ADT^A01 — Admit / Visit Notification

The message that notifies the admission of a patient into a healthcare facility. The pivot of hospital identity vigilance throughout every HL7 v2 install base.

Purpose

A01 fires when the HIS confirms the admission of a patient — whether a scheduled hospitalisation, an emergency admission, or the conversion of a pre-admission (A05) into a firm admission. The message broadcasts to all downstream consumers:

• the patient identity (PID);
• the visit context (PV1): admission type, ward, bed, attending physician;
• the admission timestamp (EVN);
• optionally the associated entities (NK1 next of kin, GT1 guarantor, IN1/IN2 insurance, AL1 allergies, DG1 admission diagnosis, ROL roles, OBX admission observations).

A01 is not suitable for pre-admissions, transfers or discharges: those events have their own messages (A05, A02, A03…) with the same segment structure but distinct trigger events.

Segment structure

Per chapter 3 of the HL7 v2.5.1 specification, the abstract structure ADT_A01 is:

ADT_A01
  MSH                    Message Header (mandatory)
  [ SFT ]                Software Segment (optional, R5+)
  EVN                    Event Type (mandatory)
  PID                    Patient Identification (mandatory)
  [ PD1 ]                Patient Additional Demographics (optional)
  [ { ROL } ]            Role (optional, repeatable)
  [ { NK1 } ]            Next of Kin (optional, repeatable)
  PV1                    Patient Visit (mandatory)
  [ PV2 ]                Patient Visit - Additional Info (optional)
  [ { ROL } ]            Role (optional, repeatable)
  [ { DB1 } ]            Disability (optional, repeatable)
  [ { OBX } ]            Observation / Result (optional, repeatable)
  [ { AL1 } ]            Patient Allergy Information (optional, repeatable)
  [ { DG1 } ]            Diagnosis (optional, repeatable)
  [ DRG ]                Diagnosis Related Group (optional)
  [ { PR1 [ { ROL } ] } ] Procedure (optional, repeatable)
  [ { GT1 } ]            Guarantor (optional, repeatable)
  [ { IN1 [IN2] [IN3] } ] Insurance (optional, repeatable)
  [ ACC ]                Accident (optional)
  [ UB1 ]                UB82 Data (optional)
  [ UB2 ]                UB92 Data (optional)
  [ PDA ]                Patient Death and Autopsy (optional)

MSH — Message Header

Mandatory segment: carries routing semantics and version. Notable v2.5.1 fields for A01:

• MSH-9: ADT^A01^ADT_A01 — message code ^ trigger event ^ structure name.
• MSH-10: message control identifier unique within the sending system (idempotency key for replays).
• MSH-11: P in production, T in test, D in debug.
• MSH-12: 2.5.1 (literal string).
• MSH-15 / MSH-16: Accept Acknowledgment Type and Application Acknowledgment Type. AL + NE is the most common combination in real-time push: ACK on receipt, no application ACK.

EVN — Event Type

Describes the clinical event that fired the message:

• EVN-1: event code, here A01 (deprecated in v2.5+ in favour of MSH-9.2 but still emitted for backward compatibility).
• EVN-2: HIS-side recording timestamp (CCYYMMDDHHMMSS).
• EVN-4: event reason (table 0062, optional).
• EVN-5: operator (HIS user) — XCN component, holds an identifier and a name.
• EVN-6: real clinical event timestamp (may differ from EVN-2 if recording is retroactive).

PID — Patient Identification

Core segment carrying the patient identity. In v2.5.1, PID has 39 fields (vs 30 in v2.3.1). Critical fields:

• PID-3: list of patient identifiers (CX repeating). Typically includes the local facility MRN (MR), the national identifier (NI, the INS-NIR in France, the SSN in the US), and possibly a regional identifier.
• PID-5: patient name (XPN type). At least one instance with usage L (Legal).
• PID-7: date of birth (TS, day precision or finer).
• PID-8: administrative sex (table 0001: F, M, O, U, A, N).
• PID-11: addresses (XAD repeating).
• PID-13: phone numbers (XTN repeating).
• PID-18: patient account number (CX) — the encounter account, not to be confused with PID-3 which carries the durable identity.
• PID-19: SSN (US-only, deprecated for PID-3 in v2.5+).

