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HL7forBusyProfessionals
YourNoSweatGuidetoUnderstandingHL7
RahulBhagat
IllustrationByCalvinHui
Anchiove2015
TableofContentsPreface
PartIScratchingthesurface
1.Introduction2.WhatisHL7?3.IntegrationConcepts4.EvolutionofHL7
PartIIDiggingDeeper
5.BasicConcepts6.MessageBuildingBlocks7.WorkingwithaMessage8.ControlSegments9.DataSegments10.OtherImportantTopics
PrefaceAfteruniversity,Igotajobwithabusy,Torontobased,healthcareconsultingcompany.Ondaytwoatwork,Iwashandedaprintoutwithcryptictextonit,andadocumentcalledinterfacespec,toreadandunderstand.
ThiswasmyintroductiontoHL7.Atthattime,Ididnotrealizethatthisobscuremessagingprotocolwouldbecomemytickettofaroffplaces,andthereasontomeetandworkwithalotofpeople.
Itdidn'ttakemelongtolearnHL7,myprogrammingbackgroundhelped.Later,IrealizedthatmyskillisinhighdemandandIbecameaconsultant.ItraveledtodifferentcitiesandworkedonvariousHL7projects.
Ialsostartedrunningintopeoplefromnon-technicalbackgroundwhowantedmetoexplainHL7intheelevatororwhilechattingintheircubicle.Therewasn’tanyintroductorybookIcouldsuggest,sotheideaofwritingonemyself.
I'mgladIcollaboratedwithCalvinHuiinwritingthisbook.HenotonlytookcareofillustrationanddesignbutalsonudgedmewhenIwasslackingafterthefirstdraft.MyfriendErikWestermannwasagreatsoundingboardandhelpedmerefinemyideas.Andthankstomanycolleagueswhohelpedmedevelopmyskills.Inparticular,DerrickLeung,whomentoredmewhenIwasjuststartingout.
Sohereitis.MyideaofanintroductorybookonHL7.Ihopeyouenjoyreadingit.
PartIScratchingthesurface
1.IntroductionAtechnicalbookusuallyimpliesadrysubject.Soitsnosurpriseauthorshaveahardtimefiguringoutwaystomakethebookinterestingtothereader.HL7isonesuchsubject.Itisasubjectthatissohighonthescaleofdrynessandnoonecomestoitwillingly.TheonlyreasonsomeonewouldreadabookonHL7isbecauseofhisorherjob.Andifyouarehere,readingthisbook,thenIassumeyouworkinhealthcareITorintendtojointheindustrysoon.
Ihavemadeeveryefforttotakeoutthedrynessofthesubjectandmakethisbookinteresting.Therearenoneedlessjargonsoresotericconceptsthrowncasuallytotripyou.Infact,youwillseeaheavyrelianceoneverydayexamplesandinclusionofbackgroundinformationtopaintacompletepicture.ButHL7andhealthcaresystemintegrationarecomplexsubjectssotherewillbetopicsthatdon'tmakesenserightaway.Pleasepersevere.Tieaknotandhanginthere.Graduallythingswillmakesense.
ThisintroductorybookonHL7goesindetailtoexplainwhatHL7is.Itgivesyouthebasicconcepts,tellsyouabouttheorganizationbehinditandhelpsyoucreateamentalmapofthevoluminousHL7specificationdocument.And,ittakesyouthroughawhirlwindtourofsomeofthemostcommonlyusedHL7messages,allinashortspanoftime.
EarlyRailroadsHL7wascreatedtosolvetheproblemsofclinicalsystemintegration.Buttotrulyunderstandtheproblemsofsystemintegration,let’sstartwithanotherintegrationproblemwesolvedcenturiesago.
The1800’swereatimewhenrailwayswerecomingofageinAmerica–justlikebatterydrivencars,dronesandothernewtechnologiesarecomingofagetoday.
Therewereliterallyhundredsofcompaniescompetingforapieceoftherailwaypie.Enterprisingcompanieswouldbuyupland,laydowntracksandrunatransportservicebetweencitieswhichhadnoothermeansoftransportationexceptforhorse-drawnwagonsor,ifonewasfortunate,steamships.
BythetimeAmericancivilwarstarted(1861),vaststretchesofthecontinentwerealreadyconnectedthroughrailandworkwaswellunderwayontheconstructionofthetranscontinentalrailroadtoconnectCaliforniawiththerestofthecountry.
