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If only C-ANCA are present in the sample, with a clearly granular staining pattern, re-sults are usually straightforward to read. Sometimes a concomitant reaction with lym-phocyte cytoplasm is seen and the neutrophil staining pattern appears less granular. The problem is best solved by demonstrating PR3-ANCA by EIA (4). If this assay is negative the pattern is most likely due to presence of A-ANCA. IIF results should always be re-ported together with EIA results or the C-ANCA results be interpreted with caution until confirmed by PR3-ANCA EIA (2).
If only P-ANCA are present in a sample, the IIF results are usually easy to interpret. Strong positive samples often show a condensed nuclear fluorescence, but at higher dilu-tions it becomes more perinuclear. The P-ANCA pattern is an artifact caused by charge interaction between a cationic lysosomal antigen and the anionic nucleus (6). If homoge-neous staining of lymphocytes and eosinophils in the whole slide is present this is due to presence of antinuclear antibodies (ANA) with chromatin specificity. If only some lym-phocytes adjacent to neutrophils show nuclear/perinuclear staining the phenomenon is likely to be due to P-ANCA reacting with redistributed MPO (or another cationic lyso-somal protein) bound to neighbouring nuclei through charge interaction. Again a positive P-ANCA result should be interpreted with caution until confirmed by EIA for MPO-ANCA, and only strong positive EIA results are then indicative of a vasculitic condition whereas weak positive MPO-ANCA results with or without ANA are common in other inflammatory diseases (2).