PV1 — Patient Visit

Describes the visit. Key fields for an A01 admission:

• PV1-2: patient class (table 0004: I=Inpatient, O=Outpatient, E=Emergency, P=Preadmit, R=Recurring, B=Obstetrics).
• PV1-3: assigned location (PL — Point of Care, Room, Bed, Facility, Location Status, Person Location Type).
• PV1-4: admission type (table 0007: A=Accident, E=Emergency, L=Labor and Delivery, R=Routine, U=Urgent, N=Newborn).
• PV1-7: attending physician (XCN).
• PV1-10: hospital service (table 0069: CAR=Cardiology, MED=Medical, SUR=Surgery…).
• PV1-17: admitting physician (XCN).
• PV1-19: visit number (CX) — the technical key of the encounter, propagated to every downstream clinical transaction (results, orders, charges).
• PV1-44: admission date/time (CCYYMMDDHHMMSS).

Optional segments

• NK1: next of kin and/or emergency contact.
• GT1: financial guarantor (often the patient themselves, sometimes employer or parent).
• IN1 / IN2 / IN3: insurance information (needed for financial flows).
• AL1: known patient allergies.
• DG1: admission diagnosis (ICD-10).
• OBX: admission observations (weight, height, vitals).
• ROL: associated roles (referring physician, care coordinator).

Real-world example

An emergency admission to ICU. Patient John A. Doe, local MRN MRN567890, admitted on 14 May 2026 at 10:25:30 by Dr Smith, attending physician, in the Cardiology service:

MSH|^~\&|HIS|HOSP01|EHR|CLINIC02|20260514102530||ADT^A01^ADT_A01|MSG00000001|P|2.5.1|||AL|NE
EVN|A01|20260514102530|||DRJONES^Jones^Sarah^^^DR.
PID|1||MRN567890^^^HOSP^MR||DOE^JOHN^A^^MR.||19720515|M|||100 MAIN ST^^ANYTOWN^CA^90210^USA||(555)555-1234|||S||ACCT123456|123-45-6789
PV1|1|I|ICU^101^A^HOSP01|EM|||DRSMITH^Smith^James^A^^DR.|||CAR|||1|||DRSMITH^Smith^James^A^^DR.|ADM|VIP||||||||||||||||||HOSP01|||||20260514102530

• MSH — sent from HIS@HOSP01 to EHR@CLINIC02, Production environment, version 2.5.1, accept-level ACK (AL), no application ACK (NE).
• EVN — type A01, recorded at 10:25:30 by user DRJONES.
• PID — patient identified by MRN567890, born 15 May 1972, male, single.
• PV1 — Inpatient (I), ICU room 101 bed A, admission type Emergency (EM), attending DRSMITH, Cardiology service (CAR).

Acknowledgment (ACK)

The receiving EHR acknowledges with an ACK^A01^ACK message whose structure is minimal:

MSH|^~\&|EHR|CLINIC02|HIS|HOSP01|20260514102531||ACK^A01^ACK|ACK00000001|P|2.5.1
MSA|AA|MSG00000001

• MSA-1: acknowledgment code (table 0008). AA = Application Accept, AE = Application Error, AR = Application Reject.
• MSA-2: message control identifier of the acknowledged message (must match the originating A01's MSH-10).

Common errors

• Malformed MSH-9: omitting the 3rd component (ADT_A01) in v2.5+. Strict parsers (HAPI, Mirth) reject the message.
• PID-3 without CX-5 (Identifier Type Code): a patient identifier without type — MR, NI, SS… — is ambiguous for the receiving system, which must guess whether it is an MRN, an INS-NIR or a SSN.
• PV1-19 different across messages: the visit number must stay stable for the entire encounter. Letting it vary breaks correlation of downstream messages (results, charges, discharge).
• Unescaped reserved characters: ^, |, ~, \ in an address or a name must be escaped using \F\, \S\, \R\, \E\.
• ISO-8859-1 vs UTF-8 encoding: HL7 v2 does not mandate an encoding; accented names or extended characters pass or break depending on what the receiver expects. Declare it in MSH-18 (UNICODE UTF-8 as the modern standard).
• EVN-2 vs PV1-44 out of sync: the HIS recording timestamp and the clinically real admission timestamp must stay consistent (otherwise efficiency KPIs drift).

Related ADT events

See also: ORM^O01 — Order Message for the prescription flows that follow admission, and FHIR Patient for the REST/JSON equivalent of PID in R5.