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International Classification of Diseases

Summary: The International Classification of Diseases (ICD) is designed for the classification of Morbidity and Mortality information for statistical purposes, and for the indexing of hospital records by disease and operations, for data storage and retrieval.

The International Classification of Disease is developed collaboratively between the World Health Organization (WHO) and 10 international centers, for purposes of ensuring that medical terms reported on death certificates are internationally comparable and lend themselves to statistical analysis. The ICD has been revised approximately every 10 years since 1900 in order to reflect changes in understanding of disease mechanisms and in disease terminology.

For further information please refer to NCHS publications on Comparability of Cause-of-Death Between ICD Revisions (

Source: ICD is published by the World Health Organization (WHO). Publications of the World Health Organization enjoy copyright protection in accordance with the provisions of Protocol 2 of the Universal Copyright Convention. For rights of reproduction or translation of WHO publications, in part or in toto, application should be made to the Office of Publications, World Health Organization, Geneva, Switzerland. The World Health Organization welcomes such applications.
In WONDER: There is a facility to help you search the ICD code book, for both the Ninth (ICD9) and Tenth (ICD10) revisions. You can search codes in hierarchical order, search for key words or phrases occurring in the code titles or in a synonyms table, and search some group categories for diseases.

Please refer to the following topics:

ICD 9th Revision
ICD9 Abstract
ICD9 Code Type
ICD 10th Revision
ICD 10 Introduction
The "Dagger and Asterisk" System in ICD10
Other Optional Dual Coding in ICD10
Brief Discussion of ICD Revision Changes
Lookup ICD Codes?

The International Classification of Diseases (ICD)
9th Revision

The 9th Revision of the ICD codes has two forms:

ICD9-CM (Clinical Modification)
The International Classification of Diseases, 9th Revision, Clinical Modification (ICD9-CM) is based on the official version of the World Health Organization's 9th Revision, International Classification of Diseases (ICD9). ICD-9-CM, Official Guidelines for Coding and Reporting can be obtained from:
Superintendent of Documents
P.O. Box 37194
Pittsburg, PA 15250-7954
FAX: (202) 512-2250

ICD9-MORT (Mortality)
ICD9 is designed for the classification of Morbidity and Mortality information for statistical purposes, and for the indexing of hospital records by disease and operations, for data storage and retrieval. This publication is maintained by the World Health Organization.

ICD9 Abstract

A classification of diseases may be defined as a system of categories to which morbid entities are assigned according to some established criteria. There are many possible choices for these criteria. The anatomist, for example, may desire a classification based on the part of the body affected whereas the pathologist is primarily interested in the nature of the disease process, the public health practitioner in aetiology and the clinician in the particular manifestation requiring his care. In other words there are many axes of classification and the particular axis selected will be determined by the interest of the investigator. A statistical classification of disease and injury will depend, therefore, upon the use to be made of the statistics to be compiled.

Because of this conflict of interests, efforts to base a statistical classification a strictly logical adherence to any one axis have failed in the past. The various titles will represent a series of necessary compromises between classifications based on aetiology, anatomical site, circumstances of onset, etc., as well as the quality of information available on medical reports. Adjustments must also be made to meet the varied requirements of vital statistics offices, hospitals of different types, medical services of the armed forces, social insurance organizations, sickness surveys, and numerous other agencies. While no single classification will fit all the specialized needs, it should provide a common basis of classification for general statistical use; that is storage, retrieval and tabulation of data.

A statistical classification of disease must be confined to a limited number of categories which will encompass the entire range of morbid conditions. The categories should be chosen so that they will facilitate the statistical study of disease phenomena. A specific disease entity should have a separate title in the classification only when its separation is warranted because the frequency of its occurrence, or its importance as a morbid condition, justifies its isolation as a separate category. On the other hand, many titles in the classification will refer to groups of separate but usually related morbid conditions. Every disease or morbid condition, however, must have a definite and appropriate place as an inclusion in one of the categories of the statistical classification. A few items of the statistical list will be residual titles for other and miscellaneous conditions which cannot be classified under the more specific titles. These miscellaneous categories should be kept to a minimum.

