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Unified Medical Language System® (UMLS®)

ICD-9-CM Diagnostic Codes to SNOMED CT Map

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ICD-9-CM to SNOMED CT Map File Derived from ICD-9-CM versionDerived from SNOMED CT version
ICD9CM_SNOMEDCT_map_201412 2013 Sept 2014 US Edition
ICD9CM_SNOMEDCT_map_201312 2013 Sept 2013 US Edition
ICD9CM_SNOMEDCT_map_201205 2012 July 2011 International Release

Introduction
Many existing electronic health record (EHR) systems 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 the 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 ICD-9-CM to SNOMED CT Map (herein referred to as “the Map”) is to facilitate the translation of legacy data and the transition to prospective use of SNOMED CT for patient problem lists. Note that this Map is not the same as, and serves different purposes from, the SNOMED CT to ICD-9-CM Map.

The most useful mappings 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, due to the differences between the two coding systems, one-to-one maps cannot be found for some ICD-9-CM codes. This difference is usually due to one of two reasons. Firstly, in ICD-9-CM, some codes are “catch-all” codes that encompass heterogeneous diseases or conditions (e.g. pneumonia due to other specified bacteria). These codes, commonly known as “NEC codes” (not elsewhere classified codes), will not have one-to-one maps because of their nature. Secondly, since SNOMED CT is more granular than ICD-9-CM in most disease areas, some ICD-9-CM diseases or conditions are further refined as more specific concepts in SNOMED CT. For such cases, it is not possible to map to a more specific SNOMED CT concept without the input of additional information.

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, and the usage frequency of the SNOMED CT concepts from the CORE Problem List Subset data. The latter information can help users to identify the more commonly used SNOMED CT targets in the one-to-many maps.

Mapping Methodology
Two lists were obtained from the Centers for Medicare & Medicaid Services (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 in the International release of SNOMED CT. 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.

Summary Statistics (201412 release)

Map TypeICD-9-CM codes% of usage of ICD-9-CM codes*
1-1 Maps 6,312 (65.4%) 66.9%
1-M Maps 2,308 (23.9%) 25.0%
No Map 1,036 (10.7%) 8.0%
Total 9,656 (100%) 100%

 

Map to CORE Subset ConceptsICD-9-CM codes% of usage of ICD-9-CM codes*
1-1 Maps to CORE Subset concepts 2,011 (20.8%) 58.1%
1-M Maps to CORE Subset concepts 816 (8.5%) 19.3%

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

Versions of Terminologies and Knowledge Sources (201412 release)

  • ICD-9-CM – the CMS data was based on the 2009 version. The 2013 version was used in the Map. 61 codes from the CMS data had become obsolete, but they were kept in the Map because obsolete codes might be present in legacy data and so their maps would still be useful
  • SNOMED CT – September 2014 release of US Edition
  • UMLS – 2014AB release
  • SNOMED CT to ICD-9-CM Cross Maps – in September 2014 release of US Edition of SNOMED CT
  • CORE Problem List Subset of SNOMED CT  – 201411 release

Data Format
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 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 still current in the 2013 version
  • IP_USAGE - % of total code usage in the in-patient file (null if ICD-9-CM code not in that file)
  • OP_USAGE - % of total code usage in the 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 in 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

License Requirements
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 IHTSDO 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.

Comments and questions
We welcome any questions, comments or suggestions that would improve the quality, accuracy and usability of the Map. Please send feedback via e-mail to Dr. Kin Wah Fung, Lister Hill National Center for Biomedical Communications, National Library of Medicine through NLM Customer Service with the subject line “ICD-9-CM to SNOMED CT Map”.