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determine WebPAS message type;

In MSH segment check the 9th field, this is meant to contain

<message type (ID)> ^ <trigger event (ID)>

(see https://www.hl7.org/documentcenter/public_temp_B5A64101-1C23-BA17-0C953B25BD420909/wg/conf/HL7MSH.htm)

our PAS is a knob and has it like this instead:

Message	Description	Message Type
A01	Admit a Patient	ADT^A01^ADT_A01
A02	Transfer a Patient	ADT^A02^ADT_A02
A03	Discharge a Patient	ADT^A03^ADT_A03
A05	Pre-admit a Patient	ADT^A05^ADT_A01
A08	Update patient Information	ADT^A08^ADT_A01
A11	Cancel Admit	ADT^A11^ADT_A09
A12	Cancel Transfer	ADT^A12^ADT_A12
A13	Cancel Discharge	ADT^A13^ADT_A01
A14	Pending Pre-Admission	ADT^A14^ADT_A01
A21	On Leave	ADT^A21^ADT_A02
A22	Return from Leave	ADT^A22^ADT_A02
A27	Cancel Pending Pre-Admission	ADT^A27^ADT_A02
A28	Add Person Information	ADT^A28^ADT_A28
A31	Update person Information	ADT^A31^ADT_A01 OR ADT^Axx^ADT_01 when using alternate “A” message
A34	Merge Person Information (Patient ID Only)	ADT^A34^ADT_A30
A44	Change U/R for a Visit	ADT^A44^ADT_A43
M02	Practitioner Message	MFN^M02^MFN_M02
S12	Notification of New Appointment Booking	SIU^S12^SIU_S12
S14	Notification of Appointment Modification	SIU^S14^SIU_S12
S15	Notification of Appointment Cancellation	SIU^S15^SIU_S12

Just check the first two fields.  The second appears to indicate the event that raised the message, for example admitting someone in PAS (admission event = A01) may send out:

preadmit message ADT^A05^ADT_A01
then update patient info ADT^A31^ADT_A01
then admission message ADT^A01^ADT_A01

The ADT events we mostly care about:

A01 Admit / visit notification
A02 Transfer a patient
A03 Discharge/end visit
A04 Registration (e.g. ED visit where not admitted)
A05 Pre-admit a patient
A08 Update patient information
A18 Merge patient information
A19 QRY/ADR - Patient query
A31 Update person information
A34 Merge patient information - patient I

full list