MSH-3 in HL7 v2: A Field Guide to Sending Application
MSH-3 is often the field an interface engine uses first to decide which routing rule to apply — before it even looks at message type — which makes it a quiet single point of failure whenever a source system gets renamed or replaced.
MSH-3 is the third field of the MSH segment — a short identifier naming the application system that originated the message, such as an ADT system or lab system.
What this field contains
Example
The 3rd field, ADTSYS, identifies the sending application as the hospital's ADT system. Synthetic message — not a real transmission.
Where this trips people up
Interface engines frequently build routing tables keyed on the exact string in MSH-3, so when a source system is upgraded, migrated, or simply renamed, every rule keyed to the old identifier stops matching at once — messages either pile up unrouted or fall through to a default rule that was never meant to handle that volume.