However,therewasoneproblem.Youcouldnotjusthoponatrainandgetoffatyourdestination,likeyoucantoday.Becausetheserailroadswerebuiltandrunbydifferentcompanies,theyuseddifferenttrackgauges(horizontaldistancebetweentworailsofthetrack).Thismeantyouhadtogetoffandchangetrainswheneveryouhitajunctionwithtwodifferentgaugewidths.Therewerewellovertwentydifferenttrackgaugesbeingusedatthetimeofthecivilwar.Thearmyhadtoconstantlyloadandunloadcargoinitsefforttogetsuppliestothetroops.Thiswasaseriousproblem!
AnditwasthereasonthatfinallymadetheAmericangovernmenttopushfortheconversionofallrailwaytrackstoastandardgauge—4feetand8.5inches,themostcommonlyusedgaugewidth.Morethanhalfoftheexistingtrackswerebuilttothiswidthsoitwaseasiesttoconverttheremainingtrackstothiswidthandachievestandardization.
Standardizationofrailtrackswasthefirststeptowardscreatinganintegratedsystemwheregoodsandpeoplecouldmovefreelyacrossthewholenetwork.Itwasfollowedbythedevelopmentofacommonsignalsystem,timezones,harmonizedtrainschedule,fixedcoachheight,astandardcoalandwatersupplysystemandonandon.
Itwasevidentthatanintegratedsystemneededastandardwayofdoingthings.
EvolutionofHealthcareITSystemsToday,weareina(somewhat)similarsituationwiththemovementofhealthcareinformation.Itcannotseamlesslyflowfromonesystemtothenext.Eachorganizationhasitsownwayofstoringandsharinginformation.Wheneverhealthinformationneedstomoveacrossorganizationboundaries,ithitstheincompatiblestandardsroadblock.Someonehastounloadandreloadtheinformation.
HealthcareITsystemshaveevolvedsimilartorailroads.Initially,hardwarecosts(thinkmulti-milliondollarmainframes)werethebiggestfactor,soonlyafewteachinghospitalswithdeeppocketshadthemeanstobuildasystem.Thesewereprimarilystand-alonesystemsmeanttoserveaspecificpurpose.Forexample,tomanagepatientpopulationinalargehospital.
Thenthehardwarecostcamedownandminicomputersarrivedonthescene.Acomputercouldbehadforlessthan$25,000anddidn’tneeda
roomtohouseit.Thisallowedsmallerplayersandevendepartmentswithinahospitaltopurchasesystemsoftheirown.Pharmaciesinstalledsystemstotrackprescriptionsanddispensedmedicationwhilelaboratoriessetupsystemstotrackrequestsfortestsandtheirresults.
Thisledtodramaticimprovementinproductivityfortheseorganizationsbuttherewasnofreeflowofinformationbetweentheclinicalsystems.Theproblemwaslackofstandardization.Informationfromonesystemhadtobeunloadedtopaperandtransportedtowheretheothersystemwas.Thenahumanoperatorwouldreloadtheinformationtotheothersystembymanuallytypingitin.
Ofcoursethiswastheworsecasescenario.Improvementsweremade.Informationwasloadedonfloppydisksandelectronicallymovedtotheothersystem.Still,therewasnofreeflowofinformationbetweensystems.Thispreventedusfromrealizingthetruepotentialofelectronicsystems.
ThensomeITvendorscameupwithasolution.Replacestand-alonesystemswithanintegratedproduct-anEHR(electronichealthrecord).IfyouarefamiliarwithCerner,EpicorMeditechthenyouknowwhatIamtalkingabout.Alargesystemwithmodulesforeverydepartment.
Thiseliminatedtheneedforhealthinformationtocrosssystemboundaries.Withinthesystem,themoduleswoulduseastandardwayofstoringandsharinginformationandthiswouldallowtheinformationtoflowseamlesslywithintheorganization.
Thisapproachworkedwell.EHRshavebeenverysuccessfulineliminatingtheproblemofintegratingsystemswithinanorganizationandtheycontinuetobeoneofthecornerstonesofthehealthcareITstructure.
