An Introduction to GEMS by Dr. Helen Walker

What are GEMS?

You will hear this term more frequently in the next couple of years. CMS and CDC created General Equivalence Mappings (GEMS) to assist in transitioning from ICD-9 to ICD-10.  GEMS is a translational dictionary used to convert ICD-9 codes to ICD-10 codes (forward mapping) and ICD-10 codes to ICD-9 codes (backward mapping). It is useful when there is lack of access to the detail in the original medical record.

Sometimes ICD-9-CM translates directly to ICD-10-CM

For example, 003.21 Salmonella meningitis in ICD-9-CM translates to A02.21 Salmonella meningitis in ICD-10-CM.

But other times there is no direct translation

For example ICD-9 code 649.53 Spotting complicating pregnancy, antepartum has no direct counterpart in ICD-10. This is because ICD-9 codes specify whether the episode of care is antepartum or postpartum while ICD-10 codes specify the stage of the pregnancy (1^st,2^nd or 3^rd  trimester).

Sometimes a single ICD-9-CM code translates into more than one ICD-10-CM codes

Histoplasma meningitis (115.11) requires 2 ICD-10-CM codes to describe it: B39.5 Histoplasmosis duboisii AND meningitis in other infectious and parasitic diseases classified elsewhere.

Other times several ICD-9-CM codes translate into just 1 ICD-10-CM code.

Coronary atherosclerosis  of autologous vein bypass graft (414.02) AND intermediate coronary syndrome (411.1) are described by just one ICD-10-CM code: (I25.710) Atherosclerosis of autologous vein coronary artery bypass graft with unstable angina pectoris.*

*Note that this combination code I25.710 solves the dilemma of which diagnosis to list as principal when a patient with CAD is admitted with angina!

Sometimes you can be fooled by the title of the codes

For example, a patient whose symptoms code to “Other chest pain” in ICD-9 (786.59) may not always code to “Other chest pain” (R07.89) in ICD-10. Why not? Intercostal pain is included in “Other chest pain” in ICD-9 but there is a distinctive code for intercostal pain in ICD-10 (R07.82).

The best practice

Although some ICD-9 codes have an equivalent ICD-10 code, many do not. In order to find the most appropriate ICD-10 codes you will need to use the medical record and an Encoder or an ICD-10-CM Code Book. Use of GEMS with mapping from ICD-9 to ICD-10 will result in codes that lack the detail that ICD-10 provides. So be familiar with the term, GEMS, but in practice use your Encoder or ICD-10 Code Book.