The Read Codes are officially, but less commonly, known as the NHS Clinical Terms. This is a clinical coding system for storing medical information in a machine-readable form: a huge thesaurus of codes for anything a doctor might need to record about a patient. The information stored mostly describes medical conditions, but can also include occupations, treatments and symptoms. Coding systems ensure that data is stored consistently and accurately so that it can be of use for statistical and auditing purposes. Or that's the idea anyway.


The Read Codes were first developed way back in 1982 for use in general practice by a GP, Dr. James Read, who practised in Loughborough. Initially he used the codes within his own practice with a list of only 25 common terms, but the list was expanded as other GPs began using the codes and requested that extra terms be included. As the Read Codes evolved, new versions were developed with data structured differently to allow more terms to be added and to deal with the more widespread use outside general practice. The 4-byte system had been developed by 1986 and contained 40,000 terms; 2 years later the 5-byte version was released with 90,000 terms - the Read Codes were quickly becoming a standard in general practice in the UK, but they were still owned by Dr. Read.

A working party established in 1987 had been looking at the possible solutions to the problem of clinical coding within the NHS and decided that a standard coding system was necessary to reap the desired benefits of consistent coding. They settled on the Read Codes as that standard, but on the condition that the development of the system could be steered in the direction that was most useful to the needs of the NHS. This meant buying the intellectual property rights to the Codes, which the NHS duly did in 1990 at a cost of £1.25 million, putting them under Crown Copyright.

The NHS then went on to expand the Read Codes into areas such as physiotherapy and community care, by forming 55 specialty working groups, each of which submitted its own set of codes for inclusion. This work formed the basis of Read Codes Version 3, released in 1994, which is of course the 5th version of the Read Codes. That is to say it's the 3rd version of the 5-byte version, which in itself is sort of the 3rd version. Confused? Good.


The modern 5-byte version of the Read Codes is structured as a hierarchy, with each level describing the term in greater detail. If the code being used is from one of the higher levels in the hierarchy, i.e. less detailed, the remainder of the 5 bytes are padded with dots. Each character is alphanumeric and case-sensitive. Here's an example from ISD Online:

G....   Circulatory system diseases
G3...   Ischaemic heart disease
G30..   Acute myocardial infarction
G301.   Anterior myocardial infarction NOS
G3011   Acute anteroseptal infarction

Each code has one preferred term to describe it in words, such as `Acute anteroseptal infarction' above, and may have any number of synonymous terms, such as `heart attack'.


The Read Codes themselves are only distributed by the NHS Information Authority in ASCII form on CDRom for use by companies developing clinical software. There was a conscious decision not to produce a dead-tree version, because the codes should ideally never be seen directly by the clinicians. Rather, each code should be displayed by its preferred term or a synonymous term and the clinical software handle the coding behind the scenes.

The NHS Information Authority estimates that around 70 percent of medical practices in the UK use a clinical system which incorporates some version of the Read Codes and usage in other areas such as dental surgeries and pharmacies is becoming increasingly prevalent.


The Read Codes are currently being merged with a clinical coding system popular in the United States, SNOMED, to form SNOMED Clinical Terms. The intention is to remove the duplication of effort in updating the two systems and to encourage developers and users of clinical software to take up SNOMED CT as a standard. This forms part of the NHS's Information for Health strategy for updating the electronic systems at all points within the NHS, eventually leading to an Electronic Health Record (EHR) for every British citizen by 2005. No, wait, 2008. No...

Sources and further information
ISD Online -
Bro Taf Health Authority -
House of Commons -