Diagnostic value of classical and atypical antineutrophil cytoplasmic antibody (ANCA) immunofluorescence patterns

Background - The 'classical' antineutrophil cytoplasmic antibody (G- ANCA) pattern seen on indirect immunofluorescence (IIF) is characterised by granular cytoplasmic staining showing central or interlobular accentuation, and is strongly associated with antiproteinase-3 antibodies (PR3-ANCA) and Wegener's granulomatosis. However, many laboratories report C-ANCA in the presence of any cytoplasmic IIF staining, regardless of pattern, which risks reducing the diagnostic value of this pattern. Aims - To classify different cytoplasmic ANCA patterns and thus determine whether stringent application of the classical criteria for C-ANCA would produce better correlation between C- ANCA and (1) PR3-ANCA enzyme linked immunosorbent assay (ELISA) results; (2) a diagnosis of systemic vasculitis (including Wegener's granulomatosis). Methods - 72 sera with cytoplasmic IIF collected over a two year period were analysed by IIF and a commercial PR3-ANGA ELISA kit. Results - Three IIF patterns were defined: 'classical/true' C-ANCA as described above (n = 27 (37.5%)); 'flat' ANCA with homogeneous cytoplasmic staining (n = 21 (29%)); and 'atypical' ANCA which included all other cytoplasmic patterns (n = 24 (33.5%)). Twenty five of the 27 true C-ANCA sera (92.5%) contained PR3-ANCA (p < 0.0001), but none of the 21 with fiat ANCA and only one of the 24 with atypical ANCA. From clinical data on 23 of the 27 true G-ANCA positive patients, 20 (87%) had evidence of Wegener's granulomatosis or systemic vasculitis (p < 0.0001 v the other two patterns). However, none of 19 sera with flat ANCA and clinical data had evidence of systemic vasculitis. Conclusions - Restricting the term 'cANCA' to the 'classical' description of central/interlobular accentuation on IIF, will improve its correlation with PR3ANCA positivity and a diagnosis of systemic vasculitis.