The World Health Organization’s International Classification of Diseases (ICD) has become the international standard diagnostic classification for reporting morbidity and mortality. In 2015, the United States transitioned from the 9th to 10th Revision. The update was necessary due to major structural limitations of the ICD-9 system. Concerns of the transition mainly centered around clinical usage and cost; however, there were concerns for overlapping codes with the same classification but different meanings between the two versions. Duplicate codes could pose an issue for big data retrospective studies that overlap between the two systems. Therefore, the goals of this study are to further explore and identify duplicate ICD codes between the systems. ICD-9-CM and ICD-10-CM code files were obtained from the Centers for Medicare & Medicaid Services. There were 14,567 ICD-9-CM codes and 91,737 unique ICD-10-CM codes tabulated. Duplicated items between the files were isolated. Four hundred sixty-nine duplicate codes were identified, consisting of 39 E Codes and 430 V Codes. These twin codes contain classifications for external causes of injury and factors influencing health status and contact with health services. Therefore, special attention should be drawn to retrospective research involving methods of injury spanning ICD-9 and ICD-10 systems.
Keywords: electronic health records, international classification of disease, retrospective research
The World Health Organization’s (WHO’s) International Classification of Diseases (ICD) has become the international standard diagnostic classification for disease.1 The classification system allows for the systematic analysis, interpretation, and comparison of morbidity and mortality data collected in different areas at different times.2 Beginning in 1893 with the first version of international classification of diseases, the system for classifying diseases has evolved from 179 to over 120,000 total codes in the most recent version.3,4
On October 1, 2015, the US Department of Health and Human Services began requiring hospitals to report diagnoses and procedures using the 10th revision of International Classification of Diseases (ICD-10-CM/PCS). The ICD-9-CM coding system includes approximately 14,000 diagnosis code, whereas the updated ICD-10-CM coding system contains nearly 70,000 diagnosis codes.5 The development of a 10th revision introduces alphanumeric codes and greater specificity than ICD-9.6 The update was crucial because of major structural limitations of ICD-9-CM that could no longer adequately accommodate important disease and procedure concepts.7 However, the transition from the 30-year-old ICD-9 coding system presented several concerns and challenges.8-13 While most of the concerns were centered around clinical usage and cost, there were concerns for overlapping codes with the same classification but different meanings between the two versions.14
From 2014-2017, there have been 39 ICD codes identified as duplicates by the Centers for Medicare & Medicaid Services (CMS). Duplicate codes could pose an issue for retrospective studies that overlap between the transition of the ICD-9 coding system to the ICD-10 coding system. The identified codes involve external cause of injury classifications (e.g., car accident); therefore, studies exploring injury research or evaluation of injury prevention strategies could be impacted. To date, no studies have investigated duplicate ICD codes and their influence on big data collection. The aims of this study are to: 1) provide background on duplicate codes published by the CMS, and 2) investigate if there are additional duplicate codes based on CMS classification.
Microsoft Excel files containing official ICD-9-CM and ICD-10-CM codes were downloaded from the CMS website.15,16 The codes were compiled into one sheet and tabulated. A function was set up to only select duplicate codes. These codes were isolated and transferred to a separate Excel file. This study is exempt from review by the Institutional Review Board because the data is publicly available.
Duplicate ICD-9-CM and ICD-10-CM Codes
14,567 ICD-9-CM codes and 91,737 unique ICD-10-CM codes were tabulated. Between the two coding systems, there were 469 duplicate codes.
Of the 469 codes, there were 39 E Codes (Table 1). The ICD-9 codes classify external cause of injury, while the ICD-10 codes classify metabolic disorders.
Four hundred thirty V Codes were identified as duplicates in addition to the E Codes. The ICD-9 V codes classify factors influencing health status and contact with health services, while the ICD-10 V codes classify external cause of injury. Examples are listed in Table 2.
Several reports highlight the impact of the ICD-10 transition, including effects on productivity, costs, reimbursement, coding accuracy, and patient care.17-22 However, there is a paucity of literature discussing the implications of the transition on research activities. In addition to the 39 ICD codes published by the CMS, there are 469 codes with the same classification but different code meanings between the two coding systems. All codes are within the E and V sections of the International Classification of Diseases. It is important for researchers and clinicians to be aware of the potential for duplicate codes, as discrepancies could lead to false information used for retrospective studies.
The discussion of possible “twinned” ICD codes began to surface around 2012 regarding the use of codes to classify the external cause of injury.23 Among the ICD-9 system, external causes of injury codes (classified as E000-E030 and E800-E999) are used as supplemental information to diagnosis codes to provide data for injury research and evaluation of injury prevention strategies.24 E codes capture how the injury, poisoning, or adverse effect happened, the intent, the person’s status, the associated activity, and the place where the event occurred.25 In ICD-10-CM, the E codes were moved to Chapter 20: External Causes of Morbidity (V01-Y99).26 In both coding systems, the external cause of injury codes are intended to be used in conjunction with diagnoses codes from other chapters in the corresponding ICD coding system to clarify or specify the nature/cause of diagnosis/condition. While there is no national requirement for mandatory external cause reporting, these codes can provide valuable data for injury research and evaluation of injury prevention strategies. Because of the overlapping nature of E and V codes (Figure 1), there is possibility of duplication between the two coding systems, specifically E800-E999 and V01-V99.
Our study findings of 39 duplicate ICD E codes are consistent with the ICD-CM Duplicate Codes posted by the CMS from 2014-2017.27,28 Of note, there were three ICD-9-CM codes (E8311, E894, E8981) that are like ICD-10-CM codes (E83110, E8940, E89810). It is important to be aware of similar codes that could lead to errors in big data collection of electronic medical records.
