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The clinical trials data collection process includes standardized practices for managing incomplete subject data. Most subject data is gathered through interactions with healthcare professionals, although some is entered directly by human subjects. HL7 has developed a standard set of NullFlavor codes represented in ISO 21090 enumeration. Clinical study records extracted directly from healthcare records may encounter this coding.

NullFlavor values are indicated in ODM clinical data through the use of a NullFlavor Annotation. 

A NullFlavor Annotation is an ODM Annotation element with a Flag child element with a codelist reference. The referenced codelist defines the set of NullFlavor values supported for the study. The Annotation element can be used at any level in the ODM clinical data hierarchy. With ODM v2.0, this means that it can be used with the StudyEventData, ItemGroupData, or ItemData elements.  

For studies that support collection of NullFlavor information, the ODM MetaDataVersion must include a CodeList element to identify the set of NullFlavor values that are allowed. For the use case where  study data will be sourced by electronic health records using Fast Healthcare Interoperability Resources (FHIR), NullFlavor values will be taken from the HL7 Version 3 NullFlavor Value Set (https://www.healthit.gov/sites/default/files/170299_f_29_hl7_v3_agender_and_nullflavor.pdf) or a defining value subset. 

When the value of a variable cannot be collected, the ItemData element will have the IsNull attribute set to "Yes" and an Annotation child element that provides the applicable NullValue Code.

When a StudyEvent identified in a data request or study workflow does not occur, a NullValue Annotation will be provided as a child element of the StudyEventData element. If a Form/Data Collection Instrument is skipped or has no values entered, the NullValue Annotation is provided as a child element of the ItemGroupData element.

See HL7, Defining Value Sets, http://build.fhir.org/ig/HL7/fhir-shorthand/reference.html#defining-value-sets; and ValueSet: NullFlavor, https://terminology.hl7.org/2.0.0/ValueSet-v3-NullFlavor.

 <ClinicalData StudyOID="ODMV2.NF.EX1.Study" MetaDataVersonOID="ODMV2.NF.EX1.MDV">
    <SubjectData SubjectKey="001" TransactionType="Insert">
        <StudyEventData StudyEventOID="vstBASE">
            <ItemGroupData ItemGroupOID="sfrmDM">
                <ItemData ItemOID="svarFHMarriage" IsNull="Yes">
                    <AuditRecord>
                        <UserRef UserOID="system"/>
                        <LocationRef LocationOID="CDISC"/>
                        <DateTimeStamp>2008-03-17T19:21:58+00:00</DateTimeStamp>
                    </AuditRecord>
                    <Annotation SeqNum="67944">
                         <Flag><FlagValue CodeListOID="STUDY.CL.NullReasonCodelist">NA</FlagValue></Flag>
                    </Annotation>
                </ItemData>
            </ItemGroupData>
        </StudyEventData>
        <StudyEventData StudyEventOID="vstWEEK1">
            <ItemGroupData ItemGroupOID="sfmECG">
                <AuditRecord>
                    <UserRef UserOID="system"/>
                    <LocationRef LocationOID="CDISC"/>
                    <DateTimeStamp>2008-03-24T19:21:58+00:00</DateTimeStamp>
                </AuditRecord>
                <Annotation SeqNum="67944">
                     <Flag><FlagValue CodeListOID="STUDY.CL.NullReasonCodelist">MSK</FlagValue></Flag>
                </Annotation>
            </ItemGroupData>
        </StudyEventData>
    </SubjectData>
</ClinicalData>