HL7 ADT_A18 (Merge Patient Information) | Integration Soup HL7 Reference Guide

HL7 ADT_A18 Merge Patient Information

An older merge event retained mostly for backward compatibility.

HL7 message structure ADT_A18 groups and segments from HL7 v2.5.1 Hide structure

These are the generated groups and segments for the version selected at the top of the page. The article explains the workflow, and this panel follows the chosen HL7 version.

Message Structure

Segment	Name	Required	Repeatable
MSH	Message Header	Yes	No
SFT	Software Segment	No	Yes
EVN	Event Type	Yes	No
PID	Patient Identification	Yes	No
PD1	Patient Additional Demographic	No	No
MRG	Merge Patient Information	Yes	No
PV1	Patient Visit	Yes	No

ADT_A18 is a legacy merge-patient-information event. In modern v2 interfaces you are more likely to see the more specific merge, move, and change events such as A40 through A45, A47, A49, A50, and A51. Still, A18 appears in older profiles and in systems whose ADT implementation predates the cleaner split of identity events.

The intent is the same dangerous-but-necessary merge workflow: one patient identity was wrong or duplicated, and downstream systems need to reconcile prior data with the correct target identity.

A small A18 example

MSH|^~\&|MPI|CITYHOSP|EHR|CITYHOSP|20260715133000||ADT^A18^ADT_A18|ADT180001|P|2.5.1 EVN|A18|20260715132900 PID|1||123456^^^CITYHOSP^MR||Smith^Jane^Anne^^Ms^^L||19800314|F MRG|999888^^^CITYHOSP^MR PV1|1|I|WARD1^101^1^CITYHOSP^^BED||||12345^Ng^Grace|||||||||||VN000345^^^CITYHOSP^VN

What systems do with it

The sender is usually an MPI, registration master, or EHR identity component. Receivers use PID as the surviving patient identity and MRG as prior identity context, then re-point or reconcile records according to local policy.

Because A18 is broad, many receivers constrain it tightly or map it internally to more explicit merge operations. A rejected A18 is often better than a silent wrong merge.

How to read the structure

The local ADT_A18 structure is compact: MSH, optional SFT if present in data, EVN, PID, optional PD1, required MRG, and required PV1. The merge direction must be documented in the interface agreement, not inferred from a friendly display name.

Implementation traps

The biggest trap is treating a legacy merge as a normal demographic update. It is not. It can move documents, results, appointments, charges, orders, and image links. Put monitoring around it, preserve audit data, and make the receiver's target/prior interpretation explicit.

Reference notes

HL7 terminology marks A18 as deprecated, and later guidance favors the more specific merge and identifier-change events. The local v2.5.1 data still includes ADT_A18 because older interfaces may need to read or document it.