HL7 EVN Segment: Event Type
The HL7 EVN segment (Event Type) identifies the trigger event that generated an HL7 message and provides event timing context. It appears immediately after MSH in ADT and MDM messages, recording who triggered the event, when it occurred, and why.
EVN Field Reference
Section titled “EVN Field Reference”| Seq | Name | Type | Opt | Description |
|---|---|---|---|---|
★ EVN-1 | Event Type Code | ID | O | Trigger event (A01, A02, T02, etc.) |
| Echoes MSH-9.2 trigger event. Technically optional in newer versions since MSH-9 already carries the event type. | ||||
★ EVN-2 | Recorded Date/Time | TS | R | When the event was recorded in the system |
| When the event was entered into the system — may lag the actual event time (EVN-6). Distinct from MSH-7 (message creation time). | ||||
EVN-3 | Date/Time Planned Event | TS | O | Scheduled date for future events |
| Used in pre-scheduling: ADT^A05 (planned admission), A15 (pending transfer), A16 (pending discharge). Useful for capacity planning. | ||||
★ EVN-4 | Event Reason Code | IS | O | Reason for the event (facility-defined) |
| Facility-specific codes (no standard HL7 table). Common examples: AMA (against medical advice), DUPLICATE (duplicate entry), OVERCROWDING (transfer reason). | ||||
★ EVN-5 | Operator ID | XCN | O | Person who triggered the event |
| Creates audit trail of who performed each action. Distinct from PV1-7 (Attending Doctor) — may be a clerk, nurse, or coordinator. | ||||
★ EVN-6 | Event Occurred | TS | O | When the event actually happened |
| When the event actually occurred (e.g., patient arrived at bed). More clinically accurate than EVN-2 (system entry time). Preferred for billing and clinical reporting. | ||||
EVN-7 | Event Facility | HD | O | Facility where the event occurred |
★ EVN-1 O Trigger event (A01, A02, T02, etc.)
Echoes MSH-9.2 trigger event. Technically optional in newer versions since MSH-9 already carries the event type.
★ EVN-2 R When the event was recorded in the system
When the event was entered into the system — may lag the actual event time (EVN-6). Distinct from MSH-7 (message creation time).
EVN-3 O Scheduled date for future events
Used in pre-scheduling: ADT^A05 (planned admission), A15 (pending transfer), A16 (pending discharge). Useful for capacity planning.
★ EVN-4 O Reason for the event (facility-defined)
Facility-specific codes (no standard HL7 table). Common examples: AMA (against medical advice), DUPLICATE (duplicate entry), OVERCROWDING (transfer reason).
★ EVN-5 O Person who triggered the event
Creates audit trail of who performed each action. Distinct from PV1-7 (Attending Doctor) — may be a clerk, nurse, or coordinator.
★ EVN-6 O When the event actually happened
When the event actually occurred (e.g., patient arrived at bed). More clinically accurate than EVN-2 (system entry time). Preferred for billing and clinical reporting.
R = Required, O = Optional, C = Conditional, W = Withdrawn (backward compatibility only)
EVN-1: Event Type Code
Section titled “EVN-1: Event Type Code”EVN-1 echoes the trigger event from MSH-9.2 (Message Type). In most messages, they are identical:
MSH|^~\&|EPIC|MAIN_HOSP|LAB_SYS|PATHOLOGY|202603011430||ADT^A01^ADT_A01|MSG00001|P|2.5.1EVN|A01|202603011430|||ADMIN^JONES^BETTY^^^RNBoth MSH-9.2 and EVN-1 say A01. However, EVN-1 is technically optional in newer HL7 versions because MSH-9 already carries the event type. Some interfaces omit EVN-1, relying on MSH-9 alone for event identification.
