9 Laboratory findings in the vasculitides

The primary vasculitides are diseases of unknown aetiology. They are characterized by inflammation of blood vessel walls. Measuring non-specific laboratory markers of inflammation is useful in the monitoring of patients with vasculitis. The diagnostic specificity of these markers is, however, restricted. In the last decade, autoantibodies reacting with myeloid granule proteins have been detected in the sera from patients with Wegener's granulomatosis, microscopic polyangiitis, Churg-Strauss syndrome, and the renal limited form of these vasculitides (i.e. idiopathic rapidly progressive glomerulonephritis). Anti-neutrophil cytoplasmic antibodies (ANCA) in the aforementioned disorders react with proteinase 3 (Pr3) or myeloperoxidase (MPO), and only incidentally to other antigens such as elastase and bactericidal-permeability increasing protein. The presence of ANCA alone, in particular perinuclear ANCA, as detected by indirect immunofluorescence, has a low specificity for those vasculitides. However, in combination with the presence of anti-Pr3 or anti-MPO antibodies as detected by enzyme-linked immunosorbent assay, sensitivity and specificity for the vasculitides is high. Several in vitro and in vivo data have suggested a pathophysiological role for anti-Pr3 and anti-MPO in the associated disorders. Measuring levels of the autoantibodies seems useful for the follow-up of patients with these vasculitides. The sensitivity and specificity of rises in ANCA levels for ensuing relapses appears somewhat lower than previously suggested. Refinement of the assays, for example, by measuring subclasses and functional characteristics of the autoantibodies, may improve their value in monitoring patients with vasculitides.