14.4 Recommendations for implementers

Recommended practices

• Use a primary LOINC term to represent the core clinical meaning of the observation.
• Send supporting qualifiers — such as method, specimen, body position, threshold, interpretation, device, and provenance — as separate attributes rather than selecting a more specific term for each combination.
• Use standardized value sets for those qualifiers. Where a LOINC axis value is post-coordinated, resolve it to one of the values published for it, as described in Section 14.2, in preference to a locally defined value.
• Check the Status of the Part you resolve to. A value published in a defining LOINC’s Answer List can be deprecated, in which case it should not be used for newly collected data.
• Maintain mappings between the pre-coordinated terms already in use locally and the corresponding post-coordinated representation, so that historical data remains comparable with newly collected data.
• Build the representation into a technology-agnostic information model that can then be implemented in FHIR, CDA, HL7 v2, or another exchange standard, rather than modeling it separately for each.
• Continue to request pre-coordinated terms where they are needed. Requests for new pre-coordinated terms are still accepted and evaluated as described in Appendix D.

Current scope

LOINC is developing content and guidance to support post-coordinated representations and their use across different health information exchange environments. This work includes establishing standardized attributes and value sets, maintaining relationships between existing pre-coordinated terms and their post-coordinated equivalents, and developing information models that can be implemented across standards such as FHIR, CDA, and HL7 v2. The information models are intended to be technology-agnostic, so that the same representation can be carried in any of these exchange standards.

That support is being introduced incrementally.

• The curly-brace naming notation that flags a post-coordinated axis value has been used in LOINC for some time, most extensively in clinical observations and, more recently, as a standing convention for microbiology molecular detection terms (see Section 14.3).
• The defining-LOINC resolution mechanism described in Section 14.2 is newer. Two prototype defining LOINCs were first released in version 2.83, both with a status of TRIAL: 115739-5 (Molgen), which carries Answer List LL7617-5, and 115740-3 (Specimen), which carries Answer List LL7618-3. Additional defining LOINCs, along with further modeling and validation guidance, will follow in subsequent releases.
• Existing pre-coordinated terms are not being deprecated as part of this work. They remain available, and post-coordination is offered as an additional way to represent detailed clinical information.

Implementers who are evaluating post-coordination for a term family that is not yet covered are encouraged to raise it through the submission process so that it can be considered as the guidance expands.