Abstract
Systemic vasculitis, although rare, is often diagnosed late and long after the onset of symptoms. The small vessel vasculitides are recognized clinically by their multisystem presentation, markers of inflammation and evidence for an acute glomerulonephritis (GN), with the most apparent organ involved directing referral to secondary care. Routine laboratory tests are usually non-specific in systemic vasculitis but the use of anti-neutrophil cytoplasmic antibodies (ANCAs) and glomerular basement membrane (GBM) antibodies can aid diagnosis, treatment and monitoring decisions. These antibodies are detected and quantified by indirect immunofluorescence (IIF) and antigen-specific enzyme-linked immunosorbent assay (ELISA), usually in combination for ANCA, and ELISA systems (or direct IIF on kidney biopsy) for GBM antibodies. The presence or absence of ANCA does not confirm or exclude the diagnosis of systemic vasculitis but negative and positive predictive values will be strongly influenced by clinical presentation. Various large studies have been unable to conclude that following serial ANCA titres has great clinical utility in each case but each patient must be considered on its own merits; for example the reappearance of ANCA in a patient who was rendered ANCA negative following treatment is more likely to indicate relapse. The adoption of consensus guidelines that direct testing towards patients with rapidly progressive GN, pulmonary haemorrhage, persistent and destructive ear, nose and upper airways problems, such as subglottic tracheal stenosis, a retro-orbital mass and cutaneous vasculitis with systemic features or peripheral neuropathy, will greatly increase the clinical utility and positive predictive value of these tests.