There are two kinds of keys in the ODM: internal and external. Internal keys are used to designate entities within the model, and to allow cross-references between entities within (and between) ODM files. Internal keys are not for human use, and are assumed to be unchanging. Internal keys include subject keys, element OIDs, and repeat keys.
To fully identify a clinical data entity, the following internal keys are needed:
| Kind of Entity | Identifying Keys |
| study | StudyOID |
| subject | above plus SubjectKey |
| study event | above plus StudyEventOID and StudyEventRepeatKey |
| item group | above plus ItemGroupOID and ItemGroupRepeatKey |
| item | above plus ItemOID |
| annotation | keys for the annotated entity plus SeqNum |
A StudyOID uniquely identifies a study. A SubjectKey uniquely identifies a subject within a study. SubjectKeys cannot be used to identify a subject across studies.
A StudyEventOID uniquely identifies a StudyEventDef within a study. However there may be several study events of a particular type for a given subject. Thus, to fully identify a particular study event, we need the StudyOID, the SubjectKey, the StudyEventOID, and a StudyEventRepeatKey.
Each form belongs to a study event, and can be identified (within that study event) by a FormOID (which gives its type) and a FormRepeatKey.
Collected data always belongs to an ItemGroup but two forms of the hierarchy maybe used within a Clinical Data element.
Each item group in clinical data can be identified (within that form) by an ItemGroupOID (which gives its type) and an ItemGroupRepeatKey.
Other items groups are reference data, and can be identified solely by an ItemGroupOID and an ItemGroupRepeatKey.
Each item belongs to an item group. However an item may be referenced only once within an item group. Thus, only the ItemOID is needed to uniquely identify an item within its item group.
External keys are any keys used by clinical personnel. These include subject randomization codes, site codes, and so on. In the ODM, external keys are represented as if they were clinical data - i.e. using one of the currently defined ItemData types. Thus external keys can be changed as needed, and are subject to normal auditing processes.