^A40
ADT^A40 — Merge Patient - Patient Identifier List
Sent to merge two patient records. The surviving patient (in PID.3) absorbs the information from the prior patient (in MRG.1). All clinical data associated with the prior patient identifier should be reassigned to the surviving patient.
When Is This Sent?
When a duplicate patient record is identified and needs to be merged into the correct record. This is the standard merge message in HL7 2.3.1 and later, replacing the deprecated A18 and A34.
Real-World Usage
Patient merges are one of the most high-risk operations in healthcare IT. A failed or partial merge can result in split patient records — some clinical data under one MRN, other data under another. Every downstream system that stores patient data must handle A40 messages. Test merge scenarios thoroughly, including reversals if the merge was done in error.
Message Structure
Segment names link to their field-level reference pages.
| # | Segment | Purpose in this message | Req | Repeat |
|---|---|---|---|---|
| 1 | MSH | Message header with ADT^A40 in MSH.9 | R | — |
| 2 | EVN | Event type with A40 trigger | R | — |
| 3 | PID | The surviving patient record — demographics and identifiers that will persist | R | — |
| 4 | MRG | The prior patient identifier(s) being merged into the surviving record (MRG.1) | R | — |
| 5 | PV1 | Visit information if this is an encounter-specific merge | O | — |
Example Message
Realistic example with fake patient data. Paste into the HL7 Message Viewer to explore interactively.
1MSH||^~\&|EPIC|HOSPITAL_A|MIRTH_PROD|PACS|20260312100000||ADT^A40^ADT_A40|MSG00025|P|2.5.1 2EVN||A40|20260312100000 3PID||1||123456^^^HOSP_A^MR||Smith^John^M^^Mr.||19850315|M 4MRG||999999^^^HOSP_A^MR
Troubleshooting Scenarios
Merge creates duplicate records instead of merging
Cause
MRG.1 identifier format doesn't match how the receiving system stores the old patient's MRN. Assigning authority or ID type mismatch is the most common cause.
Fix
Compare MRG.1 format against how the receiving system stores patient identifiers. Check: (1) assigning authority matches exactly, (2) ID type code (MR) matches, (3) no leading/trailing spaces.
Only some downstream systems process the merge
Cause
A40 isn't being distributed to all downstream systems. The merge occurs in PACS but not in the pharmacy system.
Fix
Verify A40 is included in the distribution list for every downstream system that stores patient identifiers. Create a merge event checklist and test each system.
Common Confusions
ADT^A40 (Merge patient identifier list) vs ADT^A34 (Merge patient - patient ID only) vs ADT^A44 (Move account). A40 is the standard modern merge — use this. A34 is an older version that only merges patient IDs. A44 moves an account/encounter from one patient to another without merging the patient records themselves.
Related Message Types
Segment Reference
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