroids.

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