Discovery uses NHS numbers to link separate patient records into a single person/citizen record.
Distinct patient records are linked during inbound data processing using NHS number; all records for the same NHS number are linked together regardless of the originating supplier system (EMIS, TPP, Cerner, Adastra, Vision).
Distinct records are stored separately and can be identified by the original publishing organisation; records and data from different publishing sources are not merged, deleted, or overwritten. If the NHS number on a record changes, it gets cleanly unlinked and re-linked to any other matching records. The separation is maintained in the DDS subscriber databases (Compass DB); each distinct record has its own 'patient_id' plus a "person_id" that links them together.
All current DDS publishers are PDS-linked; all NHS numbers sent to DDS have been PDS traced (although some publishers don't explicitly tell us this in their extracts). Therefore Discovery currently trust any NHS number that is sent, as well as further validating them against the NHS number format specification.
Discovery is starting work to integrate DDS with PDS so that an NHS number can be verified as correct for each patient record.