EVN-2 vs EVN-6: Timing Fields
Section titled “EVN-2 vs EVN-6: Timing Fields”The distinction between these two timestamps is a frequent source of confusion:
| Field | Name | Meaning | Example |
|---|---|---|---|
| EVN-2 | Recorded Date/Time | When the event was entered into the system | 2026-03-01 14:30 (registration clerk enters admit) |
| EVN-6 | Event Occurred | When the event actually happened | 2026-03-01 14:15 (patient physically arrived at bed) |
| MSH-7 | Message Date/Time | When the HL7 message was created | 2026-03-01 14:30 (system generates message) |
Practical Example
Section titled “Practical Example”A patient arrives at ICU bed at 2:15 PM. The nurse enters the admission at 2:30 PM. The HL7 message is generated at 2:30 PM:
MSH|...|202603011430||ADT^A01EVN|A01|202603011430|||ADMIN^JONES^BETTY^^^RN|202603011415- EVN-6 = 202603011415 (patient actually admitted at 2:15 PM)
- EVN-2 = 202603011430 (recorded at 2:30 PM)
- MSH-7 = 202603011430 (message created at 2:30 PM)
For clinical reporting and billing purposes, EVN-6 (when it happened) is typically the most accurate timestamp. EVN-2 (when recorded) reflects system entry time, which may lag the actual event.
EVN-4: Event Reason Code
Section titled “EVN-4: Event Reason Code”EVN-4 carries a facility-defined code explaining why the event occurred. Common use cases:
| Context | EVN-4 Value | Meaning |
|---|---|---|
| ADT^A02 (Transfer) | OVERCROWDING | Transfer due to bed management |
| ADT^A03 (Discharge) | AMA | Discharged against medical advice |
| ADT^A11 (Cancel Admit) | DUPLICATE | Cancelled due to duplicate entry |
| ADT^A13 (Cancel Discharge) | READMIT | Discharge cancelled, patient readmitted |
Event reason codes are facility-specific — there is no standard HL7 table. Each implementation must map codes between sending and receiving systems.
EVN-5: Operator ID
Section titled “EVN-5: Operator ID”EVN-5 identifies the person who triggered the event using the XCN (Extended Composite Name) data type:
EVN|A01|202603011430|||ADMIN^JONES^BETTY^^^RN| Component | Value | Meaning |
|---|---|---|
| XCN.1 | ADMIN | Employee/user ID |
| XCN.2 | JONES | Family name |
| XCN.3 | BETTY | Given name |
| XCN.6 | RN | Credential/degree |
This field creates an audit trail of who performed each action. It is distinct from PV1-7 (Attending Doctor) — the operator may be a clerk, nurse, or unit coordinator, not necessarily the physician responsible for care.
Key Implementation Considerations
Section titled “Key Implementation Considerations”When EVN Is Omitted
Section titled “When EVN Is Omitted”Despite the HL7 standard specifying EVN as required in ADT messages, some implementations omit it:
- Minimal ADT feeds: Systems that only populate MSH, PID, and PV1 may skip EVN
- Non-ADT messages: ORU, ORM, and SIU messages typically do not include EVN
- Backward compatibility: Older v2.1/v2.2 interfaces may not send EVN consistently
Receiving systems should handle missing EVN gracefully — fall back to MSH-7 for event timing and MSH-9.2 for event type.
EVN-3: Planned Event Date
Section titled “EVN-3: Planned Event Date”EVN-3 is used in pre-scheduling scenarios:
- ADT^A05 (Pre-Admit): EVN-3 carries the planned admission date
- ADT^A15 (Pending Transfer): EVN-3 carries the planned transfer date
- ADT^A16 (Pending Discharge): EVN-3 carries the planned discharge date
Downstream systems can use EVN-3 for capacity planning, bed management, and staffing forecasts.
Audit Trail Usage
Section titled “Audit Trail Usage”EVN-5 (Operator) combined with EVN-2 (Recorded Time) and EVN-6 (Event Time) provides a complete audit trail:
- Who: EVN-5 (ADMIN^JONES^BETTY^^^RN)
- When recorded: EVN-2 (2026-03-01 14:30)
- When it happened: EVN-6 (2026-03-01 14:15)
- What: EVN-1 / MSH-9 (A01 — Admit)
- Where: EVN-7 (MAIN_HOSP)
This audit data supports compliance requirements, patient safety investigations, and quality reporting.