What Are the Components of an HL7 Message?

HL7 terms to better understand what is HL7. Each HL7 standard message is composed of groups and segments where:

  • Groups contain segments or groups
  • Segments contain fields
  • Fields contain components
  • Components contain sub-components

Cardinality for these groups, segments, fields, components, and sub-components are defined using the following markers:

[Square Brackets] indicate something is optional; and
{Curly Brackets} indicate something is repeatable.

To explain how these markers are used to indicate segment cardinality, the ADT^A01 message fragment is defined below:

MSH Exactly one required message header
EVN Exactly one event segment
PID Exactly one patient ID segment
[PD1] Zero or one demographic segment
[ { NK1 } ] Zero to n next-of-kin segments
PV1 Exactly one patient visit segment
…
[ Start of optional, repeatable group
{ IN1 If group is in message, exactly one IN1
[ IN2 ] For each group in message, 0 or 1 IN2
[ IN3 ] For each group in message, 0 or 1 IN3
}
] End of optional, repeatable group

Visit our HL7 Resources page for additional information.

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