Updated 2005-12-08 23:58:38

Health Level Seven

Started on 13May2003 by PS

HL7 is a data interchange standard for medical applications. Philips uses version 2.2, so that is what you see here. AFAICT, basic structure has not changed much with later versions. It is completely text based, and very easy to handle in Tcl. (I used Tcl8.4 features, like [expr $str eq ""])

Communication is done over TCP sockets, where every application (at least in the Philips system I know) sits and waits until the communications server opens a single channel for message traffic and keeps that open.

Each message begins with a \xb character and ends with \x1c\xd character sequence, everything in between is the message. Segments in the message are separated by a single <CR> (\xd) character. The first segment is always the message header (MSH) segment, based on that the receiver should expect other segments.

A typical message looks like:

Where you should note that the first | is actually not only a field separator, but also an indication of what the field separator is. The four characters after that specify the sub, subsub, subsubsub and subsubsubsub field separators. [split] is our big friend with all those characters.

After you receive an unsolicited message (i.e. not an answer to a query you previously sent), you must send an ACK message in response otherwise the server will not send you any more messages.

The important number being 1437549872, the message reference from the message you just received.

Message receiver/dispatcher

My HL7 software is only a client, this dispatcher will also answer query responses with an ACK message, which is illegal.

The socket/channel must be [fconfigure $channel -translation binary -blocking 0 -buffering none] in your accept connection [proc], otherwise it won't work. See socket for examples on starting a server socket.
 proc processData { channel } {
    #Do we have a full message?
    set begin [string first \xb $::peers($channel.data)]
    set end [string first \x1c $::peers($channel.data)]
    set sender ""
    set msgid ""
    if { $end > $begin } {
	set msg [string range $::peers($channel.data) [expr $begin +1] [expr $end-1]]
	foreach line [split $msg \xd] {
	    set fields [split $line |]
	    switch [lindex $fields 0] {
		MSH {
		    set sender [lindex $fields 2]
		    set msgid [lindex $fields 9]
		    set environ [lindex $fields 10]
	if { $sender ne "" && $msgid ne "" } {
            #TODO: insert check to see if the the message is a query response, those must not
            #be answered with ACK!
	    set ack "\xbMSH|^~\\&|ASTRAIA|astraia|$sender||||ACK||$environ|2.2||\xdMSA|AA|$msgid||\xd\x1c\xd"
	    puts -nonewline $channel $ack
	    set ::peers($channel.data) [string range $::peers($channel.data) [expr $end + 1] end]

            #For debugging, I write all messages to a timestamped file, so I can replay them:
	    puts $::peers($::peers($channel).tsd) [list msg [clock clicks -milliseconds] $msg]
	    puts $::peers($::peers($channel).tsd) [list out [clock clicks -milliseconds] $ack]

            #And call the message handler:
	    processMsg $msg

	} else {
	    log "No sender/msgid $sender/$msgid"

The message parser

This handles an HL7 message, one at a time.
 proc processMsg { msg } {
    global db

    #first split the message into individual segments.
    set segments [split $msg \xd]
    foreach segment $segments {
        #as it is unlikely that we don't need to split on fields, I split
        #every segment on level one (|).

	set fields [split $segment |]

        #and for easy access everything goes into an array keyed on segment ID
        #which is, of course, a list!
	lappend seg([lindex $fields 0]) $fields
	switch [lindex $fields 0] {
	    MSH {
                #this will set some variables in my environment prefixed MSH_
                #more on this later.
		hl7_set_segment_variables $fields
            default {


    #Choose what to do based on the message type from the MSH header segment.
    switch $MSH_type {
	ADT {
	    foreach pid $seg(PID) {
		#only update:
		processPID $db $pid 1
	SIU {
	    processPID $db [lindex $seg(PID) 0] 0
	    hl7_set_segment_variables [lindex $seg(AIS) 0]
	    if { [lsearch {FE50 GR01 GR02 GR03} $AIS_code] == -1 } {
		log "Niet geintereseerd in SIU $AIS_code '$AIS_description'"
	    hl7_set_segment_variables [lindex $seg(SCH) 0]
	    hl7_set_segment_variables [lindex $seg(AIL) 0]
	    hl7_set_segment_variables [lindex $seg(PID) 0]
	    switch $MSH_eventtype {
		S12 {
                    #worker code to insert appointment details into database goes here.
		S15 {
                    #worker code to delete/cancel an appointment.
		    if { [string trim $SCH_id] ne "" } {
			ns_db dml $db "delete from diary where appointmentID=[ns_dbquotevalue $SCH_id] and application=[ns_dbquotevalue $SCH_application]"

