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.

Two lists were obtained from the CMS, covering commonly used ICD-9-CM codes in short-stay and outpatient hospitals respectively, for the year 2009. SNOMED CT maps for the ICD-9-CM codes in the lists were derived primarily from two existing knowledge sources: the synonymy between ICD-9-CM and SNOMED CT terms in the Unified Medical Language System (UMLS), and the SNOMED CT to ICD-9-CM Cross Maps published by the International Health Terminology Standards Development Organisation (IHTSDO). The choice of target SNOMED CT codes was limited to concepts in three hierarchies: Clinical finding, Situation with explicit context, and Events. One-to-one maps identified by UMLS synonymy were not manually validated. One-to-many maps that were algorithmically identified which involved less than 5 SNOMED CT targets were manually reviewed, with the intention to reduce them to one-to-one maps if possible. ICD-9-CM codes with no maps, or one-to-many maps involving a large number of targets were not manually reviewed.

Since the 201602 update of the Map, we have incorporated data from the ICD-9-CM to SNOMED CT map developed by the Nova Scotia Department of Health and Wellness. This helped us to validate (and correct in some cases) our mappings and expand the number of ICD-9-CM codes being mapped.

* % of usage is the average of in- and out-patient usage percentages

• ICD-9-CM – the CMS data was based on the 2009 version. The 2013 version was used in the Map (ICD-9-CM had not been updated since 2011). Some ICD-9-CM codes from the Nova Scotia map were also from earlier versions. Altogether, 268 codes from the Map were not current in 2013 ICD-9-CM, but they were kept because obsolete codes might be present in legacy data and their maps would still be useful.
• SNOMED CT – September 2024 release of US Edition
• UMLS – 2024AB release
• SNOMED CT to ICD-9-CM Cross Maps – extracted from 2016AA UMLS, which originated from the January 2016 International release of SNOMED CT. This was the last time that the map was updated by IHTSDO.
• CORE Problem List Subset of SNOMED CT  – 202411 release

The Map is published as two tab-delimited value files with the same file structure. The file ICD9CM_SNOMED_MAP_1TO1.TXT contains the ICD-9-CM codes with one-to-one maps, and the file ICD9CM_SNOMED_MAP_1TOM.TXT contains the ICD-9-CM codes with one-to-many maps. For completeness, the second file also contains the ICD-9-CM codes from the CMS data that do not have a map. The fields are:

• ICD_CODE – ICD-9-CM code
• ICD_NAME – ICD-9-CM description
• IS_CURRENT_ICD – whether ICD-9-CM code is current in the 2013 version
• IP_USAGE - % of total code usage in the CMS in-patient file (null if ICD-9-CM code not in that file)
• OP_USAGE - % of total code usage in the CMS out-patient file (null if ICD-9-CM code not in that file)
• AVG_USAGE – average of IP_USAGE and OP_USAGE. The file is sorted by the descending order of this value
• IS_NEC – whether the ICD-9-CM code is an NEC (not elsewhere classified) type of code, all NEC codes do not have 1-1 maps
• SNOMED_CID – SNOMED CT concept identifier, the target of the map (null for ICD-9-CM codes without a map)
• SNOMED_FSN – SNOMED CT fully-specified name (null for ICD-9-CM codes without a map)
• IS_1-1MAP – whether this is a 1-1 map
• CORE_USAGE – usage statistics in the CORE Subset (only populated for SNOMED CT concepts in the CORE Subset)
• IN_CORE – whether the SNOMED CT concept is in the CORE Subset

In accordance with NLM’s mapping assumptions, the Map can be used by users that are licensed to use both SNOMED CT and ICD-9-CM. SNOMED CT is owned by the International Health Terminology Standards Development Organisation (IHTSDO), of which NLM is the US Member. Use of SNOMED CT is free in IHTSDO Member countries including the United States, in low income countries, and for approved research projects in any country, but is subject to the SNOMED CT Affiliate license provisions (incorporated in the License Agreement for Use of the UMLS® Metathesaurus® as Appendix 2).The use of ICD-9-CM is free.

We thank William Dean and Andrew Shatto for providing us with the CMS data. We also thank the Nova Scotia Department of Health and Wellness for the Nova Scotia mapping data.

We welcome any questions, comments or suggestions that would improve the quality, accuracy and usability of the Map. Please send feedback to NLM Customer Support.