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Expert Talk
Amakomaya — Aamako Maya Apps Pvt. Ltd.

HL7 v2

HL7 version 2 is the messaging standard that most hospital systems still use to talk to each other. It predates the web, and it is nowhere near disappearing: laboratory analysers, radiology systems, admission systems and hospital information systems exchange v2 messages every day.

Anyone doing healthcare integration work meets v2 before they meet FHIR.

The shape of a message​

A v2 message is pipe-delimited text, one segment per line:

MSH|^~\&|LIS|LAB|HIS|HOSPITAL|20260115093000||ORU^R01|MSG00001|P|2.5.1
PID|1||12345^^^HOSPITAL^MR||DEVKOTA^SITA||19900412|F
OBR|1||LAB98765|718-7^Hemoglobin^LN
OBX|1|NM|718-7^Hemoglobin^LN||11.2|g/dL|12.0-15.5|L|||F
  • MSH — message header: sender, receiver, timestamp, message type, version
  • PID — patient identification
  • OBR — observation request (the order)
  • OBX — observation result (one per result value)

Each segment is a sequence of fields (|), components (^), and repetitions (~), with \& for sub-components.

Trigger events​

Messages are named by event: something happened, so a message is sent.

MessageEvent
ADT^A01Patient admitted
ADT^A03Patient discharged
ADT^A08Patient information updated
ORM^O01Order placed
ORU^R01Observation result available
SIU^S12Appointment scheduled

Delivery is usually over MLLP (Minimal Lower Layer Protocol) on a TCP socket, with an ACK message confirming receipt.

Why it is difficult​

  • Optionality. Large parts of the standard are optional, so every deployment is a local dialect. "HL7 v2 compliant" tells you very little.
  • Z-segments. Vendors add custom Z* segments for anything the standard does not cover — undocumented as often as not.
  • Thin semantics. The standard describes structure far more than meaning; terminology binding is inconsistent.
  • Interface count. Point-to-point interfaces grow as n², which is the problem integration engines exist to solve.

v2 and FHIR together​

FHIR does not delete v2 — it wraps it. A common architecture:

Lab analyser → HL7 v2 (MLLP) → integration engine → FHIR resources → FHIR server

The engine handles transport, mapping, terminology translation and error handling; downstream consumers only ever see FHIR. Mappings between v2 segments and FHIR resources are published by HL7, but local dialects still require local mapping work.

Practical advice​

  1. Get the real messages before estimating. The vendor's spec and the messages on the wire differ more often than not.
  2. Log everything, including rejects. v2 failures are silent by default.
  3. Pin the version (2.3, 2.5.1, 2.7 …) per interface.
  4. Treat identifiers carefully — PID-3 assigning authorities are where patient matching succeeds or fails.

References​

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