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CDA R2 Technical Artifacts Several articles have described the HL7 Version 3 development process, whereby the central HL7 RIM, which is the definitive source for definitions of components used in all HL7 V3 specifications, is used to derive a formal specification. 22–24 The definitive reference describing the methodology is the HL7 Development Framework (HDF), 25 which governs the process used to derive the CDA R2 object model from the RIM. The CDA schema is derived from the CDA R2 object model per the HL7 XML Implementation Technology Specification, which defines the XML conventions used by HL7 V3 specifications and algorithmically converts the object model into an XML schema. 26,27 The CDA narrative block markup, which is the XML content model of Section.text, is manually crafted (i.e., not RIM derived) and represents an extended subset of XHTML 20 that is backwardly compatible with CDA R1. The following sections describe the technical artifacts that result from application of the HDF. While many implementers are primarily interested in the CDA R2 XML schema, they will often refer to the object model to gain a deeper understanding of various aspects of the standard. CDA R2 Technical Artifacts Several articles have described the HL7 Version 3 development process, whereby the central HL7 RIM, which is the definitive source for definitions of components used in all HL7 V3 specifications, is used to derive a formal specification. 22–24 The definitive reference describing the methodology is the HL7 Development Framework (HDF), 25 which governs the process used to derive the CDA R2 object model from the RIM. The CDA schema is derived from the CDA R2 object model per the HL7 XML Implementation Technology Specification, which defines the XML conventions used by HL7 V3 specifications and algorithmically converts the object model into an XML schema. 26,27 The CDA narrative block markup, which is the XML content model of Section.text, is manually crafted (i.e., not RIM derived) and represents an extended subset of XHTML 20 that is backwardly compatible with CDA R1. The following sections describe the technical artifacts that result from application of the HDF. While many implementers are primarily interested in the CDA R2 XML schema, they will often refer to the object model to gain a deeper understanding of various aspects of the standard. HL7 V3 Data Types and Vocabulary Data types define the structural format of the data carried within an RIM class’s attribute and influence the set of allowable values an attribute may assume. Some data types have very little intrinsic semantic content (e.g., INTEGER, TIME STAMP). However, HL7 also defines more extensive data types such as GENERAL TIMING SPECIFICATION (GTS), which supports complex temporal expressions, and CONCEPT DESCRIPTOR (CD), which supports the postcoordination of codes (or, stated in another way, the combining of codes from a terminology to create a more specific concept). Several CDA components (e.g., ClinicalDocument.code and SubstanceAdministration.routeCode, further described below) are designed to carry concepts drawn from HL7-defined or HL7-recognized coding systems such as LOINC or SNOMED CT. Value sets for these components specify allowable codes, and each value set is assigned a coding strength of ‘‘Coded, No Extensions’’ (CNE), in which case, the only allowable values for the CDA component are those in the stated value set; or ‘‘Coded, With Extensions’’ (CWE), in which case values outside the stated value set can be used if necessary. Post-coordination is allowed in CDA components (such as Observation.code, further described below) that use the CD data type. For example, SNOMED CT defines a concept ‘‘cellulitis,’’ an attribute ‘‘finding site,’’ and a concept ‘‘foot structure,’’ which can be combined in Observation.code to create a post-coordinated expression. Alternatively, Observation.code 32 DOLIN ET AL., HL7 CDA R2 can be valued with the single SNOMED CT concept ‘‘cellulitis of foot.’’ While the CD data type provides syntax for combining multiple codes, it is the source terminology that specifies the rules for what codes can be combined, and the semantics for aggregating pre- and post-coordinated expressions. CDA R2 Object Model The CDA R2 object model is a technical diagram of the CDA specification. It is presented using conventions and notations that were developed by HL7 to represent the specific semantic constructs of the RIM. Although it could be represented in Unified Modeling Language (UML) 30 notation, as the RIM is, the HL7 notation provides more details about the specific constraints and class refinements being represented. The HL7 diagramming convention abbreviates some relationships, enabling diagrams to be smaller and more concise and to convey more information visually. Portions of the CDA R2 object model are shown in Figures 2 and 3.