In 2015, the US adopted ICD-10-CM, however some legacy electronic health record (EHR) systems or healthcare data may contain clinical information encoded in ICD-9-CM. To facilitate migration to SNOMED CT as the primary clinical terminology for patient problems (diseases and conditions), it is desirable that legacy ICD-9-CM data be translated to SNOMED CT. This will make it possible to compare newly collected data with historic data, and will also allow the EHR to make use of SNOMED CT to provide clinical decision support and other functions. The goal of the original ICD-9-CM to SNOMED CT Map (herein referred to as “the Map”) was to facilitate the translation of legacy data and the transition to prospective use of SNOMED CT for patient problem lists.
The most useful maps are the one-to-one maps, in which a single SNOMED CT concept can be used to represent the full meaning of an ICD-9-CM code. This allows the automatic translation of ICD-9-CM codes into SNOMED CT codes without loss of meaning. The Map tries to identify as many one-to-one maps as possible. However, not all ICD-9-CM codes can have one-to-one mapping. For example, there are the "catch-all" NEC (not elsewhere classified) codes that encompass anything not captured by existing codes (e.g. 480.8 Pneumonia due to other virus not elsewhere classified). Other reasons for not having one-to-one mapping include mismatch in granularity and composite codes (e.g., 784.2 Swelling, mass, or lump in head and neck). For such cases, it is not possible to translate an ICD-9-CM code to a SNOMED CT concept without human review.
The Map is published in two separate files, one containing the one-to-one maps, and the other the one-to-many maps. Also included in the files are the usage frequency of the ICD-9-CM codes in the CMS (Centers for Medicare & Medicaid Services) source data, and the usage frequency of the SNOMED CT concepts from the CORE Problem List Subset data. Usage frequency can sometimes be used to suggest the most likely SNOMED CT target in the one-to-many maps.