HL7 ADD Segment: Addendum

Overview

The ADD segment (Addendum) provides continuation data for a preceding segment when the contents of that segment exceed the maximum size that a sending or receiving system can accommodate in a single segment instance. ADD is paired with the DSC (Continuation Pointer) segment, which signals to the receiver that an addendum follows; the ADD segment then carries the spillover text or codes referenced by the continuation pointer.

ADD is intentionally minimal. It defines only one field — the Addendum Continuation Pointer — whose value is appended to the corresponding value in the previous segment to reconstruct the full element. Because ADD is generic, it can extend any segment in any message type; integration engines treat it as a transparent continuation token rather than as a domain-specific payload.

Used in

ADD appears wherever a segment may overflow segment-level or field-level limits in a long message. Typical hosts include lab reports with very long OBX comments, scheduling messages with extensive notes, registration flows where a free-text field is truncated, and any message that uses DSC to chain spillover content. See MSH for the enclosing message envelope and ADT for a common long-message family in which ADD is occasionally observed.

Field-by-field

The table below lists each field defined for ADD in HL7 v2.5.1, as exposed by the HAPI structures. Length is shown as "—" because the HAPI javadoc does not publish a maximum length for this field; implementations should treat it as system-dependent. Required (Req) marks fields that the standard requires for the segment to be meaningful. Repeat indicates whether the javadoc models the field as a repeating accessor. Table # cites HL7 user-defined or HL7-defined tables only where well-established.

Examples

Minimal

ADD|continuation-text-for-previous-segment

Fully populated

Because ADD defines a single field, the fully-populated form is identical in shape to the minimal form; the field simply carries the full continuation payload.

ADD|...remaining narrative for the long OBX-5 value that did not fit in the prior segment; the receiver concatenates this string with the value pointed to by the preceding DSC-1.

Annotated breakdown

ADD | continuation-text-for-previous-segment
 |    |
 |    ADD-1  Addendum Continuation Pointer (ST)
 ADD  Segment ID

In-context: ADT with a long OBX continued via DSC + ADD

MSH|^~&|LAB|VORRO|EHR|HOSP|20260610101500||ADT^A08^ADT_A01|MSG000001|P|2.5.1
EVN|A08|20260610101500
PID|1||MRN000123^^^VORRO^MR||DOE^JANE^A||19800101|F
PV1|1|I|3W^310^A^VORRO|||||||||||||||VN000456
OBX|1|TX|NOTE^Clinical Note^L||The patient reports a long-standing history of intermittent symptoms beginning roughly six weeks before this admission and tracked across several outpatient
DSC|1|F
ADD|visits; the addendum continues with the remainder of the clinician's narrative including review of systems, prior medications, and family history.

In-context: ORU with a continued OBX-5

MSH|^~&|LIS|VORRO|EHR|HOSP|20260610111200||ORU^R01^ORU_R01|MSG000002|P|2.5.1
PID|1||MRN000789^^^VORRO^MR||SMITH^JOHN^B||19720315|M
OBR|1|ORD000111|FIL000222|CBC^Complete Blood Count^L|||20260610110000
OBX|1|TX|CMT^Comment^L||Specimen received slightly hemolyzed; results released with a comment that the lipemia index was within range but the technologist
DSC|1|F
ADD|noted a mild interference flag on the analyzer that does not affect the reported values.

Notes for implementers

• ADD never appears in isolation; it follows a segment whose DSC continuation pointer references the addendum.
• Concatenation is positional: the receiver appends ADD-1 to the value indicated by the preceding DSC without inserting separators.
• Multiple ADD segments may chain together for very long content; each appended in order.
• Because ADD carries opaque text, encoding characters inside ADD-1 must be escaped using the message's escape sequences declared in MSH-2.
• ADD is generic and not bound to a particular message type; trigger-event-specific rules do not apply.