implement classic hl7v2-based ihe pdq and pix patient matching (iti-21 and iti-9) distinct from the newer fhir-based pdqm/pixm mobile profiles

Steps

1. Use the IHE PDQ (Patient Demographics Query) profile's ITI-21 transaction to let a client application query a central patient information server for demographics and encounter data using HL7 v2 query/response messages
2. Use the IHE PIX (Patient Identifier Cross-referencing) profile's ITI-9 transaction (PIX Query) to retrieve a patient's cross-referenced identifiers in other identifier domains starting from one known identifier
3. Register new patient identities with the PIX Manager as an Identity Source actor via the patient identity feed transaction so the manager maintains an accurate cross-reference before consumers query it
4. Choose PDQ when you need to search for a patient by demographics, and PIX when you already hold one identifier and need its equivalents in other domains -- the two profiles solve different matching problems and are commonly deployed together rather than interchangeably
5. Confirm whether an existing deployment actually implements the classic HL7v2-based PDQ/PIX transactions versus the newer FHIR-based PDQm/PIXm mobile profiles before assuming a REST/FHIR transport is available