Interoperability
The NHS Number and PDS: matching patients across hospital systems
The NHS Number is the national unique identifier for patients in England, Wales and the Isle of Man, and the Personal Demographics Service (PDS) is the national master database that holds it with each person's basic details. NHS England's usage policy says to use the NHS Number to uniquely identify patients and to keep any local copy of demographics regularly synchronised with PDS.
The NHS Number
Everyone registered with the NHS in England, Wales and the Isle of Man has an NHS Number. It is 10 digits, usually written 3-3-4 (for example 901 234 5678), and the 10th digit is a check digit that confirms the number is valid. The standard for it is ISB 0149. For NHS trusts, NHS England Digital says it helps create and keep a complete record for each patient, share information safely across organisational boundaries and link babies to their records for life.
What PDS holds, and how you reach it
PDS is the national master database of NHS patients: more than 80 million records, over 60 million of them for people currently registered with a GP. It holds non-clinical details such as the NHS Number, name, addresses, date of birth, contact details, registered GP, nominated pharmacy, related people and whether the record is flagged for restricted access. It doesn't hold ethnicity.
PDS has no user interface of its own. Staff reach it through applications such as a hospital's patient administration system (PAS); systems connect through the PDS FHIR API or PDS MESH. Updating a record needs strong authentication through the Care Identity Service, and PDS keeps a full audit of record access. A National Back Office resolves duplicate records and "confusion cases".
What the national usage policy says
- Use of PDS is mandatory if you need to access or manage patient demographic data in England, Wales or the Isle of Man.
- You must use the NHS Number to uniquely identify patients (ISB 0149).
- If you keep a local copy of demographic data, you must synchronise it regularly with PDS.
- For people building health and care software, the same requirement is reflected in the NHS Service Standard.
Where matching still goes wrong inside a hospital
A hospital's systems each hold their own identifiers alongside the NHS Number: a hospital number in the PAS, ids in the lab and imaging systems, a device assigned to a bed. Messages arrive in different orders and some carry fewer details than others; HL7 notes that converting v2 messages means deciding when two parts of a message refer to the same thing. When PAS records are merged, an HL7 v2 A40 event tells other systems, and each has to act on it. The safe pattern is to link identifiers deliberately, keep the history, and send uncertain matches to a person rather than merging on a guess.
What it does not mean
- An NHS Number in one system doesn't keep others correct. Local copies of demographics still have to be synchronised with PDS.
- PDS isn't a clinical record. It holds demographic and administrative details, not results or diagnoses.
- A merge in the PAS isn't a merge everywhere. Downstream systems only follow if they handle the merge events they're sent.
Where Thalix fits
Thalix links every identifier a source gives (hospital number, NHS Number, FHIR id, bedside device assignment) to one Thalix patient. When two sources disagree, the conflict is raised for a person to review; it is never merged automatically. Thalix doesn't replace PDS or the PAS: it follows what they send, and in a real deployment the hospital's own demographic and identity processes still apply.
Thalix is at pilot stage. Demos run on synthetic data, and a hospital deployment needs its own clinical safety case, information governance approval and security review. We don't claim any certification or assessment result the product doesn't have.
Questions people ask
How many digits is an NHS Number?
Ten, usually written in a 3-3-4 format. The 10th digit is a check digit used to confirm the number is valid.
Is using PDS mandatory?
NHS England Digital's usage policy says use of PDS is mandatory if you need to access or manage patient demographic data in England, Wales or the Isle of Man, and local copies must be synchronised regularly.
How do systems connect to PDS?
PDS has no user interface. Point-of-care and patient-facing applications reach it through system-to-system interfaces such as the PDS FHIR API or PDS MESH.
Sources
Each source below is an official page, read and checked in October 2026. Where a page and this guide differ, the official page wins.
- NHS numbers for health and social care organisations NHS England Digital · checked October 2026
- Personal Demographics Service NHS England Digital · checked October 2026
- HL7 table 0003: event type HL7 Terminology · checked October 2026
- Comparison: HL7 v2 messaging (FHIR R5) HL7 · checked October 2026
General information only, written for hospital digital, IT, operations and clinical safety teams. It isn't legal, regulatory, information governance or clinical safety advice, and it doesn't replace your own organisation's processes or the official guidance linked above.