Butwhataboutsharinginformationoutsidetheorganization?Healthcareorganizationsdon’tworkinisolation.Theyneedtoshareinformationwithinsurancecompaniesandsendpatientcareinformationtothegovernment.Theyhavetoconstantlycommunicatewiththeoutsideworld.
Touseourrailwayanalogy,thiswassimilartothesituationwhereeachstatecouldsetitsownstandardgauge.Youcouldtravelalloverastatewithouttheneedtoswitchtrainsbutwhenyouwantedtocrossthestateboundary,youwouldneedtodisembarkandgetonatrainthatranontheotherstate’sstandardgauge.
Clearly,EHRswereonlyalimitedsolution.Therewasalsothequestionofwhattodowithexistingstandalone
clinicalsystems.Thesesystemswerebuiltovermanyyearsthroughsubstantialmonetaryinvestment.Anorganizationwouldbeloathtoscrapallthatinvestment&hardworkandreplaceitwithanEHR.
Healthcareneededabettersolution.Itneededastandardgaugeto
connecttheseEHRs,standalonesystems,externalsystemsandsystemsthatwereyettobebuilt.Itneededtomoveawayfromconstantlyloadingandunloadinginformation.
ThesolutionwasHL7.
2.WhatisHL7?HL7isanANSIaccredited,OSIlevel7,applicationlayerprotocolforexchangingclinicalandadministrativedatabetweenhealthcaresystems.
Chancesare,ifyouarenotanetworkengineerordidnotstudycomputerscience,then“OSIlevel7,applicationlayerprotocol”probablymeansnothingtoyou.
Inlayterms,youcansaythatHL7isalanguagethatclinicalsystemsusetoexchangeinformationwitheachother.Buteventhatdoesn’ttellyouanything.WhenIwaslearningHL7,thedefinitionraiseditsownquestionsandleftmewithavaguesenseofunease.IttookafairbitofresearchtofigureoutwhatHL7is.
Soinsteadofleavingwithasenseofunease,whydon’twetakethetimeandfigureoutwhatHL7reallyis?
ApplicationLayerProtocolHL7isanapplicationlayerprotocol.Thismeansthatitdefinestherulesforexchangingdata(clinicalandadministrative)betweenapplications.
Weoftenusethewordsystemandapplicationinaninformalway,whichcloudsthedistinctionbetweenthetwo.Historicallyanapplicationwasthesameasasystem.Anoldaccountingsystem,withitshardwareandprintersandmonitorshadonlyonejoborapplication–preparingandmaintainingfinancialrecords.
Thingschangedwhensystemsbecamemorepowerfulandstartedtakingonmultipleroles.Agreatexampleisyoursmartphone.It’snotjustaphoneanymore.Makingaphonecallisjustoneofthemanyfunctionsofthedevice.Ithasnumerous“apps”orapplicationsforallsortsofthings.
Similarly,moderncomputersystemsorserversrunmultipleapplications,includingclinicalapplications.Whenapplicationscommunicatewitheachother,theyhavetodosothroughtheirsystem.Basically,applicationscreateamessageinalanguagethatisunderstoodbytheircounterpartapplications–inourcaseHL7–andhanditovertotheirsystemfordelivery.Thesystemdoesn’tunderstandthemessage.Itsjobistoreliablydeliverthemessagetothedestinationsystem.
HL7isonesuchspecializedapplication-to-applicationlanguage/messagingrulebook/protocol–whateveryoucallit–forcommunicationbetweenclinicalapplications.
OSILevel7HL7isalsoanOSI(OpenSystemInterconnection)Level7protocol.Thisisjustaformalwayofsayingthatitisanapplicationlayerprotocol.
Now,wearegoingtodiscussOSIanditslevelsandthatmeanssplashingthroughpacketbased,networkcommunication.Ifyouarenotinterestedinit,Iwouldsuggestskippingovertothenextchapter.
OSIisareferencemodelthatnetworkingguysusetomakesenseofthenetworkcommunicationmodelandhowthingsreallyhappenatthebitandbytelevel.
ItisnotdifficulttounderstandtheOSImodel.Thesecretisproperbackgroundknowledgeandanunderstandingofthekeyconcepts.Let’sseeifwecandothatinafewshortpageshere.
HistoricalBackgroundUsingelectricityforcommunicationstartedwithSamuelMorse,theinventorofthetelegraph.Hecreatedasimplecircuitwithabattery,abowlofmercuryandtwolongwiresgroundedatends.
