General Equivalence Mapping System (GEMS): The GEMS is a tool that can be used to convert data from ICD-9-CM to ICD-10-CM and vice versa. The GEMS is a comprehensive translation dictionary that can be used to accurately and effectively translate any ICD-9-CM based data, including the following:

Tracking quality
Recording morbidity/mortality
Calculating reimbursement
Converting any ICD-9-CM to ICD-10-CM

There are two types of GEMS conversion
One-to-one mapping
One-to-many mapping

One-to-one mapping:
Some ICD-9-CM codes map easily to ICD-10-CM in a simple one-to-one conversion.

Example, the ICD-9-CM code 733.6 (Tietze’s syndrome) maps directly to the ICD-10-CM code M94.0.

One-to-three mapping:

ICD-9-CM code 649.51 (spotting complicating pregnancy) requires information
about weeks in pregnancy to map.

ICD-10-CM has three options:

O26.851 (spotting complicating pregnancy, first trimester)

O26.852 (spotting complicating pregnancy, second trimester)

O26.853(spotting complicating pregnancy, third trimester).

One-to many mapping

In an extreme rare example, the ICD-9-CM code 733.82 (other disorders of bone and cartilage, nonunion of fracture), there are 2530 corresponding ICD-10-CM codes due to the degree of specificity required in ICD-10.