HL7 v2 message types

ADT^A01^ADT_A01 sits in MSH-9 of every admission message. It looks like the same thing written three times. It is not.

Reading MSH-9

• ADTthe message type: what kind of message this is. ADT covers admissions, discharges and transfers.
• A01the trigger event: the real-world thing that caused it to be sent. A01 is a patient being admitted.
• ADT_A01the message structure: which segments to expect and in what order.

The second and third look alike here and often differ. One structure usually serves several events, so an A04 registration and an A08 update both arrive shaped asADT_A01. The structure is what a parser needs; the event is what tells you why the message exists.

The ones you will meet

There are 123 message types. Most integrations touch these 7.

• ADTAdmit, Discharge, Transfer

  ADT carries patient administration events: a patient arriving, moving between wards, being discharged, or having their demographics corrected. It is the busiest interface in most hospitals, because almost every other system needs to know who the patient is and where they are.

• ORUObservation Result, Unsolicited

  ORU delivers results that nobody asked for at that moment: a laboratory sending back a completed panel, a device reporting a reading, a radiology system releasing a report. The receiving system did not query; the producing system pushed.

• ORMOrder Message

  ORM places, changes and cancels orders. It is the message that says "do this test" and, through the order control code in ORC-1, the message that later says "actually, do not".

• SIUScheduling Information Unsolicited

  SIU reports what has happened to an appointment: booked, rescheduled, cancelled, or the patient failed to attend. Each of those is a separate trigger event on the same message type.

• MDMMedical Document Management

  MDM moves clinical documents and their status: a dictated note becoming available, a transcription being edited, a document being replaced. The document itself travels in OBX segments beneath a TXA header.

• DFTDetailed Financial Transaction

  DFT posts charges and financial transactions against a patient account: a procedure performed, a supply used, an adjustment made. The transactions themselves are FT1 segments.

• ACKGeneral Acknowledgment

  ACK is the reply that says a message arrived and what happened to it. MSA-1 carries the acknowledgment code and MSA-2 echoes the control ID of the message being answered.

Every message type

123 in all, with the name the standard gives each one in table 0076. Clicking one shows the structures it can take and the events that fire them.