Cathepsin-AbELISAisasolidphaseenzymeimmunoassayemployingnativehumanCathepsinGforthequantitativedetectionofantibodiesagainstCathepsinGinhumanserum.Theassayisatoolinthediagnosisofautoimmunesystemicvasculitis.AntibodiesagainstCathepsinG,aneutralserineproteinase,belongtothegroupofanti-neutrophilcytoplasmicantibodies(ANCA)whicharedirectedagainstcytoplasmiccomponentsofneutrophilicgranulocytesandmonocytes.ForthedetectionofANCAsindirectimmunofluorescencetestonethanolfixedneutrophilshasbeentheestablishedmethodsofar.ItbecameapparentthatsomeANCAscreateacytoplasmicfluorescencepattern(thuscalledcANCA)whileotherscreateaperinuclearpattern(thepANCA).Asbothpatternsmaycovermultipleantigens,immunofluorescencedoesnotsufficeforasatisfyingdifferentialdiagnosisofvasculitis;thuseachIFTshouldbeverifiedwithspecificELISAtests.Proteinase3(PR3)andMyeloperoxidase(MPO)havebeenidentifiedasthemajorcANCAandpANCAantigens,respectively,butothercellularcomponentslikeLactoferrin,CathepsinGandElastasecauseperinuclearstaining,tooandthereforeareincludedintothegroupofpANCAs.DetectionofANCAsisausefullaboratorydiagnostictestforcertainsmallvesselvasculitidesandsomenon-vasculiticclinicalsyndromes,suchasinflammatoryboweldisease(IBD).AntibodiesagainstMPOcorrelatewithidiopathicorvasculitisassociatednecrotizingcrescenticglomerulonephritis.Theyarefoundfrequentlyin70%ofpatientswithmicroscopicpolyangiitisaswellas5-50%ofpatientswithChurg-Strausssyndrome.AutoantibodiestoPR3areaspecificSEROlogicalMarkerforWegener`sgranulomatosis(WG).AntibodiesagainstLactoferrinandCathepsinGwereidentifiedinasubgroupofpatientswithinflammatoryboweldisease.However,theseANCAspecificitiesdonotappeartocorrelatewithdiseaseactivity.