General Equivalence Mappings (GEMs) were developed by CMS and CDC, with collaboration of the AHIMA and the AHA, as a tool to assist with the conversion from ICD-9-CM codes to ICD-10-CM and the conversion of ICD-10-CM codes back to ICD-9-CM. GEMs act as a translation dictionary to bridge the language gap between ICD-9-CM and ICD-10-CM/PCS. They help users understand, analyze, and manage the translation of one code set to the other. They also help users create their own applied mappings as needed.

The GEMs are more complex than a simple one-to-one crosswalk. They reflect the relative complexity of the code sets clearly so that applications that use them can be managed effectively. They are public domain, general purpose reference mappings designed to give all sectors of the healthcare industry that use coded data the tools to:
• Convert large databases and test system applications
• Link data in long-term clinical studies
• Develop application-specific mappings
• Analyze data collected before and after the transition to ICD-10-CM/PCS

The GEMs were developed to serve a specific, limited, short-term need—to allow the industry to migrate systems, applications, and data from ICD-9-CM to ICD-10-CM/PCS. They are intended to be used primarily for translations of code lists or code tables used by an application or other coded data when codes in one code set are the only source of information.