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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 health care professionals though some is entered directly by humans subjects. HL7 has developed a standard set of Null Flavor codes represented in an ISO 21090 enumeration.  Clinical study records extracted directly from healthcare records may encounter this coding.

NullFlavors 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  of NullFlavors supported for the study. The Annotation element can be used at any level in the ODM clinical data hierarchy. For ODM V2, it means that means 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  CodeList to identify the set of NullFlavors that are handledallowed.  For the use case where the study data will be sourced by electronic health records using FHIR resources,  NullFlavors NullFlavors will be taken from the HL7 Version 3 Null Flavor Value set 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 Null Value 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 Null Value Annotation is provided as a child element of the ItemGroupData element.

http://build.fhir.org/ig/HL7/fhir-shorthand/reference.html#defining-value-sets

https://terminology.hl7.org/2.0.0/ValueSet-v3-NullFlavor