General Equivalency Mappings (GEMs)

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Centers for Medicare and Medicaid Services (CMS) developed General Equivalency Mappings (GEMs) with the cooperation of the National Center for Health Statistics (NCHS), the American Health Information Management Association (AHIMA), the American Hospital Association (AHA) and 3M Health Information Systems (2).

GEMS were developed due to concerns about mapping codes from one coding system to another. GEMs provide information on the how a code in one system is coded in the other system. There are crosswalks from ICD-9-CM to ICD-10-CM and the reverse. Mappings are also available between ICD-10-PCS and ICD-9-CM. Not all codes can be mapped directly to the other system, generally due to the higher level of specificity of ICD-10-CM codes.

With passage of the American Recovery and Reinvestment Act (ARRA) on February 17, 2009, hospitals will be required to bill Medicare using International Classification of Diseases-Clinical Modification (ICD-10-CM) codes rather than current ICD-9-CM codes. ICD-10-CM codes are mandatory for Medicare billing beginning October 1, 2013 (1). The Centers for Medicare and Medicaid Services (CMS) set an aggressive timeline that requires many system changes to allow for implementation of ICD-10-CM by that date.

Transition from ICD-9-CM to ICD-10-CM brings a number of system upgrade challenges. Where ICD-9-CM is limited to 5 alpha and/or numeric characters, ICD-10-CM codes have up to seven alpha and/or numeric characters. This presents challenges to all parties that bill or that receive bills for Medicare patients. Current systems need to be updated to allow for this change. Hospitals bill multiple types of parties, including, but not limited to Medicare, managed care, employer sponsored programs, self-insured entities, automobile coverage, workers’ compensation and so forth. The change in coding to ICD-10-CM codes is not required for these other entities; therefore, hospitals may need to bill under both coding systems if they do not choose to change to ICD-10-CM.

To facilitate the transition, Medicare made the GEM mapping files available without cost on the CMS website. It should be noted that GEM mappings are not a substitute for coding, but provide data to aid the transition between the two coding systems.


  1. Centers for Medicare & Medicaid Services (CMS). [2010]. [cited 2010 Nov 22] Available from URL:
  2. Butler R. The ICD-10 General Equivalence Mappings: Bridging the Translation Gap from ICD-9. Journal of AHIMA 2007; 78 (9): 84-86.
  3. International Classification of Diseases-10, Clinical Modification (Draft). 1st ed. Salt Lake City. Ingenix; 2010.
  4. International Classification of Diseases-9, Clinical Modification. 9th ed. Salt Lake City. Ingenix; 2010.
  5. International Classification of Diseases-10, Procedure Coding System (Draft). 1st ed. Salt Lake City. Ingenix 2010.

Submitted by: Barbara Kelly-Mahaffey