Null Values in HL7V2.x

In HL7V2.x the null value is indicated by ¦""¦. When a system receives this it interprets that the particular attributes value has been changed to null as the sending system has deleted the value of that particular data element in its database. It also indicates that the data element is available in the sending system’s database but currently not valued. The receiving system on receiving it should change the current value to null for that data element. However the absence of a field (¦¦) from sending system indicates that the value of the absent element may not have changed since the last event and the receiving system should retain the current value. The absence of a field (¦¦) may also indicate that the particular data element is not available in the sending system’s database.

A few tips on interpretation of "null" value and "not available" in HL7V2.x messages for application programmers

1.If the HL7 NULL value is present in message received and if the application processing allows a field to be changed to null for this source, the Interface needs to null existing application data elements.

2. If an optional non-repeating field or entire optional segment is absent in a received message, the Interface must not null the existing application values.

3. The application Interface should not support partial updates of repeating data. If some fields in a set of repeating fields are absent or null, the Application Interface may inappropriately overlay valid data or retain unwanted values. The Application Interface must not “error” the message.

4. The Application Interface must not support partial updates of repeating prioritized segments. If some segments in a set of repeating ordered segments are absent, the Application Interface may inappropriately overlay valid data or retain unwanted values. The Application Interface must not “error” the message.

5. When a message is sent the Application Interface must value all available changed and unchanged data elements in a segment

6.The Application Interface must send all instances of a repeating field.

7. The Application Interface must send all instances of repeating prioritized segments. For segments that repeat but can be uniquely identified and updated as an atomic unit, the Application Interface may send only the segment that has changed.

8.The Application Interface must send a null value for attributes that are present in the application database but have a null value. The option to send the HL7 null value need to be configurable by feed and not by field. The Application Interface must interpret zeros in a date field as null and zeros in an Application coded value field as null; however, the Application Interface must interpret zeros in a numeric field as a valid value.