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.