Once you have a good sample of HL7 messages, the next step is to use Chameleon to define the different message types, and to specify what data you wish to extract from each message type. This information is summarized in a Chameleon Visual Message Definition (VMD) file.

In this section of the tutorial, you will set up a VMD file for your application. The following sample HL7 message will be used:

MSH|^~\&||.|||199908180016||ADT^A04|ADT.1.1698593|P|2.5
PID|1||000395122||LEVERKUHN^ADRIAN^C^^^||19880517180606|M|^^^^^||6 66TH AVE NE^^WEIMAR^DL^98052||(157)983-3296|||S||12354768|87654321
NK1|1|TALLIS^THOMAS^C|GRANDFATHER|12914 SPEM ST^^ALIUM^IN^98052|(157)883-6176
NK1|2|WEBERN^ANTON|SON|12 STRASSE MUSIK^^VIENNA^AUS^11212|(123)456-7890
PV1|1|E|EMG-W^^|1||||||||||ER||||ER||H|||||||||||||||||||OVL||REG|||199908180015
GT1|1||SMITH^JAMES^M||12914 164TH AVE NE^^RICHMOND^ON^98052|(157)883-6176|||||F|535-52-9776||||WEISS JENSON|.^^WELLINGTON^ON^.|(206)340-9577 
IN1|1|PRE2|001|LIFE PRUDENT BUYER|PO BOX 23523^WELLINGTON^ON^98111|||19601||||||||THOMAS^JAMES^M|F|||||||||||||||||||ZKA535529776

An HL7 message is a sequence of segments, each of which begins with a three-letter label that identifies the segment type. These segments are terminated by a segment terminator, which is normally a carriage return character.

Each segment in a message is divided into composites, or fields, and the fields are separated by pipe characters ('|'). As you can see, many of the fields in the sample message are empty, and this is frequently the case in the real world as well. Fields may be further subdivided into subfields, delimited by the caret ('^'), and subsubfields, delimited by the ampersand ('&').

Every HL7 message begins with a MSH segment that contains basic information about the message, such as the time it was issued and the message type. In the sample message shown above, the MSH segment indicates that the message type is ADT^A04 (Register Patient), which is the message type that your application will be processing.

As you can see, HL7 messages are comprised of readable characters. However, they are not really designed to be read by people. It is easy to lose your place when counting pipe characters, and it is impossible to determine the purpose of any information contained in the message. For example, if a field contains a name, there is no way to tell whether it is the name of the patient, the next of kin, the physician, or someone else. This purpose could be determined by consulting the appropriate segment definition in the HL7 standard, but this quickly becomes tedious.

With Chameleon, these problems disappear. The Chameleon Message Browser is a handy utility that makes both the structure and the content of HL7 messages plainly evident.

	For more information on HL7 messages, see HL7 Overview.