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IHE PIX (Patient Identifier Cross-Referencing) and IHE PDQ (Patient Demographics Query) are foundational IT Infrastructure (ITI) integration profiles maintained by Integrating the Healthcare Enterprise (IHE) to resolve patient identity across multi-facility health systems, regional Health Information Exchanges (HIEs), and diagnostic networks. When a patient receives care across independent hospitals, laboratories, and radiology centers that each assign their own local Medical Record Number (MRN), IHE PIX and PDQ provide standardized transaction contracts around an enterprise Master Patient Index (MPI / EMPI) so clinical systems can cross-reference disparate patient IDs and search demographic records using either classic HL7 v2 messaging or modern HL7 FHIR RESTful APIs (PIXm and PDQm).
What Is the Difference Between IHE PIX and IHE PDQ?
Although PIX and PDQ are almost always deployed together on top of the same underlying EMPI database within a healthcare ”patient identifier cross-reference domain” (affinity domain), they solve two distinct clinical lookup problems:
- IHE PIX (Patient Identifier Cross-Referencing — ”I have an ID; give me the other IDs”): Used when a clinical system (the PIX Consumer) already knows a patient’s local identifier in Identifier Domain A (for example, Clinic MRN `C-99812` under Assigning Authority OID `2.16.840.1.113883.3.100.1`) and needs the PIX Manager to return the corresponding list of equivalent patient identifiers assigned to that same person in Identifier Domains B and C (such as the Hospital Inpatient MRN and the Regional HIE Enterprise ID). Crucially, a standard PIX Query returns only cross-referenced identifiers (`PID-3`), not full demographic details.
- IHE PDQ (Patient Demographics Query — ”I have demographic traits; find matching patient candidates”): Used when a clinical registration desk, imaging modality, or portal application (the Patient Demographics Consumer) does not yet know the patient’s enterprise identifier and needs to query the central Patient Demographics Supplier using demographic search parameters—such as partial or exact family/given name, date of birth, administrative sex, address, telephone number, or mother’s maiden name—to retrieve a list of candidate patient records complete with their demographic attributes and domain identifiers.
How Do HL7 v2 ADT, QBP^Q23 (PIX), and QBP^Q22 (PDQ) Messages Operate Across Affinity Domains?
In the classic HL7 v2.3.1 / v2.5.1 IHE ITI Technical Framework, PIX and PDQ coordinate five specific transactions (`[ITI-8]`, `[ITI-9]`, `[ITI-10]`, `[ITI-21]`, and `[ITI-22]`) over MLLP/TLS sockets:
| IHE Transaction ID | Transaction Name & Actors | HL7 v2 Message Type & Trigger Events | Operational Function in the Affinity Domain |
|---|---|---|---|
[ITI-8] |
Patient Identity Feed (Patient Identity Source → PIX Manager / PDQ Supplier) |
ADT^A01 (Admit), ADT^A04 (Register), ADT^A05 (Pre-admit), ADT^A08 (Update Demographics), ADT^A40 (Merge Patient ID List) / ACK |
Pushes local registration updates and MRN merges (`MRG` segment in `A40`) into the central EMPI so the cross-reference registry stays continuously synchronized. |
[ITI-9] |
PIX Query (PIX Consumer → PIX Manager) |
QBP^Q23 (Get Corresponding Identifiers) → RSP^K23 |
Consumer sends a known `PID-3` (`ID^^^&OID&ISO`) in segment `QPD-3` and optional target domain OIDs in `QPD-4`; Manager returns matching cross-domain IDs in `RSP^K23`. |
[ITI-10] |
PIX Update Notification (PIX Manager → PIX Consumer) |
ADT^A31 (Update Person Information) → ACK |
Proactively notifies subscribed downstream clinical systems whenever a patient’s cross-reference link list changes (e.g., after an EMPI link/unlink or `A40` merge). |
[ITI-21] |
Patient Demographics Query (PDQ Consumer → PDQ Supplier) |
QBP^Q22 (Find Candidates) → RSP^K22 |
Consumer sends demographic filter fields (e.g., `@PID.5.1^[email protected]^19780415`) in `QPD-3` with continuation pointer `DSC` support; Supplier returns matching `PID` segments in `RSP^K22`. |
[ITI-22] |
Patient Demographics and Visit Query (PDQ Consumer → PDQ Supplier) |
QBP^ZV1 → RSP^ZV2 |
Extends PDQ to combine demographic criteria with active encounter/visit attributes (`PV1` admission class, ward, or attending physician). |
Note on HL7 v3 Variants and Cross-Community Discovery: For SOAP/XML health information networks built on HL7 Version 3, IHE defined PIXV3 (`[ITI-44]`, `[ITI-45]`, `[ITI-46]`) and PDQV3 (`[ITI-47]`) using `PRPA_IN201309UV02` / `PRPA_IN201305UV02` interactions, while cross-community federation between separate HIEs utilizes IHE XCPD (`[ITI-55]` Cross-Community Patient Discovery).
