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Anti-Myeloperoxidase antibodies in patients with Microscopic polyangiitis V D Pradhan, S S Badakere, A R Pawar, A F Almeida Abstract Background & objectives: Anti-neutrophil cytoplasmic antibodies in active necrotizing and crescentic glomerulonephritis are associated with systemic vasculitides like Wegener's granulomatosis, Microscopic polyangitiis and Churg Strauss Syndrome. This study shows the incidence of ANCA with specificities to Myeloperoxidase and Proteinase3 in MPA cases and gives the correlation of ANCA with Birmingham Vasculitis Activity Score. Material & methods: Eighteen cases of MPA were diagnosed as per Chapel Hill Consensus Criteria. ANCA was detected by indirect immunofluorescence microscopy using fluorescence and Confocal Laser Scanning Microscopes. Anti-MPO and anti-PR3 were identified by commercial ELISAs and anti-MPO subclass and IgG isotypes were also detected. Results: MPA patients showed a male preponderance with BVAS ranging from 17-30. Systemic involvement was seen in 88.9%, lower respiratory tract involvement in 77.8% and upper respiratory tract in only 33.3% cases. All these patients had perinuclear pattern on IIF, where titers ranged from 80-640 and ELISA showed anti-MPO; values ranging from 20-80 units/ml. IIF and ELISA showed a good correlation (r=0.77). Two patients having FPGN had dual specificities and had both anti-MPO and anti-PR3 which could be picked up only by ELISA. A good correlation (r=0.78) was observed between BVAS and ANCA levels as well. IgG ANCA was detected in 88.7% and 11.1% had IgG+IgM and IgG1+IgG4 ANCA was detected in 50% patients. Conclusion: p-ANCA with anti-MPO is highly specific for MPA; both IIF and ELISA should be carried out for true positivity and to identify rare cases of dual specificities. Confocal laser scanning microscopy is useful in identifying ANCA patterns especially when ANA is also positive. ANCA testing with BVAS assessment will surely help in early diagnosis and estimating the severity of this life threatening disease. Keywords Anti-neutrophil cytoplasmic antibodies, Microscopic polyangitiis, Indirect Immunofluorescence, Enzyme Linked Immunosorbent Assay, perinuclear-ANCA, Anti-Myeloperoxidase

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