Fields

A field is a string of characters. HL7 is not concerned with how a system actually stores data within an application. When fields are transmitted, they are sent as character strings. Except where noted, HL7 data fields may take on the null value. Sending the null value, which is transmitted as two double quote marks (""), is different from omitting an optional data field. The difference appears when the contents of a message is used to update a record in a database rather than create a new one. If no value is sent, (that is, it is omitted) the old value should remain unchanged. If the null value is sent, the old value should be changed to null.

Data field characteristics are as follows:

• Position: The ordinal position or sequence of the data field within the segment.

• Maximum Length: Maximum number of characters that one occurrence of the data field may occupy.

• Type of Data: Restrictions on the contents of the data field. There are a number of data types defined by HL7, for example TX (text data), SN (structured numeric), PT (processing time), DT (date), and so on.

• Optionality: Whether the field is required, optional, or conditional in a segment.

• Repetition: Whether the field may or may not repeat, or the number of times the field may repeat.

• Table: The manner in which HL7 defines the valid values for tables varies depending on institution, data type, reference, and so forth.

• ID Number: Small integer that uniquely identifies the data field throughout the Standard.

• Name : Descriptive name for the field.