Identifiers used for linkage
| Identifier | Description and processing undertaken | Data set |
| NHS number | NHS numbers are 10-digit identifiers assigned to people registered for NHS care in England, Wales, or the Isle of Man. They are assigned to patients soon after birth (since year 2002) or the first time they receive NHS care or treatment.30
Processing: removed non-numeric characters and blanks. Invalid values: 10-digit numbers that are all the same; dummy value ‘2333455667’; format ‘n00000000n’ (eg, ‘6000000006’).15 Valid values: Not invalid (above) and satisfying the checksum digit check.31 | NCHDA, PICANet, ICNARC-CMP, HES/ONS |
| Hospital patient ID | Hospitals use their own local patient identifiers, which in combination with the centre ID constitute a unique patient identifier that we refer to as ‘hospital patient identifier’. A patient can have multiple hospital identifiers across their records for example, associated with care in different hospitals at different times. Processing: standardised the centre ID values, and removed blanks, leading zeroes and leading/trailing special characters from the local patient identifiers.15 Valid values: any value was considered valid. | NCHDA, PICANet |
| Date of birth (DoB) | Date of birth of the patient is available as recorded in the data sets Processing: standardised the format to day/month/year (eg, 17/11/2007). Invalid values: Any date after 01/04/2017 or before 01/01/1895. Equal to either 01/01/1901 or 31/12/1899.15 Valid values: Not invalid (see above) and a feasible date. | NCHDA, PICANet, ICNARC-CMP, HES/ONS |
| Name/surname |
Processing: converted to upper case; removed prefixes and titles (eg, MISS, MSTR, MASTER, MRS, MS, MR, MAST, DR, SGT, SHEIKHA, SULTANA, SHEIKH, SULTAN), removed generic values (eg, BABY, INFANT, TWIN, TRIPLETS, BOY, GIRL, NAME1, NAME2). Removed special characters (apostrophes and accents). Valid values: non-empty values (after processing the fields). | NCHDA, PICANet |
| Postcode |
Processing: converted to uppercase, removed blanks and special characters (only alphanumeric characters allowed). Valid values: postcodes included in the historical list of postcodes from the Organisation Data Service32 and not corresponding to country postcodes (starting with ‘ZZ’) and not from an NHS trust site.33 | NCHDA, PICANet, ICNARC-CMP, HES/ONS |
HES, hospital episode statistics; ICNARC-CMP, Intensive Care National Audit & Research Centre Case Mix Programme; NCHDA, National Congenital Heart Disease Audit; ONS, Office for National Statistics (mortality); PICANet, Paediatric Intensive Care Audit Network.