Clinical Statements
The article Clinical act statements, provides an overview of the clinical statements that form the “core” of a CDA document.

Clinical statements are contained within entry elements, which in turn are contained within section elements, which in turn comprise the structured CDA Body, located in the XML path: ClinicalDocument/component/structuredBody.

Clinical statements begin with an XML element of: act, encounter, procedure, observation, substanceAdministration, supply, or organizer.

The article What goes inside a clinical statement XML entry?, overviews some of the sub-elements and attributes that all clinical act statements have in common.

“Observation” Clinical Statements
In the underlying HL7 v3 model on which CDA is based, an “observation” is defined as: An act that is intended to result in new information about a subject.

The distinguishing characteristic of the observation clinical act statement element is that it contains a sub-element named value. The combination of the code sub-element (that is common to all clinical statement elements) and the value sub-element (that is unique to the observation clinical statement element) conveys the substance of the “observation”.

“Observation” vs. “Procedure”
Clinical statements represented with the observation element are very closely related to those that are represented with the procedure element. Refer to The procedure element for more information about the procedure element.

In some texts, an “observation” is presented as a “procedure” (in the informal sense) that does not alter the physical state of the patient, and results in an observed value. As explained in the article just referenced regarding procedure elements (The procedure element), although it is formally correct in the current version of the HL7 standards, it’s challenging to define the use of the procedure element based on the criteria of whether or not it “alters physical state”.

So, as a practical matter, it’s probably best to focus on defining the observation element based on whether or not what is primarily being communicated is a “value” that was observed. If so, the observation element should be used. If not, a procedure element or act element should be used (as per the discussion in The procedure element about what distinguishes a “procedure” from the more generic “act”, in the CDA model).

classCode and moodCode Attributes of observation
The classCode attribute of the observation element is generally set to the value “OBS”. This is not a default value and must be set explicitly. In C-CDA templates that are applied to the observation element, there typically is a declared rule that states that the value “OBS” must be assigned to the classCode attribute.

The moodCode attribute of the observation element can take the following values: “INT” (intended), “DEF” (definition/observation), “EVN” (already took place), “GOL” (goal), “PRMS” (promised), “PRP” (proposed), or “RQO” (request/order) . Refer to The moodCode attribute, for additional information.

Sub-Elements of observation
For the most part, the observation clinical statement element supports the same sub-elements as the act clinical statement element. This includes sub-elements used to express entry relationships (refer to Act relationship elements) and participations (refer to Participation elements).

However, there are several sub-elements that are unique to observation:

  • derivationExpr: A text string that indicates how the value of one observation was derived from another. Only relevant if observations are related to each other via an entryRelationship sub-element indicating that one observation is derived from another. Rarely used.
  • methodCode: Used with some code systems for providing additional detail about the means or technique used to ascertain the observation represented in the code sub-element of observation. If the method used to ascertain the observation is implicit in the code sub-element, it does not need to be provided. Of course, there may not be any conflict between the code sub-element and the methodCode sub-element
  • targetSiteCode: Takes one of the SNOMED-CT codes for describing the parts of the body (from the relevant SNOMED-CT “code hierarchies” related to body structure). Represents the anatomical site or system that is the focus of the observation.
  • referenceRange: This is an act relationship sub-element (refer to Act relationship elements) that is used exclusively with observations. It associates an observationRange clinical act with the observation. The observationRange is itself a special kind of observation clinical statement that allows normal and abnormal ranges to be indicated for the value sub-element of its observation element parent. The subject of “observation ranges” will be covered in a future CDA PRO Know article.
  • interpretationCode: This is a coded element that typically uses codes from the code system named ObservatonInterpretation that includes codes for concepts such as “normal”, “abnormal”, “below normal”, “change up”, “resistant”, “susceptible”, etc. that provide a rough qualitative interpretation of the observation.

The value Sub-Element of observation
As noted above, the main unique sub-element of the observation clinical act statement element, is the value sub-element. The combination of the code sub-element (that is common to all clinical statement elements) and the value sub-element (that is unique to the observation clinical statement element) conveys the substance of the “observation”.

As explained in What is the role of the xsi:type attribute in CDA?, the value sub-element of an observation element can take on many different XML Schema data types. It is formally defined as being of type “ANY”. The XML Schema for CDA defines a datatype this data type as one which is not intended to be used directly, only to be the generic base datatype for deriving more specific datatypes. Anywhere this datatype is used, it is required that the xsi:type attribute be used. Thus, the xsi:type attribute must always be used with the value sub-element of an observation element.

Other CDA PRO Know Articles Referenced In This Article