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.....