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2.8.18 HD - hierarchic designator

Components: <namespace ID (IS)> ^ <universal ID (ST)> ^ <universal ID type (ID)>

The HD is designed to be a more powerful application identifier. It is also designed to be used either as a local version of a site defined application identifier or a publically assigned UID. Syntactically, the HD is a group of two application identifiers: one defined by the first component, and one defined by the second and third components.

The HD allows any site to act as an assigning authority (on a local or user-defined basis), even if it technically doesn’t have the right to issue new ids within an identification scheme. HDs which have defined third components (defined UID types) must be unique within the series of ID’s defined by that component.

Note: The HD is used in fields that in earlier versions of HL7 used the IS data type. Thus, a single component HD (only the first component valued) will look like a simple IS data type for older systems expecting a single component in the place of the HD data type.

If the first component for the HD data type is present, the second and third components are optional. If the third component is present, then the second must also be present (although in this case the first is optional). The second and third components must either both be valued (both non-null), or both be not valued (both null).

2.8.18.1 Namespace ID (IS)

Refer to user-defined table 0300 - Namespace ID for suggested values.

2.8.18.2 Universal ID (ST)

The HD’s second component, Universal ID (UID), is a string formatted according to the scheme defined by the third component, Universal ID type (UID type). The UID is intended to be unique over time within the UID type. It is rigorously defined. Each UID must belong to one of the specifically enumerated schemes for constructing UID’s (defined by the UID type). The UID (second component) must follow the syntactic rules of the particular universal identifier scheme (defined by the third component).

2.8.18.3 Universal ID type (ID)

The third component governs the interpretation of the second component of the HD. If the third component is a known UID refer to HL7 table 0301 - Universal ID type for valid values, then the second component is a universal ID of that type.

Table 0301 - Universal ID type

Value

Description

DNS

An internet dotted name. Either in ASCII or as integers

GUID

Same as UUID.

HCD

The CEN Healthcare Coding Scheme Designator. (Identifiers used in DICOM follow this assignment scheme.)

HL7

Reserved for future HL7 registration schemes

ISO

An International Standards Organization Object Identifier

L,M,N

These are reserved for locally defined coding schemes.

Random

Usually a base64 encoded string of random bits.

The uniqueness depends on the length of the bits. Mail systems often generate ASCII string "unique names," from a combination of random bits and system names. Obviously, such identifiers will not be constrained to the base64 character set.

UUID

The DCE Universal Unique Identifier

x400

An X.400 MHS format identifier

x500

An X.500 directory name

Note: X400, X500, and DNS are not technically universally valid for all time. Names can be de-registered from an existing user and registered to a new user.

Examples:

1.2.34.4.1.5.1.5.1,1.13143143.131.3131.1^ISO
14344.14144321.4122344.14434.654^GUID
falcon.iupui.edu^DNS
40C983F09183B0295822009258A3290582^RANDOM
LAB1     Local use only: an HD that looks like an IS data type.
PathLab^UCF.UC^L   A locally defined HD in which the middle component is itself

structured. This can be considered the combination of
'PathLab' with the locally defined UID system "L".
LAB1^1.2.3.3.4.6.7^ISO  An HD with an ISO "Object Identifier" as a suffix, and a 

locally defined system name.
^1.2.344.24.1.1.3^ISO  An HD consisting only of an ISO UID.

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