Because of its four character code structure, READ Codes version 1 was more commonly known as 4-Byte READ. The first release was in April 1986; the final official release of 4-Byte READ occurred in April 2009.
===READRead version 2 (5-byte READ)===
4-Byte READRead could only encode a monoaxial hierarchy with a maximum of 4 hierarchical levels. The operational NHS requirement to provide a direct crossmap to both [[ICD|ICD-9-CM]] and [[OPCS|OPCS-4]] implied an additional hierarchical level was required. Accordingly, a new scheme was devised with exactly the same technical properties as 4-Byte READ except that the code structure was extended to 5-Bytes. This became known as READ2, or 5-Byte READ. The first release of 5-Byte READ occurred sometime prior to January 1991. The October 2010 release contained 82,967 discrete 5-byte codes (although the actual number of discrete clinical concepts that may be represented is estimated to be slightly lower - 82,593 - because of duplicate entries).
A later extension of READRead version 2 product family was the co-publication of a drug and appliance dictionary. This follows the same technical structure (5-character alphanumeric codes with first character lower case alpha organised in a monohierarchy). Released every four weeks, the October 2010 release contained 52,316 codes.
A popular misconception is that all 4-Byte codes are also present in 5-Byte, where they will also carry the same meaning. Whilst in the majority of cases any 4-Byte code of the general form 'wxyz' will be equivalent to a 5-Byte code of the form 'wxyz.', there are notable exceptions. The 4-Byte code [E333 Fear of flying], for example, corresponds to 5-Byte [E202A Fear of flying]; no [E333.] code exists in 5-Byte READ at all.
