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Segments

Each line in a message is referred to as a Segment, with each one having its own semantic purpose. This means that it contains information of a specific type. For example:

• MSH segment contains information about the Sender and Receiver of the message, the type of the message, a time stamp, etc.
• EVN contains information about the type of message; for example, A04 (Register a patient). Information contained in EVN is duplicated in MSH, so starting from HL7 version 2.3 this segment is excluded from all message definitions.
• PID contains demographic information about the patient such as name, id codes, address, etc.
• PV1 contains information regarding the patient's stay in the hospital, such as location assigned, referring doctor, etc.

As you can see, the sample message in the previous section contains MSH, EVN, PID, NK1 and PV1 segments. There are more then 120 segments defined in version 2.3 of the standard.

	Segment Definition Segments are the units that comprise a message. A segment is defined as a sequence of fields that also may or may not repeat. An HL7 message definition also states whether each segment is mandatory or not.

Segments consist of fields that are composites. Composites are delimited by "|" (pipe) characters. Each field has its own unique purpose and is defined by the HL7 standard for each segment.

The PID segment in HL7 version 2.3 consists of thirty composite fields. Only two of them are mandatory: Patient Name and Patient ID. As you can see, some fields consist of a single value (e.g. field 8), while the other fields consist of a number of values delimited by ^ (e.g. field 5, patient name).

	In order to be as flexible as possible and achieve a consensus, the HL7 committees were forced to define many fields as optional. The downside of this decision is that you cannot be certain that particular information will be present in a given message. This is one of the reasons why the same message may vary significantly from vendor to vendor.