Ifhedippedawireinthebowlofmercury,itcompletedthecircuitandcurrentflowedthroughit.Tosendashortburstofelectricity,hewoulddipthewireandpullitoutquickly.Thiswaslikesendingelectric“smokepuffs”totheotherend.
ThisbasicideawasrefinedintothetelegraphandMorsecode.Thecodehadtwoletters–adotandadash.Adotwasashortpuffofelectricityandadashwasalongerpuff(about3timesthedurationofadot).Dotsanddasheswerecombinedtorepresentlettersandvoila!Wehadelectroniccommunication.
.....Thisisapreview.RestoftheChapterisnotshown.....
4.EvolutionofHL7Weliveinaworldofstandards.Asnewtechnologiesemerge,anecessaryconditionfortheirwideadoptionisstandardization.Wenoticeitwhenthereisaproblemwiththestandard.Asmustbeevidenttoanyoneinaforeigncountrynotabletopluginalaptopbecausethepoweroutletisdifferent.
Standardsemergefromdifferentsources.Astandardcouldbeimposedbythegovernment,aswasthecasewiththeconversionfromtheimperialtothemetricsystemofmeasurement.BoththeUSAandCanadastartedtheconversionbutintheStates,thegovernmentdefundedtheMetricBoard,stallingtheirconversion.InCanadahowever,theconversionwascompletedandwestartedmeasuringdistanceinkilometersandtemperatureinCelsius.
Anothersourceofstandardistheindustryitself.Companiesattheforefrontofanewtechnologyvieforcompetitiveadvantagebypromotingadoptionoftheirtechnology.WehaveallheardaboutthefamousvideotapestandardsbattlebetweenBetamaxandVHS.Sony’sBetamaxwasasuperiortechnologybutVHSbecamethestandardbecauseJVCwasrelentlessinpromotingittoelectronicsmanufacturers.
Athirdsourceofstandardismarketforces,whichleadstoade-factostandard.ThiswasthecasewithTCP/IP.Itbecamethedominant,andultimatelystandard,network-communicationprotocolasaresultofgradualadoptionbyuniversitiesandbusinesses.BythetimetheISOmodelwasdeveloped,itwastoolateforaswitchover.TCP/IPwasalreadybakedin.
Andfinally,thereisthedeliberateapproachwhereexpertsgettogetherwiththespecificintentofcreatingastandard.ThisishowtheHL7standardcameintobeing.
.....Thisisapreview.RestoftheChapterisnotshown.....
PARTIIDiggingDeeper
6.MessageBuildingBlocksTotheuninitiated,thesightofanHL7messageisoftenintimidating.Abrewofsymbolsandcharacters,itlookslikesomethingoutoftheMatrixthatisbeyondthecomprehensionofmeremortals.Buttobehonest,HL7reallyisquietsimpleandstraightforward,onceyouknowhowtoreadit.Andforthat,youwillneedtolearnaboutthebuildingblocksofanHL7message.
Let’staketheexampleofregisteringanewpatient.Whenthestaffatthefrontdeskcompletesthepatientregistrationandhitsenter,ittriggersanevent:A04(Registerpatient).ThiscausesthesystemtogenerateanewADT^A04HL7message,whichlookssomethinglikethis.MSH|^~\&|SENDER_APP|SENT_BY|RECEIVER_APP|RCVD_BY|201310201500||ADT^A04|MSG_ID001|P|2.5|||AL
EVN|A04|201310201500|||ID221^Dude@Terminal
PID|1||PAT416^^^HEALTH_ID||SEBELUS^KANSAS||194801150600|M|||123SESAME.ST^^TORONTO^ON^A1A2B2^CANADA||(416)888-8088||ENGLISH|M||PAT_AC_721914
NK1|1|SEBELUS^MARY|SPOUSE|||(416)888-9999|(647)123-1234|C|20131020
PV1|1|O|ROOM10^BED12^OUTPATIENT|ELECTIVE|||S21195^DRIKOFF^FRANCIS^^^DR^MD||C90023^PAYNE^TRACY^^^DR^MD|SUR||||1|||S21195^DRIKOFF^FRANCIS^^^DR^MD||37323|SELF||||||||||||||||||||||||201310201500
PV2|||DAYSURGERY
AL1|1|FA^PEANUT||PRODUCESMILDRASH
SeewhatImean?Makesnosense.Butsoonitwill.
