TheRheumatoidFactorIgA/IgG/IgMisasolidphaseenzymeimmunoassaywithhighlypurifiedFcfragmentofhumanimmunoglobuline(IgG)fortheseparatequantitativeandqualitativedetectionofIgG,IgMandIgArheumatoidfactors(RF)inhumanserum.Theassayisanaidinthediagnosisofrheumatoidarthritis(RA).Rheumatoidfactors(RF),firstdescribedin1940asantibodiesreactingwithgammaglobulins,areautoantibodiesdirectedagainsttheC-terminalpartoftheconstantregionoftheIgGheavychain,theIgGFc.Althoughnamedafterthediseasetheywereinitiallyassociatedwith,RFsarefoundbothinthehealthypopulationandseveraldiseases.ThediseasescommonlyassociatedwithhighRFconcentrationsarerheumatoidarthritis(RA;50-90%)andSjögren`ssyndrome(75-95%).Theyarealsofoundinsystemiclupuserythematosus(SLE;15-35%),systemicsclerosis(20-30%)polymyositis/dermatomyosisits(5-10%),cryoglobulinemia(40-100%)andmixedconnectivetissuediseases(MCTD;50-60%).AlthoughthepresenceofIgMRFintheserumhasbeenregardedasthemostimportantSEROlogicalindicatorforRA,thusincludedintheACRlistofcriteriaforthediagnosisofthisdisease,RFofIgGandIgAsubclassareimportantfordiagnosisaswell.Determinationoftheseisotypesprovidesadditionalinformationwithregardtodiagnosis,differentialdiagnosisandfollow-upofRAincomparisontoconventionaltechniquessuchaslatexagglutinationtestandnephelometry.WhilstRFofsubclassIgMaremostsensitivefordiagnosisofRA,thusmostsuitableforscreening,RFofsubclassIgGaremostspecificforRAandlikesubclassIgAcorrelatewithclinicalparametersanddiseaseactivity.Thepresenceofallthreesubclassestogetheris100%specificforRA.RFinSLEareassociatedwithsiccasyndrome,hypergammaglobulinemia,hightiterofanti-nuclearantibodies,anemiaandusuallySS-AandSS-Bantibodiesappearance.AllthreesubclassesarefoundinSLE.EspeciallythesubclassIgAdefinesasubgroupofSLEpatientscharacterizedbydistinctautoimmunephenomenaandhighdiseaseactivityintheabsenceofnephritis.