The segment parser

Using [upvar], [split] and [lindex], it is really very easy to assign each field to a variable:
 proc hl7_set_segment_variables { segment } {
    #segment contains a list of fields from a single segment (line) from the message.
    #this means, it has already been [split $line |]

    #the first element contains the segment ID:
    set seg [lindex $segment 0]

    #fields is a list with list indices (starting at one, because $seg is at 0!) and
    #desired field names. If a field is to be devided in subfields, the field name is
    #a list of field names and the field will be split again, this time with the level 2
    #character (usually the carret: ^)
    #because the messages I need to parse go no deeper than two levels, this code doesn't
    #go any further, but the same can of course be done for the next levels.

    #Note that my segments dictionaries are not complete!
    switch $seg {
	MSH {
	    set fields {
		1 encodingchars
		2 sendingapplication
		3 sendingfacility
		4 receivingapplication
		5 receivingfacility
		6 datetime
		8 {type eventtype}
		9 msgid
		10 processingid
		11 version
		12 sequencenumber
		13 continuationptr
	SCH {
	    set fields {
		1 {id application}
		16 {fillercode fillername}
		20 {enteredbycode enteredbyname}
		25 statuscode
	AIL {
	    set fields {
		1 setid
		2 actioncode
		3 {resourceid roomnumber}
		5 locationgroup
	AIS {
	    set fields {
		1 setid
		3 {code description codingsystem}
		4 startdatetime
		7 duration
		8 durationunits
		9 allowsubstitution
		10 fillerstatus
	PID {
	    set fields {
		1 setid
		2 {externalid externalid_checkdigit}
		3 {pid pid_checkdigit}
		4 alternateid
		5 {eigennaam roepnaam initialen achtervoegsels voorvoegsels titulatuur}
		7 birthdate
		8 sex
		11 {straatnaam huisnr woonplaats provincie postcode land}
		13 phonehome
		14 phonebus
		15 languagepatient
		16 maritalstatus
		17 religion
		23 birthplace
		26 citizenship
	default {
            #error or return for unknow segment types?

    #Now set the variables in the callers environment:
    foreach {fieldno names} $fields {
	if {[llength $names] == 1} {
            #A single variable name, no further split needed.

            #Use [upvar] to create/set a variable in the callers environment
	    upvar ${seg}_$names var

            #hl7string does nothing more than return an empty string when the field has
            #the 'empty' marker "" in it.
            #HL7 makes a difference between NULL values and empty string.
	    set var [hl7string [lindex $segment $fieldno]]
	} else {
            #More than one fieldname: split another level
            #Even better would have been the same syntax for each sublevel
            #and then call a function like [setsubsegment <segmentid> <fieldnames> <sepchars-for-remaining-levels>]

	    set subfields [split [lindex $segment $fieldno] ^]
	    set i 0
	    foreach name $names {
		upvar ${seg}_$name var
		set var [hl7string [lindex $subfields $i]]
		incr i

I have some helper functions too:
 proc hl7string { s } {
    if { $s eq {""} } {
	return ""
    return $s

 proc hl7dateToISO { date } {
    set dpart [string range $date 0 7]
    set tpart [string range $date 8 13]
    set res ""

    if { [string length $dpart] == 8 } {
	append res "[string range $dpart 0 3]-[string range $dpart 4 5]-[string range $dpart 6 7] "
    if { [string length $tpart] == 6 } {
	append res "[string range $tpart 0 1]:[string range $tpart 2 3]:[string range $tpart 4 5].000"
    } elseif { [string length $tpart] == 4 } {
	append res "[string range $tpart 0 1]:[string range $tpart 2 3]:00.000"

    #log "Date $date $dpart/$tpart $res"
    return $res

 proc removezeros { s } {
    regexp {^0+(.+)$} $s -> s
    return $s

-- PS.

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