HL7 terms to better understand what is HL7. In HL7 v2.x, the OBX is a segment used to transmit a single observation or observation fragment. It represents the smallest indivisible unit of a report. The principal mission of an OBX is to carry information about observations and results in HL7 standard report messages (i.e., ORU or MDM).
Sometimes, the OBX segment is found as a part of an order message (ORM or RDE). In this case, the OBX carries clinical information needed by the receiving system. For example, an OBX is needed to report the menstrual phase information which should be included on an order for a pap smear to a cytology lab.
The OBX segment can be used to contain encapsulated data, e.g., a CDA document or a DICOM image or any of the MIME (Multimedia Internet Mail Extension) support formats such as JPEG, GIF, and FAX.
If the actual observation value is not sent in an OBX but exists somewhere else, the OBX segment can contain a reference pointer. The reference pointer can be a hyperlink which the receiving system would use to access the observation information, document or image.
Essentially, the OBX segment is used to transmit patient clinical information in a variety of formats.