HL7 v2 segments

A segment is one line of an HL7 message. It starts with a three-letter code saying what the line is about, and everything after that is fields separated by pipes.

What a segment is

Here is a short admission message. Four lines, four segments, and the code at the start of each one tells you what you are looking at.

Read in order, those four lines say one thing. St Joseph admitted Eleanor Cartwright on 14 March 2024 and put her in bed A of room 312 on ward 3 West.

Required and optional

A message type states which segments it expects and whether each one has to be there. An R segment must be present, an O segment is optional, and a C segment is required only when some stated condition holds. The same marks appear against every field inside a segment.

For an admission, that leaves four segments you cannot omit, which is the message above. The ADT page shows the full structure and what each of the optional ones would add.

The seven to learn first

The standard defines 151 segments. Almost all ordinary traffic is these 7. Each page has a worked example, notes on the fields that cause trouble, and a FHIR mapping.

• MSHMessage header

  MSH is the first line of every HL7 v2 message. It sets the delimiters the rest of the message uses. It also names the sender, the receiver, the time and the kind of message.

• PIDPatient identification

  PID carries the identity of one patient: who they are, what identifiers the sending system knows them by, and the demographics that travel with those identifiers.

• PV1Patient visit

  PV1 describes one visit. It says where the patient is, what kind of stay it is, who is responsible for them and what the visit is called.

• OBRObservation request

  OBR carries one request for an observation: what was ordered, who ordered it, when the specimen was taken, and in a result message, the report that answered it.

• OBXObservation result

  OBX carries exactly one observation: what was measured, what the answer was, what units it is in, and how confident the sender is that the answer is final.

• ORCCommon order

  ORC carries the parts of an order that stay the same whatever is being ordered. Its most important field is the order control code, which says whether the order is being placed, cancelled, held or reported on.

• IN1Insurance

  IN1 carries one insurance policy for the patient. It names the payer, the plan, the person who holds the policy and how that person relates to the patient.

HL7 segments list

144 more, running from allergies to waveform data. Their pages carry the field table as published.