HL7 v2 trigger events

A01 means a patient was admitted. R01 means a lab result is ready. Codes like these ride in every message and say what happened to cause it.

What a trigger event is

When a patient is admitted, the admitting system puts A01 into MSH-9.2 and sends the message. The receiver reads that code and knows to open a new visit rather than update one or close it. Change the code to A03 and the very same segments now say the patient has gone home.

That is the whole job of a trigger event: the segments carry the patient's details, and this one code says what to do with them.

The event is not the same as the message structure it fires, which is the part that catches people out. One structure usually serves several events. ADT_A01 carries A01, A04, A08 and A13, so the shape of the message tells you how to parse it, and the event tells you what to do about it.

The ones you will meet

There are 266 trigger events. Most integrations start with these 7.

• A01Admit/Visit Notification

  A patient has been admitted and given a bed. This is the event that starts an inpatient stay, and it is the one most systems treat as "this person is now here".

  Use A04 instead if the patient is being registered for an outpatient or emergency visit and is not occupying a bed.

• A03Discharge/End Visit

  The patient has left. The visit is over, and the message usually carries the discharge time and disposition.

• A04Register a Patient

  A patient has been registered for a visit without being admitted to a bed: an outpatient clinic, an emergency attendance, a day case.

  Carries the same segments as A01. What differs is the meaning, not the shape of the message.

• A08Update Patient Information

  Something already sent has changed: a corrected name, a new address, an updated insurer. The receiver is expected to overwrite what it holds.

  A08 updates visit and patient information together. A31 updates the person record without reference to a particular visit.

• R01Unsolicited Observation Message

  Results are being sent without anyone having asked for them at that moment: a lab releasing a panel, a device reporting a reading. The producing system pushes; the receiver did not query.

• O01general order message

  An order is being placed, changed or cancelled. Which of those it is comes from the order control code in ORC-1, not from the trigger event.

• S12SIU

  A new appointment has been booked.

Every trigger event

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