SegmentTheprimarybuildingblockofamessageisasegment.Asegmentissimplyarowofdatainthemessage.So,forthemessageabove,thefirstsegmentstartswithMSHandendsonlinetwowithAL.Itisactuallyjustonerowofdata,whichwaswrappedovertothesecondline.ThereisalinebreakafterALandthatmeansendofthesegment.ThesecondsegmentstartswithEVNonlinethreeandendsat“Terminal”onthesameline,followedbyalinebreakandsoon.Anewsegmentalwaysstartsonanewline.
ThefirstthreecharactersofeachsegmentisthesegmentID.ThesegmentIDisanacronymorthenametagofthesegment.
Onceweknowthesegmentname,weknowtheinformationinthat
segment.Thisisbecausethemainpurposeofasegmentistogrouprelatedinformationtogether.
Inourexamplehere,therearesevensegments(IDsbolded).MSHistheMessageHeadersegment,EVNistheEventsegment,PIDisthePatientIdentificationsegmentandsoon.WithoutevenlookingatthecontentofthePIDsegment,Icantellyouitcontainsthenameofthepatient,hishealthID,dateofbirth,phonenumber,address-basicallyalltheinformationthatcanbeusedtoidentifythepatient.Hencethenameofthesegment-PatientIdentification.MSH|^~\&|SENDER_APP|SENT_BY|RECEIVER_APP|RCVD_BY|201310201500||ADT^A04|MSG_ID001|P|2.5|||AL
EVN|A04|201310201500|||ID221^Dude@Terminal
PID|1||PAT416^^^HEALTH_ID||SEBELUS^KANSAS||194801150600|M|||123SESAME.ST^^TORONTO^ON^A1A2B2^CANADA||(416)888-8088||ENGLISH|M||PAT_AC_721914
NK1|1|SEBELUS^MARY|SPOUSE|||(416)888-9999|(647)123-1234|C|20131020
PV1|1|O|ROOM10^BED12^OUTPATIENT|ELECTIVE|||S21195^DRIKOFF^FRANCIS^^^DR^MD||C90023^PAYNE^TRACY^^^DR^MD|SUR||||1|||S21195^DRIKOFF^FRANCIS^^^DR^MD||37323|SELF||||||||||||||||||||||||201310201500
PV2|||DAYSURGERY
AL1|1|FA^PEANUT||PRODUCESMILDRASH
MessageStructureSegmentsinamessagearealwaysorganizedinaspecificorder.Thisorderiscalledthemessagestructure.Differentmessagetypeshavedifferentmessagestructuresbutsomethingsarealwaysthesame.Forexample,everymessagestartswithanMSHsegment.
Iftheorderofsegmentsinamessageisnotexactlylikeitsmessagestructure,thenthatmessagewillbecomeinvalid.Itwillberejectedbythereceivingsystem.
YoucangettheabstractmessagestructureofanymessageintheHL7specificationdocument.TheabstractmessagestructureofanADT^A04messageisinChapter3oftheHL7specificationdocumentwhereeventA04isdiscussed.
HereisapartialabstractmessagestructureofanADT^A04message.Itisjustatablewiththreecolumns:segmentID,segmentnameandthechapterwherethatsegmentisexplained.
Ifyoucomparetheexamplemessagetoitsabstractmessagestructure,thesegmentorderdoesnotmatchbetweenthetwo.Intheexamplemessage,theSFTsegmentismissingafterMSH;PD1&ROLaremissingtoo.
Doesthatmeantheexamplemessageisinvalid?No,it’snotbecausethe[]and{}bracketsaroundthosesegmentsmakethemeitheroptionalorrepeatable.
Optional/Repeatable/MandatoryTherearetwokindsofbrackets:square[]andcurly{}.IfasegmentIDisenclosedwithin[squarebrackets],itmeansthesegmentisoptional.Wecanchoosewhethertoincludethatsegmentinthemessageornot.Thesesegmentsaregenerallyforoptionalinformation,suchasPD1(additionalpatientinformation).
