ThrombinScreenELISAisasolidphaseenzymeimmunoassayemployingnativehumanthrombinforthequantitativedetectionofIgG,IgAandIgMantibodiesagainstthrombininhumanserum.Theassayisatoolinthediagnosisoftheanti-phospholipidsyndrome(APS).Thrombinisnotanormalconstituentofthecirculatingblood.Itisgeneratedbythecatalyticcleavageofitsplasmaprecursor,prothrombin(factorII),bytheactivatedStuartfactor(factorXa).Thisisthefinalstepoftheintrinsicandextrinsicpathwaysofcoagulation.Thetransformationrequiresthepresenceofanactivatedcofactor,factorVa,releasedfromfactorVbythrombinitself,andwhosebindingtoprothrombinacceleratestheactivityoffactorXainanon-enzymaticmanner.Thrombinisaglycoproteinformedbytwopeptideschainsof36and259amino-acidslinkedbydisulfurebonds.Threeimportantsiteshavebeenidentifiedonthesurfaceoftheenzyme:Thecatalyticsitethatconferstothemoleculeitsserineproteaseactivity,theexositeoneresponsIBLeforthebindingofthesubstrate(fibrinogenorthrombinreceptor)andtheexositetworesponsibleforthebindingofantithrombinIIIandinactivationofthrombin.Thrombinis,however,morethanasimpleplasmaenzyme.Itspropertiestostimulateplateletsandcausethemtoexpandaggregateandreleasecomponentsofthealphaanddensegranuleswererecognizedearlieron.Thepresenceofanti-thrombinantibodieshasbeenassociatedwiththeclinicalfeaturesofthesocalledantiphospholipidsyndrome(APS).Anti-Thrombinantibodiesseemtocorrelatewithdeepvenousthrombosis.