Systemic vasculitis is a rare condition and diagnosis is often difficult. Any organ system can be affected and patients can present with an insidiously progressive illness, a remitting/relapsing pattern, or acute multisystem failure. The early institution of aggressive and potentially toxic immunosuppression may be necessary to minimise permanent organ damage, and, despite the difficulties, rapid and unambiguous diagnosis is sometimes of great importance. The development of assays for antineutrophil cytoplasm antibodies (ANCAs) over the past decade has provided clinicians with a laboratory test for vasculitis. Classification of the vasculitides is confusing and has been the subject of a recent international consensus conference.1The conditions most associated with the presence of ANCAs are small vessel necrotising vasculitides such as Wegener’s granulomatosis, microscopic polyangiitis (microscopic polyarteritis), and pauci-immune crescentic glomerulonephritis. ANCAs were first described in 1982 in eight Australians with segmental necrotising glomerulonephritis,2 and the association with vasculitis was soon recognised.3 In 1985 van der Woude et al reported that ANCAs were specific for Wegener’s granulomatosis, and that the titre was associated with the degree of disease activity.4 In 1988 ANCAs were first described in children, in three cases of necrotising glomerulonephritis.5 Subsequently ANCAs have been reported, in both adults and children, in numerous conditions characterised by vasculitis and inflammation.6 Recent work has also suggested that they may be pathogenic in vasculitis. ### INDIRECT IMMUNOFLUORESCENCE (IIF) IIF detects the binding of antibodies to antigens in the cytoplasm of whole ethanol fixed neutrophils. It is subjective, semiquantitative, and requires experience to interpret. However, it is a rapid and standard technique and is widely available.7 Two major staining patterns can be distinguished: granular cytoplasmic staining with central accentuation, known as C-ANCA, and perinuclear staining, known as P-ANCA. A C-ANCA pattern usually corresponds to antibodies against proteinase 3,8 and P-ANCA staining …