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In HL7 messaging, when a receiving application accepts and consumes the data in an HL7 message, it is expected to send an acknowledgment (ACK) message back to the sending application. If the sending application does not receive the ACK, it may continue sending the same message until an ACK is received.

If the receiving application encounters problems when processing the message, it sends a negative acknowledgment (NACK) message instead. NACKs are of two types.

NACK TypeIndicates a problem with...
Application Error (AE)

The message structure or the message itself. The sending application must correct this problem before resending the message.

Application Reject (AR)

Either of the following:

  • One of the fields in the MSH segment, such as MSH.9, MSH.11, or MSH.12
  • The receiving application

Structure of an ACK

MSH    Message Header
   Message Acknowledgment


Structure of a NACK

MSH    Message Header
   Message Acknowledgment



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