QSU^Q01 — Cancel Subscription / Acknowledge Message

Server-side, in continuous query / unsolicited update mode: QSU notifies the client that its subscription is cancelled by the server.

Purpose

QSU is emitted by an HL7 server when it unilaterally terminates a subscription previously registered by the client (typically via a QSB message). Typical cases:

• subscribed-event quota reached;
• security policy: the client is no longer authorised to receive this data;
• server maintenance or decommissioning;
• time-based subscription expiry.

The QSB → QSU pattern is the HL7 v2 ancestor of FHIR R5 Subscription: instead of a REST push, the server emits messages triggered by events.

Segment structure

QSU_Q01
  MSH                    Message Header (mandatory)
  [ SFT ]                Software Segment (optional)
  MSA                    Message Acknowledgment (mandatory)
  [ { ERR } ]            Error (optional, repeatable)
  QID                    Query Identification (mandatory)

MSH — MSA

• MSH-9: QSU^Q01^QSU_Q01.
• MSA-1: return code (table 0008).
• MSA-2: identifier of the original subscription.

QID — Query Identification

• QID-1: query identifier (same as the one in the originating QSB).
• QID-2: server-side logical query name.

Real-world example

The EHR server cancels subscription SUBSCRIB-001 of the cardiac monitor (the session has reached its expiry):

MSH|^~\&|EHR|HOSP01|MONITOR|CLINIC02|20260515153000||QSU^Q01^QSU_Q01|MSG00015001|P|2.5.1
MSA|AA|SUBSCRIB-001
QID|SUBSCRIB-001|Subscribe-OBX-CARDIAC^Continuous query for cardiac OBX^L

Full subscription lifecycle

1. QSB — client subscribes to an event type.
2. ACK — server confirms subscription.
3. Server then emits unsolicited messages to the client on each event (for instance ORU for lab results).
4. QSU — server cancels the subscription when it chooses.
5. QSX — client may also cancel unilaterally.

Acknowledgment (ACK)

MSH|^~\&|MONITOR|CLINIC02|EHR|HOSP01|20260515153001||ACK^Q01^ACK|ACK00015001|P|2.5.1
MSA|AA|MSG00015001

Common errors

• QSU without QID: the client does not know which subscription to cancel.
• MSA-2 does not match the originating QSB: ambiguous on the client side.
• Continuing to send messages after QSU: once cancelled, no further unsolicited message must be emitted for that QID.

See also: QRY^A19, ADR^A19, and FHIR Subscription.