The finding of 430 additional ICD V codes to the published CMS duplication list has not been discussed in previous literature. Within the ICD-9 system, V codes are utilized to classify occasions when circumstances other than a disease or injury are recorded as a diagnosis or problem, such as an encounter to act as an organ donor. As previously mentioned, the V codes among the ICD-10 system are a part of the external causes of morbidity and mortality. This includes environmental events and circumstances as the cause of injury, such as a motor vehicle accident. Most ICD-10-CM External Causes of Morbidity (V01-Y99) codes have a requirement for a seventh character (A, D, or S) to indicate whether the injury or condition being treated is the initial encounter (A), subsequent encounter (D), or sequela (S). For example, V41.7 encodes a diagnosis for person on outside of car injured in collision with pedal cycle in traffic accident. However, the code is invalid if it has not been coded to the full number of digits required for that code.29 In this case, the full code would require seven characters, such as V14.7XXA. While the identified duplicate V codes among ICD-10-CM are considered invalid, this finding is important, as it is possible for incomplete codes to arise during the translation of codes depending on the conversion process utilized.
Moreover, this finding can have implications outside the realm of direct patient care with retrospective research projects that utilize ICD conversions. Studies that involve external cause of injury codes spanning both ICD systems can be implicated. For an example, a study aimed to examine hip fractures due to osteoporosis excludes fractures due to high velocity trauma. To accomplish this, all hip fractures associated with an external cause of injury code indicating high velocity trauma (V01-Y99) would be removed, such as V700 (driver of bus injured in collision with pedestrian or animal in non-traffic accident). If the dataset does not distinguish between ICD-9 or ICD-10, the code of interest might be removed by mistake. In this case, the ICD-10-CM definition of V700 is “routine general medical examination at a health care facility.” Routine medical examination is a much more common event than a bus driver being injured by colliding with an animal and would falsely inflate the ICD-10 code for that high-trauma event. Thus, not only would fractures be inaccurately excluded from the study, but a very large portion of the fractures would also be excluded. Because of this possibility, it is important for large-scale projects be able to differentiate between which codes are ICD-9-CM and ICD-10-CM. Being able to distinguish between ICD-9 and ICD-10 codes is necessary; however, this brings attention to another issue of using outside resources for code conversion and lack of consistency.
To ease the burden of researchers who need to translate their cohort from ICD-9-CM to ICD-10-CM, the CMS created and maintains the General Equivalent Maps (GEMs) as a tool for conversion between the two versions.30 The GEMs provide information linking codes from one system with codes in the other system, often times described as “crosswalks.”31 The GEM crosswalks are bidirectional with “forward maps” converting ICD-9-CM to ICD-10-CM and “backward maps” converting ICD-10-CM to ICD-9-CM. However, the complex relationship between the conversion ICD-9 and ICD-10 codes is not one-to-one mirror images of one another; therefore, the use of GEMs requires informed consideration.32-36 As a result of the challenging nature of accurate ICD conversions, many researchers rely on automated conversions, such as coding conversion websites. There is variation among conversion website services, and even highly automated conversions require detailed review.37 For instance, ICD-9-CM codes and ICD-10-CM codes, respectively, are generally referenced as numeric codes without a period (e.g., E030) and alphanumeric codes with a period between the numbers (e.g., E03.0). However, there is variable listing of both coding systems with and without periods. This study excluded periods from both coding versions to be consistent with the methodology utilized by the duplicate codes files published by the CMS.38,39 The complicated translation between GEMs and variations between automated conversion systems highlight the importance of detailed attention as inconsistency can potentially have an impact on study findings.
There are several limitations of this study. First, we are unable to directly assess the impact of duplicate codes in other studies. As there are differences in the accuracy of conversions with forward or backward mapping,40 our study findings encourage studies with overlapping ICD codes to disclose the methods of conversion. Furthermore, while there were several studies investigating the accuracy of conversions among specialties of medicine,41,42 there is a lack of research regarding the impact of ICD-10 transition pertaining to injury related research. We postulate that this may be due to E and V codes being classified as supplemental codes and not necessary for diagnosis. Future studies are warranted to investigate the accuracy of the ICD-10 transition among injury related research, especially due to the potential for duplicate codes to impact the accuracy of findings among this area of research. Lastly, ICD-9-CM and ICD-10-CM codes were used without periods, which could impact the generalizability of the results to conversions utilizing periods among ICD-10-CM codes. However, we believe this should not limit our findings, as this method was consistent with the duplicate codes file published by the CMS.
With the implementation of another revision, ICD-11, it is important that all data is understood. This topic becomes even more important with the possibility of three different coding systems spanning bioinformatics and hospital administrative data.
ICD-9-CM and ICD-10-CM can be obtained from CMS website with the following links, respectively, https://www.cms.gov/Medicare/Coding/ICD9ProviderDiagnosticCodes/codes and https://www.cms.gov/Medicare/Coding/ICD10/2015-ICD-10-CM-and-GEMs.
Conflicts of Interest
The author declares no conflict of interest.
This research received no external funding.
I would like to acknowledge and thank Bruce Benes for assisting with excel formatting and functions.
The following are available online: Table S1: Master List of Duplicate ICD-9-CM and ICD-10-CM Codes.
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Gregory Benes (firstname.lastname@example.org) is a medical student at the Louisiana State University Health Sciences Center School of Medicine.