Concept: ICD-10 to ICD-9-CM Conversion

Introduction

This concept provides information on converting International Classification of Diseases, 10th Revision, with Canadian Enhancements (ICD-10-CA) diagnosis codes and Canadian Classification of Health Interventions (CCI) codes to International Classification of Diseases, 9th Revision, with Clinical Modifications (ICD-9-CM) codes, including the reason why this conversion should be applied, the data files that contain ICD / CCI coding and the relevant dates when coding systems in these data sets was changed, and discussion on the conversion process including the quality of conversion. Additional information is provided along with links to more detailed documentation available on the MCHP internal network (internal access only) and other conversion tools that are available on-line.

Why Convert from ICD-10 / CCI to ICD-9-CM?

The main reason for converting ICD-10 / CCI codes to ICD-9-CM codes is so that you are using a consistent set of ICD codes in a particular study. This consideration is particularly important depending on the time frame of your study. ICD-10-CA/CCI coding is dramatically different than ICD-9-CM (or earlier versions of ICD codes) so that consistency is important! Please see the links provided above to the ICD and CCI term definitions for more detailed formatting information used in these coding systems.

Data Sets / Time Frames For Considering Conversion

There are two data sets that contain ICD / CCI codes that should be considered for conversion, depending on the time frame of research. Beginning in 2000, ICD-10 coding is used in the Vital Statistics Mortality Registry data. Starting in fiscal year 2004/05, Hospital Abstracts data for Manitoba facilities are coded using ICD-10-CA/CCI. If your research uses these data sets, and the time frame of your study includes data on either side of these dates, then a conversion of codes from ICD-10/CCI to ICD-9-CM is necessary so that a consistent set of ICD codes can be used in the study.

NOTE: The ICD codes in the Medical Services (Physician Claims) data are coded using 3-digit ICD-9-CM codes and therefore do not need to be converted.

Conversion Process

Broad or general conversions can be made between ICD-10-CA diagnosis codes and CCI intervention codes to ICD-9-CM codes using ICD conversion files developed by the Canadian Institute for Health Information.

PLEASE NOTE: The ICD Conversion Files developed by CIHI are available for use by researchers who have approvals to use the Hospital Abstracts data and Vital Statistics Registry data held in the MCHP Data Repository, as part of an agreement between Manitoba Health and the Canadian Institute for Health Information (CIHI). The CIHI ICD conversion files are proprietary and cannot be provided to third parties by MCHP as part of this agreement, but may be acquired directly from CIHI. For more information on the CIHI ICD conversion files, please see the following CIHI Online Store web page: Conversion Tables (for use with ICD-10-CA/CCI) or contact CIHI directly.

MCHP has developed the ICD conversion tables into SAS® formats. There are four formats available in the MCHP format library. Two of these formats are used for converting ICD-10 and CCI codes to the corresponding ICD-9-CM codes. The other two formats provide a "quality of fit" or grade (good/excellent, fair and poor) for each of the codes converted. The conversion formats should always be used in conjunction with the grade formats. Missing and or poor conversions (grade) will need to be checked manually or clarified with Manitoba Health/CIHI. NOTE: A great deal of valuable information is lost when translating to ICD-9-CM especially with CCI codes.

If researchers are interested in specific conditions or utilization patterns for particular diagnoses or interventions they are encouraged to look at the specific individual coding instead of using 'broad' or general conversion formats.

NOTE: CIHI plans to update ICD-10-CA every two years. ICD-10 is supported by the World Health Organization (WHO) and within Canada by Statistics Canada.

1. Converting Hospital Discharge Abstracts Diagnosis Codes

Since the hospital abstracts use ICD-10-CA/CCI codes, the conversion formats can be applied directly to the coded data with few or no missing values. The hospital abstracts beginning in 2004/05 now contain 25 diagnoses and 20 intervention fields.

See the SAS code below for an example of converting ICD-10 to ICD-9-CM for Hospital Separation Abstracts.

2. Converting Vital Statistics Mortality Registry Diagnosis Codes

In the Vital Statistics Mortality Registry data, "Primary Cause of Death" is coded using ICD-10 (four characters) in the variable named PRIMARYCAUSEDEATH. It is important to remember that vital statistics is coded following the WHO convention for ICD-10 not the CIHI ICD-10-CA. At the four character level they are supposed to be identical but the CIHI 2001 conversion files do not contain all of the four character codes when ICD-10-CA is coded completely to a 5 or 6 character length.

The conversion for diagnosis is not perfect for the vital statistics file. This is likely due to differences between ICD-10 and ICD-10-CA. The CIHI conversions do not appear to contain some WHO codes. This appears to happen when CIHI has added one or two characters to the WHO code and then required that ICD-10-CA to be coded to that level. This means the four digit level is not a valid ICD-10-CA code (e.g. C349 does not exist but C3490 does).

The code that is used to capture these differences is available in the SAS code link below. This is done by adding one, or two, additional 0's to the ICD-10 code. The assumption has been made that the first code (0) in the list is the most common and has actually been coded - THIS IS NOT ALWAYS THE CASE. Eventually a complete format should be made or bought that includes all of the codes. When the converted ICD-9 codes are used for mortality data it may be most appropriate to limit the data to the three or four character level in any case.

See the SAS code below for an example of converting ICD-10 to ICD-9 for Vital Stats mortality data.

When one or two '0's are added to the ICD-10 code there is generally only one ICD-9-CM code that matches the whole category.

There are a few additional codes that are not captured using the above. Where there have been modifications the grade should be noted as '3'. These modifications have been supplied in the above format and should be reviewed prior to running any new year of data. The additional codes include:

ICD-10	ICD-9-CM	Notes
A16__	011,012	Tuberculosis
B20_	042	HIV related codes CIHI codes as B24
B21_	042	HIV related codes CIHI codes as B24
B22_	042	HIV related codes CIHI codes as B24
B23_	042	HIV related codes CIHI codes as B24
D37_	2351	Neoplasm of uncertain or unknown behavior of oral cavity and digestive organs
E148	250	Unspecified Diabetes
G82_	3441	Paraplegic
G938	3488	Other conditions of the brain
J38_	4783	Paralysis of vocal cords or larynx
J46_	493	Asthma


Each conversion from ICD-10-CA/CCI to ICD-9-CM has been graded. As mentioned above, these grades represent the "goodness of fit" for the particular conversion. If you are converting files, you should create new variables for both the ICD-9-CM codes and the grade. Test the grade variable with a SAS® PROC frequency. Poor fitting observations should be checked.

Additional Information

More information on ICD-10-CA/CCI codes and the ICD Conversion Files can be found on the MCHP internal web site - see the LINKS section below (internal access only). This includes:


• tabular and alphabetical index documents for different years of ICD-10-CA/CCI codes; and
  
• documentation on the ICD / CCI conversion SAS® formats and related grade of conversion.

OTHER CONVERSION TOOLS

Other tools to convert ICD-10 to ICD-9 coding are available on-line, but these do not necessarily follow the same conversion rules developed by CIHI for Canadian data. One such conversion tool is available from the National Bureau of Economic Research (NBER) web site, and is designed for use with ICD coding systems that are used within the United States. Please see the following web page CMS' ICD-9-CM to and from ICD-10-CM and ICD-10-PCS Crosswalk or General Equivalence Mappings for more information.

NOTE: The CMS (Centers for Medicare and Medicaid Services) conversion tool available on the NBER web site has not been validated at MCHP.