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Understanding ICD-10: Crosswalks and General Equivalence Mappings

By Jennifer Bresnick

In the pursuit of more specific diagnoses and better documentation, ICD-10 will bring 140,000 different codes, more than eight times the number currently in use with ICD-9.  While not every practice will need to use every code, the sheer amount of new information can be daunting to contemplate.  To aid providers in transitioning to ICD-10 by October 1, 2014, CMS and the National Center for Health Statistics (NCHS) have created General Equivalence Mappings (GEMs), otherwise known as “crosswalks” to translate one code set to the other.

GEMs are not a substitute for learning how to use ICD-10 codes, nor do they simply and automatically translate one code to another in a completely reliable way.  But they can be a useful tool for comparing and translating data during the transition process, and can be beneficial for research and informatics before and after ICD-10 comes into effect.

What are the General Equivalence Mappings?

There are two sets of GEMs: one for the diagnostic codes of ICD-10-CM and one for the procedure codes contained in ICD-10-PCS.  These freely available files, created by CMS and NCHS, are the authoritative source for “forward” code mapping between ICD-9 and ICD-10, as well as “backwards” mapping from ICD-10 back to ICD-9.  The GEMs were developed to provide a consistent way to translate data, and are intended to be used during the transition process, not as an ongoing dictionary.  They will not be updated or maintained after the transition date has passed.

GEMs can be used by anyone with a need to convert large batches of data, which will include payers, providers, researchers and informatics professionals, coders, vendors, and anyone with historical ICD-9 data that needs to be usable in the future.  They have been used to translate ICD-9-CM codes in the Official ICD-10-CM Coding Guidelines, and helped produce the ICD-10 Reimbursement Mappings to ensure that providers will get paid for claims coded in ICD-10.

Why do we need them?

GEMs should be used for converting large databases of ICD-9 codes, linking data in long-term clinical studies that span the transition date, or analyzing data collected before and after the transition.  They should not be used for smaller data sets like an individual clinical chart.  Primarily, they are intended to be used for translations of code lists or code tables used by an application when codes in one code set are the only source of information.  They can also be useful for calculating equivalent reimbursements across code sets and refining reimbursement and quality applications.

What are the limitations of GEMs?

The two code sets are organized into clusters of two to four related codes.  Some ICD-9 clusters map to one ICD-10 code, and some ICD-10 codes map to a cluster of ICD-9 codes.  GEMs can’t provide a simple one-on-one translation.  If the codes sets were that similar, there would be no reason to upgrade to ICD-10 in the first place.  Vendors may use GEMs in their data translation algorithms, but any vendor that relies exclusively on GEMs should be avoided, since they are an incomplete method of translation.

There are more than 8,000 ICD-10 codes that map backwards to more than one equally plausible ICD-9 code.  Because accurate documentation requires the selection of the most specific applicable code available, human eyes are needed to ensure that an ICD-10 code actually matches the patient’s condition.  If a vague or inaccurate code is selected, payers may not reimburse for the accompanying services.  Most codes have an approximate match with one additional choice that needs to be made to get to the most specific code.  This doesn’t require much added effort for a medical coder, but it does mean that GEMs are only useful for automated crosswalking up to a point.

In other words, GEMs are useful in the same way that an English-to-Spanish dictionary is useful.  One can look up English words and their Spanish translations, just like crosswalking from ICD-9 to ICD-10, but that doesn’t mean that all the Spanish words in existence have a direct English equivalent.  Nor does every word mean exactly the same thing in every context.  Reading a dictionary isn’t equal to spending a year in Spain, just as GEMs are no substitute for becoming familiar with the nuts and bolts of ICD-10.

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