HL7 PID segment has three fields for Patient Identifiers, PID.2 - Patient ID (External ID), PID.3 - Patient Identifier List (Internal ID), PID.4 - Alternate Patient ID.
HL7 recommended use of these fields have changed in different revisions. How a healthcare system recognize & use these different fields is totally a system dependent. For any HL7 interface development, it is advisable to consult source system administrator during analysis.
In this article, I will discuss these three fields & their applications
PID.2 Patient ID (External ID)
This field generally identifies a patient across facilities. When we have a multi facility setup or we receive messages from external systems, this field is stores patient id from that system.
These days many organizations are in process of implementing EMPI(Enterprise Master Patient Index) and they use PID.2 to store EMPI which can identify a patient across facilities/hospitals.
This field has been deprecated & term "External Id" associated with this field has been removed from HL7 v2.3.1 onwards and it is recommended to use PID.3 for all kind of patient identifiers.
This field still remains for backward compatibility and older systems still use this.
PID.3 Patient Identifier List
This filed contains list of identifiers which are used to identify a patient within or across healthcare facility. Multiple repetitions are used to store different identifiers.
Most common use ot PID.3 in healthcare systems is to store MRN (Medical Record Number) of a patient. But repetitions allow us to store multiple identifiers like MRN, Billing Number, SSN etc. HL7 Table 0203 - Identifier Type lists all possible identifier types.
The repetitions: assigning authority, assigning facility, and identifier type code attributes of PID.3 allow for distinctive identifier representation.
Components: <ID (ST)> ^ <check digit (ST)> ^ <code identifying the check digit scheme employed (ID)> ^ <assigning authority (HD)> ^ <identifier type code (IS)> ^ <assigning facility (HD)>
Subcomponents of assigning authority: <namespace ID (IS)> & <universal ID (ST)> & <universal ID type (ID)>
Subcomponents of assigning facility: <namespace ID (IS)> & <universal ID (ST)> & <universal ID type (ID)>
A Sample PID.3 field
PID|1||12345^^^ADT1^MR^MCM~123456789^^^ADT1^SS^MCM~1-123-123^^^ADT1^PI^MCM||JONES^WILLIAM^A^III||19610615|M||C|1200 N ELM STREET^^GREENSBORO^NC^27401 1020|GL|(919)379 1212|(919)271 3434||S||PATID12345001^2^M10^ADT1^AN^A|123456789
Following is the detail of different repetitions used in highlighted PID.3 in above PID segment & table lists meaning of different Identifier Type Codes
|Identifier Type Code||Meaning||Field Value||Assigning Authority||Assigning Facility|
|PI||Patient Internal Identifier||1-123-123||ADT1||MCM|
PID.4 Alternate Patient ID
This field has been depricated from HL7 v2.3.1 onwards and it is recommended to use PID.3 for all kind of patient identifiers.
It was used to store Patient ID when we have more than one identifier for patient.
Note: I have never seen HL7 feed which sends PID.4 in my projects.