Negative Acknowledgments, known as NACKs, let the sending system know that there was an error while processing the message.

If there were problems processing the message, a negative ACK message is sent to the sending system.

The HL7 standard defines two types of negative ACK messages that can be sent:

• An AE (Application Error) message indicates that there was a problem processing the message. This could be related to the message structure, or the message itself. The sending application must correct this problem before attempting to resend the message.
• An AR (Application Reject) message indicates one of two things: either there is a problem with field 9, field 11 or field 12 of the MSH segment, or there is a problem with the receiving application that is not related to the message or its structure.