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Understanding the LOINC Document Ontology

The LOINC Document Ontology was developed to identify, characterize, and organize documents exchanged between healthcare systems. Created in 2000 by the HL7/LOINC task force, the LOINC Document Ontology enables documents with the same type of content and information to be assigned the same LOINC code for external data exchange, even though they may have very different local names in their respective systems.

A document in LOINC is named according to LOINC’s six-axis terminology model, but the term in some of the axes would look quite different from those you’d see in a lab test or a clinical observation. In general, a LOINC document would be named as follows:

 Component Property  Time  System  Scale  Method
 <Type of Service> <Kind of Document>  Find  Pt  <Setting>  Doc  <SMD><Role>


Thus, while the LOINC axis of Property would be Finding (Find), the Time axis is set to Point in Time (Pt) and Scale is Document (Doc), the other axes would be filled in with the following five attributes:
CTPrep

• Component
   o Type of Service: Service or action provided
      Example: Consultation, history and physical, discharge summary
   o Kind of Document: General structure of the document
      Example: Note, Letter, Consent Form

• System
   o Setting: Extension of CMS’s definition (not equivalent to location)
       Example: Hospital, Outpatient Clinic, Emergency Department

• Method
   o Subject Matter Domain (SMD): Domain of the content in the document
      Example: Cardiology, Physical Therapy, Oncology
   o Role: Professional classification of document author
      Example: Physician, Nurse, Case Manager, Patient

For a new document, LOINC requires that the Kind of Document and at least one additional attribute be defined. As noted above, combinations within the Component and Method axes are allowed when appropriate. Let’s take a look at LOINC document 84200-5 Mental Health Hospital Admission Evaluation Note as an example:

  Component  Property  Time System   Scale  Method
 <Type of Service> <Kind of Document>  Find  Pt  <Setting>  Doc  <SMD><Role>
 Admission evaluation note  Find  Pt    Doc  Mental Health


The Component axis specifies a combination of the Type of Service (admission and evaluation) being provided and the Kind of Document (note) being used. The System axis, which specifies the Setting in which the document is used, is left null to indicate that the document could be used in multiple settings (i.e., hospital, outpatient clinic, emergency department, etc.). The Method value of “Mental Health” indicates the Subject Matter Domain in which the document is used. Note that the combinatory attribute Role is left null, meaning the document could be used by multiple roles (physician, nurse, case manager, etc.).

It is useful to understand how documents are structured in LOINC, as it is the preferred code system for HL7 Clinical Document Architecture (CDA) and for compositions of FHIR. CDA templates for clinical notes require LOINC codes to identify document types when used for Meaningful Use. Additional information for implementing the LOINC Document Ontology can be found on the LOINC Document Ontology website and in the HL7 CDA implementation guides  . If you have any questions regarding LOINC Document Ontology or need help with terminology implementation or mapping, please contact us.