IfthesegmentIDisenclosedwithin{curlybrackets},thenthatsegmentisrepeatable.Wecanhavemorethanoneinstanceofthatsegmentinarealmessage.CurlybracketsareforsegmentslikeNK1(NextofKin).Ifapatienthasgivencontactinformationfortwonextofkin(spouseandsister),thentheinformationforeachnextofkinwillneedaseparateNK1segmentinthemessage.
IfasegmentIDisenclosedinboth[{squareandcurly}]bracketsthenthatmeansthesegmentisbothoptionalandrepeatable.IfasegmentIDisnotsurroundedbyanybracketthenthatmeansitisamandatorysegment.Thatsegmenthastobepresentinthemessage.SosegmentslikeMSH,EVN,PID,PV1,withnobrackets,havetobepresentinarealmessage.
Basedonthisknowledgewecanseewhytheexampleisavalidmessage.Allthemissingsegments,SFT,PD1andROLaresurroundedbysquarebrackets.Andthatmeansthosesegmentsareoptional.Wecanchoosetoleavethemout.
.....Thisisapreview.RestoftheChapterisnotshown.....
8.ControlSegmentsFromthelastchapter,ifyourememberthediscussionabouttheanatomyofamessage,controlsegmentsarethesegmentsintheheadofamessage.Theyonlycarrymeta-datainformationaboutamessage.
ThereareaboutadozencontrolsegmentsdefinedbyHL7.Theyareallexplainedinchapter2oftheHL7spec.Fortunately,weonlyneedtoknowaboutafewofthemtoaccountforthevastmajorityofcases.Forexample,therearecontrolsegmentsforbreakingaverylargemessageintosmallerpiecesandcontrolsegmentsforbatchingtogetheralargenumberofmessages.Thesecontrolsegmentsarenotusedthatfrequentlyandforageneralunderstanding,youcanskipthem.
Therearefivecontrolsegmentsthatyoureallyshouldknowabout–MSH,EVN,NTE,MSA&ERR.WewillstartwithMSH,theubiquitouscontrolsegmentthateverymessagebeginswith.Itisthemostimportantcontrolsegment.Ifyoudecidefiveistoomanyforyourprecioustimeandyouareonlygoingtoreadaboutone,thenletthisbetheone.
MessageHeaderSegment(MSH)Themessageheadersegment(MSH)isthemostimportantcontrolsegment.EveryHL7messagestartswiththissegment.WhenanHL7messageisreceivedbyasystem,itistheMSHthattellsthereceivingsystemwherethismessagecamefrom,theinformationitcontainsandhowitissupposedtobeacknowledged.
Thisisasegmentyouwanttoknowwell.Togetabetterunderstandingofthecontentsofthissegment,let’suse
theMSHsegmentfromourexampleA04messageandexploreitscontents.MSH|^~\&|SENDER_APP|SENT_BY|RECEIVER_APP|RCVD_BY|201310201500||ADT^A04|MSG_ID001|P|2.5|||AL
Ifyoubreakthesegmentdownintoitsseparatefields,itgetseasiertofigureoutthecontent.Remember|isusedtoseparatefields.
MSH-1:|(FieldSeparator)
MSH-2:^~\&(EncodingCharacters)MSH-3:SENDER_APP(SendingApplication)
MSH-4:SENT_BY(SendingFacility)
MSH-5:RECEIVER_APP(ReceivingApplication)
MSH-6:RCVD_BY(ReceivingFacility)
MSH-7:201310201500(Date/TimeofMessage)
MSH-8:
MSH-9:ADT^A04(MessageType)
MSH-10:MSG_ID001(MessageControlID)
MSH-11:P(ProcessingID)
MSH-12:2.5(VersionID)
MSH-13:
MSH-14:
MSH-15:AL(AcceptAcknowledgementType)
Notethatsomeofthefieldsareempty(e.g.MSH-8).Thisisperfectlyfine.Remember,noteveryfieldinasegmentisrequiredtohaveavalue.IfyourefertothesegmentattributetableofMSH,youcanconfirmthatallmissingfieldsareoptional.
Now,hereisalittleinsiderinformation.Thereareonlyafewfieldsineachsegmentthatarereallyimportantandregularlyused.Thatiswhyyouseetheusualpipepattern(||||)inHL7messages.Theconsecutivepipesarenothingbutemptyfields.
Sokeepingwithourtradition,andsavingyouprecioustime,wearegoingtodiscussonlythemostimportantfieldsinasegment.
