HL7 ACK Message: Acknowledgment & Errors
HL7 ACK (Acknowledgment) messages confirm receipt and processing of HL7 v2 messages. Every production HL7 interface relies on acknowledgment handling for reliable message delivery — without proper ACK processing, messages can be silently lost or duplicated.
ACK Message Structure
Section titled “ACK Message Structure”| Segment | Name | Required | Purpose |
|---|---|---|---|
| MSH | Message Header | Yes | Sender, receiver, message type, version |
| MSA | Message Acknowledgment | Yes | Accept/reject code and correlation ID |
| ERR | Error | No | Detailed error information (when MSA-1 is AE or AR) |
The ACK message is the simplest HL7 v2 message type — just 2-3 segments. However, its correct implementation is critical to interface reliability.
MSA Segment: Message Acknowledgment
Section titled “MSA Segment: Message Acknowledgment”| Field | ID | Data Type | Description |
|---|---|---|---|
| Acknowledgment Code | MSA-1 | ID | AA, AE, or AR |
| Message Control ID | MSA-2 | ST | Echoes MSH-10 of the original message |
| Text Message | MSA-3 | ST | Optional error description |
| Expected Sequence Number | MSA-4 | NM | Sequence tracking (rare) |
| Error Condition | MSA-6 | CE | Deprecated — use ERR segment |
MSA-1: Acknowledgment Codes
Section titled “MSA-1: Acknowledgment Codes”| Code | Meaning | Action Required |
|---|---|---|
| AA | Application Accept | Message processed successfully — no action needed |
| AE | Application Error | Message parsed but could not be processed — review error and potentially resend |
| AR | Application Reject | Message rejected — structural or protocol problem, do not resend without fixing |
MSA-2: Control ID Correlation
Section titled “MSA-2: Control ID Correlation”MSA-2 echoes the MSH-10 (Message Control ID) of the original message. This is how the sending system correlates an ACK to the message that triggered it:
Original: MSH|...|MSG00001|P|2.5.1Response: MSA|AA|MSG00001If MSA-2 does not match any pending MSH-10, the sender should log a correlation error and alert operators.
ERR Segment: Error Details
Section titled “ERR Segment: Error Details”When MSA-1 is AE or AR, the ERR segment provides diagnostic information:
| Field | ID | Data Type | Description |
|---|---|---|---|
| Error Code and Location | ERR-1 | ELD | Deprecated in v2.5+ |
| Error Location | ERR-2 | ERL | Segment^Sequence^Field^Component |
| HL7 Error Code | ERR-3 | CWE | Error category from HL7 Table 0357 |
| Severity | ERR-4 | ID | W (Warning), I (Information), E (Error) |
| Application Error Code | ERR-5 | CWE | Application-specific error code |
| Application Error Parameter | ERR-6 | ST | Additional error context |
| Diagnostic Information | ERR-7 | TX | Human-readable error description |
| User Message | ERR-8 | TX | Message suitable for display to end user |
ERR-3: Common HL7 Error Codes (Table 0357)
Section titled “ERR-3: Common HL7 Error Codes (Table 0357)”| Code | Meaning | Description |
|---|---|---|
| 100 | Segment sequence error | Segments in wrong order |
| 101 | Required field missing | Required field not populated |
| 102 | Data type error | Field value doesn’t match expected data type |
| 103 | Table value not found | Coded value not in expected table |
| 200 | Unsupported message type | Receiving system doesn’t handle this message type |
| 201 | Unsupported event code | Trigger event not supported |
| 202 | Unsupported processing ID | Processing ID (P/T/D) not accepted |
| 203 | Unsupported version ID | HL7 version not supported |
| 207 | Application internal error | Unspecified processing failure |
Original vs Enhanced Acknowledgment
Section titled “Original vs Enhanced Acknowledgment”HL7 v2 supports two acknowledgment modes, controlled by MSH-15 and MSH-16:
Original Mode (Most Common)
Section titled “Original Mode (Most Common)”A single ACK for each message. MSH-15 = AL (Always) means the receiver sends one ACK. MSH-16 = NE (Never) means no separate application-level ACK. This is the configuration used by the vast majority of HL7 interfaces.
Original Acknowledgment Mode
Sender Source System
Receiver Destination System
ADT^A01 MSH-15=AL, MSH-16=NE
ACK (MSA-1=AA) Single acknowledgment
sender receiver
ADT^A01 MSH-15=AL, MSH-16=NE receiver sender
ACK (MSA-1=AA) Single acknowledgment Enhanced Mode
Section titled “Enhanced Mode”Two separate acknowledgments — one for transport, one for application. Enhanced mode adds complexity but provides confirmation that the message was both received (transport) and successfully processed (application). In practice, enhanced mode is rare.