How Do IHE PIXm ($ihe-pix) and PDQm Modernize Cross-Enterprise Patient Matching Using HL7 FHIR R4?
To eliminate custom MLLP socket listeners and HL7 v2 pipe-delimited parsing in modern web, mobile, and cloud EHR architectures, IHE published two HL7 FHIR R4 mobile profiles—IHE PIXm (Patient Identifier Cross-referencing for Mobile) and IHE PDQm (Patient Demographics Query for Mobile):
1. IHE PIXm: FHIR Patient Identity Feed `[ITI-104]` and `$ihe-pix` Operation `[ITI-83]`
- Mobile Patient Identifier Cross-reference Query (`[ITI-83]`): Instead of sending an HL7 v2 `QBP^Q23` message, a PIXm Consumer invokes the custom FHIR extended operation
$ihe-pixon the `Patient` resource endpoint:GET https://empi.example.org/fhir/Patient/$ihe-pix?sourceIdentifier=urn:oid:2.16.840.1.113883.3.100.1|C-99812&targetSystem=urn:oid:2.16.840.1.113883.3.200.9The PIXm Manager validates the `sourceIdentifier` (`system|value`) and returns an HL7 FHIR
Parametersresource containing matching `targetIdentifier` slices (`Identifier` data types) and `targetId` references (`Patient/[id]`) restricted to the requested `targetSystem` namespaces. - Patient Identity Feed FHIR (`[ITI-104]`): Replaces HL7 v2 `ADT` feeds (`[ITI-8]`) with standard FHIR `PUT [base]/Patient/[id]` (create/update) and `DELETE` or `Patient.link` (`replaced-by`) interactions to maintain cross-referenced identifiers in the PIXm Manager.
2. IHE PDQm: Standard FHIR Patient Search `[ITI-78]` and `$match` `[ITI-119]`
- Mobile Patient Demographics Query (`[ITI-78]`): Replaces HL7 v2 `QBP^Q22` (`[ITI-21]`) with standard RESTful FHIR search parameters on the `Patient` resource (`GET` or `POST [base]/Patient/_search`), querying parameters such as `family`, `given`, `birthdate`, `gender`, `address-postalcode`, `telecom`, and `identifier`:
GET https://empi.example.org/fhir/Patient?family:exact=Smith&given=Elena&birthdate=eq1978-04-15The PDQm Supplier returns a FHIR
Bundle(`type = “searchset”`) of matching `Patient` resources. - Probabilistic Demographics Match (`[ITI-119]` — `Patient/$match`): Because standard FHIR `GET /Patient` search parameters apply deterministic Boolean filtering, IHE PDQm v3.0+ added transaction `[ITI-119]` invoking the HL7 FHIR
POST [base]/Patient/$matchoperation. The client submits an input `Parameters` resource containing a skeleton `Patient` resource (plus `onlyCertainMatches` and `count` flags), and the EMPI executes probabilistic Fellegi-Sunter or machine-learning linkage, returning a `searchset` Bundle where each candidate `Patient` entry carries a numeric match confidence score in `entry.search.score` (`0.0` to `1.0`) and an assessment grade (`certain`, `probable`, `possible`, `certainly-not`).
Frequently Asked Questions
What is the difference between IHE PIX (Patient Identifier Cross-Referencing) and IHE PDQ (Patient Demographics Query)?
IHE PIX is an identifier-to-identifier lookup: a system that already knows a patient’s local ID in one clinical domain queries the PIX Manager to retrieve that same patient’s corresponding IDs across other hospitals or HIE domains (without demographic payloads). IHE PDQ is a demographics-to-patient search: a system without a known ID queries the PDQ Supplier using demographic traits (name, birth date, sex, address, phone) to retrieve candidate patient demographic records and their associated IDs.
How do HL7 v2 ADT^A01/A04/A08/A40, QBP^Q23 (PIX), and QBP^Q22 (PDQ) messages operate across affinity domains?
Participating registration systems push ADT^A01/A04/A05/A08 demographic updates and ADT^A40 patient merges to the central EMPI via the [ITI-8] Patient Identity Feed. Downstream systems then query the EMPI using QBP^Q23 / RSP^K23 ([ITI-9] PIX Query) to cross-reference domain MRNs, or QBP^Q22 / RSP^K22 ([ITI-21] PDQ Query) to search candidate patients by demographic parameters, while ADT^A31 ([ITI-10]) pushes identifier link updates to subscribed consumers.
How do IHE PIXm ($ihe-pix) and PDQm modernize cross-enterprise patient matching using HL7 FHIR R4?
IHE PIXm replaces HL7 v2 QBP^Q23 with the RESTful FHIR operation GET [base]/Patient/$ihe-pix?sourceIdentifier=system|value ([ITI-83]), returning cross-referenced Identifier structures inside a FHIR Parameters resource. IHE PDQm replaces HL7 v2 QBP^Q22 with standard FHIR GET [base]/Patient demographic searches ([ITI-78]) and probabilistic EMPI scoring via POST [base]/Patient/$match ([ITI-119]).
