IntroductionHeparin-inducedthrombocytopenia(HIT)isanacute,immune-mediatedprocessthatmayresultinlife-threateningthrombosis.HITiscausedbyplatelet-activating,heparin-dependentantibodies,whoformcomplexesofheparintogetherwitheitherplateletfactor-4(PF-4)(mostoftheclinicalcases),interleukin-8(IL-8)orneutrophil-activatingpeptide2(NAP-2).Detectionoftheseantibodiesisanimportantlaboratoryfunction.
Theantibodiesareinitiallyformedafter5ormoredayswhenapatienthasbeenonheparintherapy.Animmuneresponsetoaheparindosemaybeobservedwithinminutesorhoursifthepatienthashadpreviousexposuretoheparinandantibodiesarealreadycirculating.
ThediagnosisofHITisdifficultbecauseitssignsarenon-specificandduetotheabovementionedstructureitisveryimportanttolookforthepresenceandfunctionofthecomplexspecificantibodies.
DiagnosticsFordetectionofHITtwodifferentkindofassayscanbeused,theimmunologicalandfunctionalassay.Thefunctionalassaysreproducethein-vivopathophysiology,andhighlycorrelatewiththeclinicalpresentationofHIT.Thisisincontrasttotheimmune-detectionantigen(antigenemia)assays,whichmeasureantibodiesreactivetotheheparinplatelet-factor4(PF4)complexes.However,onlyafractionofpatientspositiveinthesetestsdevelopclinicalHIT.Ontheotherhand,pathogenicantibodiesmayalsoreactwithotherheparincomplexes,suchasheparin-interleukin-8,orheparin-neutrophilactivatingpeptide2,thus,notbeingdetectedbythoseassays.
AnimmunologicalassaycanbetheEnzyme-LinkedImmunosorbentAssay(ELISA)ortheGelTest(PaGIA),whichissimilartoELISA.Thedisadvantageofthesetestsisthefactthattheyarenon-functionalassays,theygivemorethan10%falsenegativeanda20%disagreementwithSRA.
AfunctionalassayistheSEROtonin-releaseassay(SRA),whichissensitiveandspecific, butnon-feasIBLebecauseoftheuseofrADIoisotypes.AnotherfunctionalassayistheAggregationAssaybutithasalowsensitivityandalongtimetoperform.
PrincipleoftheHITAlert™KitRecently,theAmericanCollegeofChestPhysicians(ACCP)haspostedupdatesonrecommendationsfortestingofHIT.ThisshouldfurtherincreasetheneedofhighperformancetestingforHIT.TheHITAlert™Kitissuchatest.
FortheHITAlert™Kitdonorplatelets(PRP)areused,whichareincubatedinthepresenceofpatientserumandinthepresenceorabsenceofheparin.WhenpathogenicantibodiesarepresenttheactivationofthedonorplateletsisshownusingaplateletactivationMarker.Byincubatingthesampleswithanantibodyagainstplateletsandtheactivationmarkerthisreactioncanbevisualizedusingflowcytometry.
Results
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Figure:ResultsofHITAlert™Kit:(A)dotplotofPEpositiveplatelets,(B)negativepatient,(C)positivepatient.