IntheMSHsegment,owingtothefactthatitcontainsmostofthemeta-datainformation,therearemanyimportantfields.Itistheheaviestcontrolsegment.Someoftheseimportantfieldsarerequiredandothersareoptional,buttheyalmostalwayshaveavalue.
IfyourefertothesegmentattributetableoftheMSHsegment,HL7requiresthatthefollowingfieldsalwayshaveavalue.MSH-1:FieldSeparator
MSH-2:EncodingCharacters
MSH-7:Date/TimeOfMessage
MSH-9:MessageType
MSH-10:MessageControlID
MSH-11:ProcessingID
MSH-12:VersionID
Itiseasytofindoutwhichfieldsarerequiredinasegment.JustgotothesegmentattributetableandlookfortheletterRintheoptionality(OPT)column.
Besidestheserequiredfields,therearesomeotherfields(below)inMSH,whichareoptionalbutregularlyused.TheyareimportantandIthinkyoushouldknowaboutthem.MSH-3:SendingApplication
MSH-4:SendingFacility
MSH-5:ReceivingApplication
MSH-6:ReceivingFacility
MSH-15:AcceptAcknowledgementType
MSH-16:ApplicationAcknowledgementType
Keepinmindthoughthatthisisonlymypersonalopinion.Otherscanarguethatthereareotheroptionalfieldsthatareimportantandsomeherearenot.I’mnotdenyingit.Butfrommyexperience,IbelievethesearetheimportantfieldsintheMSHsegment.
Nowlet’sgetfamiliarwiththesefieldsbecausethenameofthefielddoesn’ttellyouevenonetenthofthestory.
MSH-1:FieldSeparatorUsually,thefirstfieldinasegmentisthefieldthatfollowsthesegmentID.Sotechnically“encodingcharacters”shouldbethefirstfieldofMSHsegment.ButwithMSH,thereisananomaly.Thefirstfield(MSH-1)alwaysdefinesthesymbolthatwillbethefieldseparator(delimiter)fortheentiremessage.IfyourememberthediscussionaboutpipedelimitersinChapter6,|isthefieldseparatorinHL7messagesandtherefore,thefirstfieldofMSH.Butitdoesn’thavetobe.Youcanchoosetohaveacomma(,)oranyothersymbolastheseparator.Ifyouchoosetouseacomma,thesegmentwilllooksomethinglikethis.MSH,^~\&,SENDER_APP,SENT_BY,RECEIVER_APP,RCVD_BY,201310201500,,ADT^A04,MSG_ID001,P,2.5,,,AL
ThiswouldbeaperfectlylegitimateHL7segment.However,|hasbecomesuchadefactostandardthatnoonereallyusesanythingbut|asthefielddelimiter.Butit’sgoodtoknowthatwehavethepowertochangeit.
MSH-2:EncodingCharactersEncodingCharactersarethefoursymbols^~\&thatHL7reservesformessageconstruction.
Thesecharactershavespecialmeaning,whichallowsapplicationsreadinganHL7messagetodistinguishbetweencomponentsandsubcomponentsofafield,readrepeatingfields,andtranslatesymbols.
Theencodingcharacters,inorder,are-ComponentSeparator(^),RepetitionSeparator(~),EscapeCharacter(\)andSubComponentSeparator(&).Thepositionofeachcharacterisfixedinthefield.Firstthecomponentseparatorthentherepetitionseparatorandsoon.
ByhavingthesesymbolsinMSH-2,wearebasicallysayingthatinthisHL7message,^willbeusedtoseparatecomponents,~willbeusedtoseparatemultipleoccurrencesofafield,\willbeusedforspecialcharactersand&willbeusedtoseparatesubcomponents.
Butshouldn’tthisbehardcodedinsystemsthatreadHL7messages,insteadofincludingitineverymessage?
Goodpoint.ThereasonencodingcharactersareincludedineveryHL7messageisbecausethesecharactersarecustomizabletoo,justlikethefieldseparator|.
HL7givesyoutheoptionofselectingyourownencodingcharacters.Ifyoudon’tlike^andwouldratherhave#asthecomponentseparatorinyourmessagesthenallyouhavetodoisreplace^with#inMSH-2.Asaresult,yourencodingcharacterswouldbe#~\&.The#symbolwillnowbethecomponentseparator.
