Health Level 7, set of international standards
Old version 2.x (non-XML):
MSH header segment
PID Patient identity
PV1 patient visit information
Tilde ~ default repetition separator
Caret ^ component separator
Pipe | field separator
Ampersand & for subcomponent separator
ADT-A01 message type and triggerm used when a patient is admitted
ADT A08 used when patient information is changed, such as address or name change, when no other trigger is needed
OBR
MSH (header) contains the type and trigger information
ACK is the expected response after system receives an HL7 message (as opposed to NACK?)