Introduction
ThemostimportantclinicaluseoffetalRBCdetectionisthediagnosisandquantitationoffetomaternalhemorrhage(FMH). FMHoccursnormallythroughoutpregnancyinminuteamounts,withincreasingvolumesduringthelaterstagesofgestation.IfthereisasignificantdifferenceintheRBCantigenicitybetweenthefetusandmother,thiscanresultinallosensitizationofthematernalimmunesystemeitherbeforeorafterparturition. ThematernalantibodiestothefetalRBCantigensmaybeclinicallysilentorcauselife-threateningautoimmunesequelaeforthecurrentorsubsequentpregnancies(e.g.:erythroblastosisfetalisorearlyabortion). SuchsensitizationcanoccurwithanyRBCantigenmismatch,butthehighestfrequencyandprofoundclinicalconsequencesoccurwithRhorD-antigenmismatches.
DetectionandenumerationofFetalRBCsisanessentialpartofthemanagementofthosepatientswithFMHtreatedwithRhimmuneglobulin(RhIG)preparations. TheuseofRhimmuneglobulinprophylaxisisauniversalpractice,butdosingamountsandscheduleshaveregionalvariations.Hence,thesensitivityandspecificityofdetectionassaysforFMHisacriticalfactorintherapeuticefficacyandsubsequentclinicaloutcome.
ThemostwidelyusedassayforFMHdetectionhasbeenthevisualmicroscopiccountingKleihauer-Betke(KB)method,whichisbaseduponthedifferencesinsolubilitypropertiesinacidconditionsoffetalhemoglobin(HbF)fromadulthemoglobin.WhiletheKBmethodiseasilyperformedbymostclinicallaboratories,itlackssensitivityandexhibitspoorreproducibilityorprecision(CVsof50-100%).Flowcytometricmethodshavebeendevelopedusingtheantigenicdifferencesorquantitativeassessmentoffetalhemoglobin(HbF)todistinguishfetalRBCsfromadultRBCs.Thesemethodsaremorepreciseandlesssubjective.Nonetheless,manylaboratorieshavecontinuedtousetheKBmethodduetothelimitedavailABIlityofFlowCytometry.