Enhanced Acknowledgment Mode
Sender Source System
Receiver Destination System
ADT^A01 MSH-15=AL, MSH-16=AL
ACK (MSA-1=CA) Accept ACK (transport)
ACK (MSA-1=AA) Application ACK (processing)
Confirms message was both received AND processed
sender receiver
ADT^A01 MSH-15=AL, MSH-16=AL receiver sender
ACK (MSA-1=CA) Accept ACK (transport) receiver sender
ACK (MSA-1=AA) Application ACK (processing) Confirms message was both received AND processed Sample ACK Messages
Section titled “Sample ACK Messages”Successful Acknowledgment (AA)
Section titled “Successful Acknowledgment (AA)” ACK — Application Accept (AA)
Header Patient Clinical Order/Result Financial
| Field | Name | Value | Note |
|---|---|---|---|
MSH-3 | Sending Application | LAB_SYS | Responding system |
MSH-9 | Message Type | ACK^A01^ACK | ACK responding to an ADT^A01 |
MSH-10 | Message Control ID | ACK00001 | Unique ID for this ACK message |
| Field | Name | Value | Note |
|---|---|---|---|
MSA-1 | Acknowledgment Code | AA | Application Accept — message processed successfully |
MSA-2 | Message Control ID | MSG00001 | Echoes MSH-10 of the original message for correlation |
Error Acknowledgment (AE)
Section titled “Error Acknowledgment (AE)” ACK — Application Error (AE)
Header Patient Clinical Order/Result Financial
| Field | Name | Value | Note |
|---|---|---|---|
MSH-9 | Message Type | ACK^A01^ACK | |
MSH-10 | Message Control ID | ACK00002 |
| Field | Name | Value | Note |
|---|---|---|---|
MSA-1 | Acknowledgment Code | AE | Application Error — parsed but could not process |
MSA-2 | Message Control ID | MSG00002 | Correlates to the original message |
MSA-3 | Text Message | Patient MRN not found in system | Human-readable error description |
| Field | Name | Value | Note |
|---|---|---|---|
ERR-3 | HL7 Error Code | 101^Required field missing^HL70357 | HL7 Table 0357 error code |
ERR-4 | Severity | E | E=Error, W=Warning, I=Information |
ERR-7 | Diagnostic Information | PID-3 contains unknown identifier |
Reject Acknowledgment (AR)
Section titled “Reject Acknowledgment (AR)” ACK — Application Reject (AR)
Header Patient Clinical Order/Result Financial
| Field | Name | Value | Note |
|---|---|---|---|
MSH-9 | Message Type | ACK^A01^ACK | |
MSH-10 | Message Control ID | ACK00003 |
| Field | Name | Value | Note |
|---|---|---|---|
MSA-1 | Acknowledgment Code | AR | Application Reject — structural/protocol problem, do not resend |
MSA-2 | Message Control ID | MSG00003 | |
MSA-3 | Text Message | Unsupported message type |
| Field | Name | Value | Note |
|---|---|---|---|
ERR-3 | HL7 Error Code | 200^Unsupported message type^HL70357 | Receiving system does not handle this message type |
ERR-4 | Severity | E | |
ERR-7 | Diagnostic Information | MDM messages not configured for this channel |
Key Implementation Considerations
Section titled “Key Implementation Considerations”Retry Logic and Timeouts
Section titled “Retry Logic and Timeouts”When a sender does not receive an ACK within the configured timeout:
- Wait: Allow adequate time (typically 30-120 seconds depending on message type)
- Retry: Resend the original message with the same MSH-10 control ID
- Limit retries: Cap at 3-5 attempts before alerting operators
- Exponential backoff: Increase delay between retries (30s → 60s → 120s)
The receiving system should use MSH-10 deduplication to handle retransmitted messages — processing the first instance and returning AA for subsequent duplicates without reprocessing.
AE vs AR: When to Resend
Section titled “AE vs AR: When to Resend”| ACK Code | Should Resend? | Rationale |
|---|---|---|
| AA | No | Message processed successfully |
| AE | Maybe | Data issue — fix the message content and resend, or investigate the error. Same message will likely fail again. |
| AR | No | Structural/protocol rejection — message cannot be processed as-is. Fix the root cause before resending. |
Acknowledgment Queuing
Section titled “Acknowledgment Queuing”Integration engines (Mirth Connect, Rhapsody, etc.) typically manage ACK processing automatically:
- Outbound channels: Track pending ACKs per message, trigger retry on timeout or AE/AR
- Inbound channels: Generate ACK immediately after parsing (AA) or after processing (AA/AE depending on configuration)
- Queue depth monitoring: Alert when unacknowledged messages exceed thresholds — indicates downstream system issues
Common Pitfalls
Section titled “Common Pitfalls”- Missing MSA-2 correlation: Sending an ACK without echoing MSH-10 makes it impossible for the sender to match the response
- Ignoring AE responses: Treating all non-AR responses as success leads to silently lost data
- Timeout too short: Network latency and processing time vary — timeouts under 30 seconds cause unnecessary retransmissions
- No deduplication: Failing to check MSH-10 for duplicates can result in double-processing (duplicate orders, duplicate charges)
Related Resources
Section titled “Related Resources” MSH Segment Reference MSH-15 and MSH-16 control acknowledgment behavior — accept and application acknowledgment type fields.
HL7 Integration Services HL7 v2 integration engineering — acknowledgment handling, error monitoring, and interface reliability.
HL7 Resource Center Message types, segments, reference guides, and integration tools.