It is this element of grouping in a statistical classification that distinguishes it from a nomenclature, a list or catalogue of approved names for morbid conditions, which must be extensive in order to accommodate all pathological conditions. The concepts of classification and nomenclature are, nevertheless, closely relate din the sense that some classifications (e.g. in zoology) are so detailed that they become nomenclatures. Such classifications, however, are generally unsuitable for statistical analysis.

The aims of a statistical classification of disease cannot be better summarized than in the following paragraphs written by William Farr (1) a century ago:

"The causes of death were tabulated in the early Bills of Mortality (Tables Mortuaires) alphabetically; and this course has the advantage of not raising any of those nice questions in which it is vain to expect physicians and statisticians to agree unanimously. But statistics is eminently a science of classification; and it is evident, on glancing at the subject cursorily, that any classification that brings together in groups diseases that have considerable affinity, or that are liable to be confounded with each other, is likely to facilitate the deduction of general principles.

"Classification is a method of generalization. Several classifications may therefore, be used with advantage; and the physician, the pathologist, or the jurist, each from his own point of view, may legitimately classify the diseases and the causes of death in the way that he thinks best adapted to facilitate his inquiries, and to yield general results.

"The medical practitioner may found his main divisions of diseases on their treatment as medical or surgical; the pathologist, on the nature of the morbid action or product; the anatomist or the physiologist on the tissues and organs involved; the medical jurist on the suddenness or the slowness of the death; and all these points well deserve attention in a statistical classification.

"In the eyes of national statists the most important elements are, however, brought into account in the ancient subdivision of diseases into plagues, or epidemics and endemics, into diseases of common occurrence (sporadic diseases), which may be conveniently divided into three classes, and into injuries, the immediate results of violence or of external causes."

ICD9 Code Type - Letter Codes in WONDER

"Letter codes" are used to specify the type or category of ICD9 codes. For example, the "Group" (72 Causes of Death grouping) for measles, code G-011, contains the "Nature of Disease" codes for measles, N-055, including 055.0, 055.1, 055.2, 055.7, 055.8 and 055.9. Generally, the best rule is specify the range(s) of ICD codes contained in a group code, instead of specifying the group code value. Thus, the query criteria should specify ICD9 055 - 055.9 for measles, instead of ICD9 G011.

Datasets include limited ICD9 Codes, thus when you searching for a specific code, it may not be included in a particular dataset. For example, cancer incidence data do not include accident codes. The following list describes the ICD9 Codes and associated datasets.

Compressed MortalityICD9N - Nature of disease
 E - External causes of injury 
 G - Group Codes
SeerICD9-CMN - Nature of disease
NHDSICD9-CMN - Nature of disease
 E - External causes of injury 
 V - Health Status
 P - Procedures

International Classification of Diseases (ICD)
10th Revision

(c) World Health Organization, 1992

Publications of the World Health Organization enjoy copyright protection in accordance with the provisions of Protocol 2 of the Universal Copyright Convention. For rights of reproduction or translation of WHO publications, in part or in toto, application should be made to the Office of Publications, World Health Organization, Geneva, Switzerland. The World Health Organization welcomes such applications.

Three sections follow:

Introduction to ICD10
The "Dagger and Asterisk" System
Other Optional Dual Coding

Introduction to ICD10

A classification of diseases may be defined as a system of categories to which morbid entities are assigned according to established criteria. There are many possible axes of classification and the one selected will depend upon the use to be made of the statistics to be compiled. A statistical classification of diseases must encompass the entire range of morbid conditions within a manageable number of categories.

The Tenth Revision of the International Statistical Classification of Diseases and Related Health Problems is the latest in a series that was formalized in 1893 as the Bertillon Classification or International List of Causes of Death . A complete review of the historical background to the classification is given in Volume 2. While the title has been amended to make clearer the content and purpose and to reflect the progressive extension of the scope of the classification beyond diseases and injuries, the familiar abbreviation 'ICD' has been retained. In the updated classification, conditions have been grouped in a way that was felt to be most suitable for general epidemiological purposes and the evaluation of health care.

Work on the Tenth Revision of the ICD started in September 1983 when a Preparatory Meeting on ICD-10 was convened in Geneva. The programme of work was guided by regular meetings of Heads of WHO Collaborating Centres for Classification of Diseases. Policy guidance was provided by a number of special meetings including those of the Expert Committee on the International Classification of Disease - Tenth Revision, held in 1984 and 1987.