Butthiswholediscussionispointless!Overtheyears,thesesymbolshavebecomeadefactostandard.I’llbet,manyfolkswhohavebeenworkingwithHL7foryears,don’tknowthatyoucanchangethesesymbols.Ihavenevercomeacrossamessagewhereadifferentsetofsymbolswereused.
Beforewemovetothenextfield,youneedtoknowmoreabouttheothertwoencodingcharacters–theRepetitionSeparator(~)andtheEscapeCharacter(\).
RepetitionSeparator(~):Thisisthesymbolthatseparatesmultiplevaluesinafield.Rememberthesectiononsegmentattributetableinchapter7?Somefieldsarerepeatableandtheycanhavemultiplevalues.~isthesymbolthatisusedtoseparatethosevaluesinafield.
IntheMSHsegment,fieldMSH-18andfieldMSH-21arerepeatable.Thismeans,wheneverthosefieldshavetwoormorevalues,thevalueswillbeseparatedbythe~symbol.Ifasystemreadingthemessagecomesacrossthe~symbol,itwillknowrightawaythatwhatfollowsisthenextvalueofthefield.
EscapeCharacter(\):HL7reservesencodingcharactersformessageconstructionandtheyhaveaspecialmeaninginthemessage.Whathappensifyouneedtouseoneofthosespecialcharactersaspartofthedata?The
applicationreadingthemessageisgoingtobeallconfused!!Inrealworldapplications,themosttroublesomeofthesespecial
charactersistheampersandsymbol(&).Itisusedfor“and”(asinBen&Jerry’s)andisalsoacommonlyusedsymbolinprogramminglanguageslikeHTML(whichcouldbeembeddedinanHL7message).So,soonerorlater,youareboundtocomeacrossthe&symbolinthebodyofanHL7message.
Whathappensifthesecharactersarepartofthedata?Let’sconsideranexample.
Ben&Jerry’sDiagnosticCentersendstheresultofatestasanHL7messagetotheorderinghospital.Thehospitalsystemreceivesthemessageandstartsreadingittoparsethedata(pulloutfieldvalues)andsaveittoadatabase.WhenthesystemgetstotheSendingFacilityfield(MSH-4),itwillread“Ben”andthenrunintothe&symbol.Atthatpoint,thesystemisbeingtoldthatthenameofthefacilityhasasub-component.Facilitynamesdon’thavesub-components(ifyoucheckthesegmentattributetable),soinalllikelihoodthesystemdoesn’thaveacorrespondingfieldinthedatabasetosavethevalue“Jerry’sDiagnosticCenter”.
Thisisarecipeforapplicationfailure.Let’sassumethisisafuturistic,can-handle-anythingkindofsystem,buteventhenthesystemisonlysaving“Ben”asthenameofthesendingfacility,whichisincorrect.Thedoctorreadingthelabreportwillseethat“Ben”sentthetestresult.Idon’tknowhowmuchfaithshewillhaveinthereport.
Sowhatdowedo?Wecan’taskBen&Jerry’sDiagnosticCentertochangeitsname.
Thisiswheretheescapecharactercomestotherescue.Ifcharacters,whichhavespecialmeaninginHL7,needtobetransmittedaspartofthedata,thenalloneneedstodoisreplacethecharacterwithitscorrespondingescapesequence.Thesystemreadingitwillreadtheescapesequenceandreplaceitwiththatspecialcharacterbeforesavingit.
Anescapesequenceisnothingmorethanoneormorecharacterssurroundedbytheescapecharacter(\).EveryspecialcharacterinHL7hasacorrespondingescapesequence.Therearemanyescapesequencesforformattingandhighlightingtext,andyoucanevencreatecustomescapesequences.
Herearetheescapesequencesforthesymbolswehavealreadydiscussed:
GuesshowBen&Jerry’sDiagnosticCenterwillbeencodedinanHL7message.Youwillreplacetheencodingcharacter&withitscorrespondingescapesequencesothatyouwillhave“Ben\T\Jerry’sDiagnosticCenter”encodedinthemessage.Thereceivingsystemwillrecognize\T\asanescapesequenceandreplaceitwiththe&symbolwhenthedataissavedlocally.
.....Thisisapreview.RestoftheChapterisnotshown.....
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