The purpose of the GEMs is to create a useful, practical, code to code translation reference dictionary for both code sets, and to offer acceptable translation alternatives wherever possible.
The I-10 and I-9 GEMs are used to facilitate links between the diagnosis codes in I-9 volume 3 and the new I-10 code set. The GEMs are the raw material from which providers, health information vendors and payers can derive specific applied mappings to meet their needs. This is covered in more detail in section 2.
The I-10 to I-9 GEM contains an entry for every I-10 code. Not all I-9 codes are contained in the I-10 to I-9 GEM; the I-10 to I-9 GEM contains only those I-9 codes which are plausible translations of the I-10 codes. The translations given are based on the I-10 code looked up, the source system code in the I-10 to I-9 GEM.
The I-10 to I-9 GEM can be used to convert I-9 based systems or applications to I-10 based applications, or create one-to-one backward mappings (also known as a crosswalk) from incoming I-10 based records to I-9 based legacy systems. This is accomplished by using the I-10 to I-9 GEM, but looking up the target system code (I-9) to see all the source system possibilities (I-10). This is called reverse lookup.