ADT^A18 — Merge Patient Information

Patient record merge: A18 declares that two separately-tracked identities belong to the same person. Retained in production but superseded in v2.4+ by A40 (Merge Patient — Patient Identifier List).

Purpose

A18 resolves identity-vigilance: the PID's MRN survives; the identifiers in MRG (MRN, account, visit) are declared obsolete. Clinical references pointing to the MRG values must be re-pointed to the surviving PID.

A18 is less precise than A40: A40 uses PID/PD1 + MRG per patient and pinpoints which identifier domain is being merged. Many US EHRs (Epic, Cerner) have migrated to A40 but keep A18 parsers for legacy sources.

Segment structure

Structure ADT_A18: MSH, EVN, PID (survivor), MRG (to be merged).

• PID: new master record — surviving identity post-merge.
• MRG-1: Prior Patient Identifier List — old identifiers now alias.
• MRG-3: Prior Patient Account Number.
• MRG-7: Prior Patient Name.

Real-world example

DOE Jonathan (MRN999888) merged into DOE John (MRN567890):

MSH|^~\&|HIS|HOSP01|EHR|CLINIC02|20260516110000||ADT^A18^ADT_A18|MSG00018001|P|2.5.1|||AL|NE
EVN|A18|20260516110000|||DRJONES^Jones^Sarah^^^DR.
PID|1||MRN567890^^^HOSP^MR||DOE^JOHN^A^^MR.||19720515|M
MRG|MRN999888^^^HOSP^MR||ACCT999888||||DOE^JONATHAN^^^MR.

Acknowledgment (ACK)

MSH|^~\&|EHR|CLINIC02|HIS|HOSP01|20260516110001||ACK^A18^ACK|ACK00018001|P|2.5.1
MSA|AA|MSG00018001

Common errors

• PID/MRG inverted: if the surviving record sits in MRG instead of PID, the master identity is lost.
• No downstream propagation: PACS, LIS, pharmacy must all re-attach their orphaned results to the surviving PID.
• A18 on a patient with an open encounter: some EHRs reject — require a window where no new clinical message arrives during the merge.
• A18 used in v2.5+ when A40 is expected: ambiguity over the merged domain (local MRN? national ID? account?).

Related ADT events

See also: ADT^A24, FHIR Patient.link.