NHS numbers are the unique numbers allocated to registered users of the three public health services in England, Wales and the Isle of Man; the three health systems use a shared numbering scheme. It is the key to the implementation of the electronic health record and its use is now compulsory in all approved NHS software.
It replaced the previous system founded on wartime identity card numbers which in England and Wales used letters and digits (e.g. "JRDAN 269"); Scotland used numbers based on households with individuals further identified within the household (e.g. STUV123:3), and this meant that it was hard to validate a specific number. The numerical part of ID/NHS numbers allocated to persons born after the war in England and Wales matched the birth register entry number (i.e. a person whose birth was entry number xy would have an ID/NHS number in the format LLLLxy). Between 1969 and July 1995, the old-style NHS number was used on a baby's birth certificate as the reference number for the certificate.
The new format used is a ten digit number in a 3-3-4 format with the final digit being a check digit. Examples given include 943-476-5919. The format includes an error detecting checksum, which is the role of 10th and final digit. Ignoring the check digit, each of the first 9 digits is multiplied by 11 minus its position. For example, the first digit in the number 943-476-5919 is 9. This would be multiplied by 10, and the 9th digit (1) would be multiplied by 2. The result of this calculation is summed. In this example, the result is 299. The remainder when dividing this number by 11 is calculated, which would be 2 in this case. Finally, this number is subtracted from 11 to find the checksum 9.
A checksum of 11 is replaced by 0 in the final NHS number. If the checksum is 10 then the NHS number is not valid.
A person gets an NHS number at birth, or when they first make contact with the NHS by registering with a GP. It comes from a record being made on the Personal Demographics Service, a national patient database.
Number ranges and co-ordination with Scotland and Northern Ireland
Scotland's equivalent is called a CHI number; a similar system is also used in Northern Ireland with each one of the three using the same format but with non-overlapping number blocks thus preventing the issue of the same number by more than one system.
Currently issued numbers for England, Wales and the Isle of Man are from 400-000-000 to 499-999-999 and 600-000-000 to 708-800-001. Unavailable number ranges include 320 000 001 to 399 999 999 (allocated to the Northern Ireland system) and 010 101 0000 to 311 299 9999 (used for CHI numbers in Scotland).
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