These examples are a small portion of possible HL7 messages that may be seen in sending data from one system to another. Understanding the basics can assist in troubleshooting some application issues. First an ADT message has to be sent to the coding system to register the patient. It must include a MSH segment, an EVN segment and a PID segment. The MSH segment is the header that all records must have and identifies the system it is coming from. The A04 trigger event identifies this as an outpatient registration record. The receiving system should create an error message if this patient already exists in the system. The PID segment has the patient demographic information. The PV1 segment has information about the patients stay. Each field divider |, indicates a new sequence. The area after the first | in each segment is numbered. So you would refer to the fields in the PID segment as PID-1, PID-2, PID-3, etc, following the field dividers.