Thrombosis with Thrombocytopenia Syndrome (also termed Vaccine-induced Thrombotic Thrombocytopenia) · Incidence is extremely low. · Urgent medical ...JumptoMainContentAmericanSocietyofHematologyCOVID-19ResourcesThrombosiswithThrombocytopeniaSyndromeEducationPublicationsAgendaforNematologyResearchRecommendationsAboutUsPrecisionMedicineGenomeEditingandGeneTherapyImmunologicTreatmentResearchSupportandFundingMeetingsCareersThrombosiswithThrombocytopeniaSyndrome (alsotermedVaccine-inducedThromboticThrombocytopenia)(Version1.6;lastupdatedAugust12,2021)InputfromJamesB.Bussel,MD;JeanM.Connors,MD;DouglasB.Cines,MD;CynthiaE.Dunbar,MD;LauraC.Michaelis,MD;LisaBaumannKreuziger,MD;AgnesY.Y.Lee,MD,MSc;andIngridPabinger-Fasching,MD.KeyTakeawaysThrombosiswithThrombocytopeniaSyndrome(TTS)DefinitiveDiagnosis(mustmeetallfivecriteria):COVIDvaccine4to42dayspriortosymptomonset# Anyvenousorarterialthrombosis(oftencerebralorabdominal)Thrombocytopenia(plateletcount<150x109/L)*PositivePF4“HIT”(heparin-inducedthrombocytopenia)ELISAMarkedlyelevatedD-dimer(>4timesupperlimitofnormal)  Incidenceisextremelylow. RiskofdeathandseriousoutcomesofCOVID-19,includingthrombosis,faroutweighriskofTTSpossiblyassociatedwithhighlyefficaciousvaccines.UrgentmedicalevaluationforTTS isindicatedifanyofthefollowingdevelop4to42daysaftervaccination:SevereheadacheVisualchangesAbdominalpainNauseaandvomitingBackpainShortnessofbreathLegpainorswellingPetechiae,easybruising,orbleedingIfTTSissuspected,performimmediateCBCwithplateletcountandimagingforthrombosisbasedonsymptoms.Ifthrombocytopeniaorthrombosisarepresent,recommendurgentconsultationfromhematologistwithexpertiseinhemostasis.AvoiduseofheparinuntilTTShasbeenruledoutoruntilanalternativeotherplausiblediagnosishasbeenmade.Initialwork-up(anormalplateletcountislessconcerningforTTS*):CBCwithplateletcountandperipheralsmearImagingforthrombosisbasedonsigns/symptomsPF4-ELISA(HITassay);drawbloodpriortoanytherapiesFibrinogenandD-dimerInitiatetherapywithintravenousimmunoglobinandno