In addition to the technical contributions provided by many specialist groups and individual experts, a large number of comments and suggestions were received from WHO Member States and Regional Offices as a result of the global circulation of draft proposals for revision in 1984 and 1986. From the comments received, it was clear that many users wished the ICD to encompass types of data other than the 'diagnostic information' (in the broadest sense of the term) that it has always covered. In order to accommodate the perceived needs of these users, the concept arose of a 'family' of classifications centred on the traditional ICD with its familiar form and structure. The ICD itself would thus meet the requirement for diagnostic information for general purposes, while a variety of other classifications would be used in conjunction with it and would deal either with different approaches to the same information or with different information (notably medical and surgical procedures and disablement).

Following suggestions at the time of development of the Ninth Revision of the classification that a different basic structure might better serve the needs of the many and varied users, several alternative models were evaluated. It became clear, however, that the traditional single-variable-axis design of the classification, and other aspects of its structure that gave emphasis to conditions that were frequent, costly or otherwise of public health importance, had withstood the test of time and that many users would be unhappy with any of the models that had been proposed as a possible replacement.

Consequently, as study of the Tenth Revision will show, the traditional ICD structure has been retained but an alphanumeric coding scheme replaces the previous numeric one. This provides a larger coding frame and leaves room for future revision without disruption of the numbering system, as has occurred at previous revisions.

In order to make optimum use of the available space, certain disorders of the immune mechanism are included with diseases of the blood and blood-forming organs (Chapter III). New chapters have been created for diseases of the eye and adnexa and diseases of the ear and mastoid process. The former supplementary classifications of external causes and of factors influencing health status and contact with health services now form part of the main classification.

The dagger and asterisk system of dual classification for certain diagnostic statements, introduced in the Ninth Revision, has been retained and extended, with the asterisk axis being contained in homogeneous categories at the three-character level.

The "dagger and asterisk" system in ICD10

ICD-9 introduced a system, continued in ICD-10, whereby there are two codes for diagnostic statements containing information about both an underlying generalized disease and a manifestation in a particular organ or site which is a clinical problem in its own right.

The primary code is for the underlying disease and is marked with a dagger (+); an optional additional code for the manifestation is marked with an asterisk (*). This convention was provided because coding to underlying disease alone was often unsatisfactory for compiling statistics relating to particular specialties, where there was a desire to see the condition classified to the relevant chapter for the manifestation when it was the reason for medical care.

While the dagger and asterisk system provides alternative classifications for the presentation of statistics, it is a principle of the ICD that the dagger code is the primary code and must always be used. Provision should be made for the asterisk code to be used in addition if the alternative method of presentation may also be required. For coding, the asterisk code must never be used alone. Statistics incorporating the dagger codes conform with the traditional classification for presenting data on mortality and morbidity and other aspects of medical care.

Asterisk codes appear as three-character categories. There are separate categories for the same conditions occurring when a particular disease is not specified as the underlying cause. For example, categories G20 and G21 are for forms of Parkinsonism that are not manifestations of other diseases assigned elsewhere, while category G22* is for "Parkinsonism in diseases classified elsewhere". Corresponding dagger codes are given for conditions mentioned in asterisk categories; for example, for Syphilitic parkinsonism in G22*, the dagger code is A52.1+.

Some dagger codes appear in special dagger categories. More often, however, the dagger code for dual-element diagnoses and unmarked codes for single-element conditions may be derived from the same category or subcategory.

The areas of the classification where the dagger and asterisk system operates are limited; there are 83 special asterisk categories throughout theclassification, which are listed at the start of the relevant chapters.

Rubrics in which dagger-marked terms appear may take one of three different forms:

(i)    If the symbol (+) and the alternative asterisk code both appear in the rubric heading, all terms classifiable to that rubric are subject to dual classification and all have the same alternative code, e.g.

     A17.0+      Tuberculous meningitis (G01*)
    Tuberculosis of meninges (cerebral) (spinal)
    Tuberculous leptomeningitis

(ii)    If the symbol appears in the rubric heading but the alternative asterisk code does not, all terms classifiable to that rubric are subject to dual classification but they have different alternative codes (which are listed for each term), e.g.

     A18.1+      Tuberculosis of genitourinary system
    Tuberculosis of:
    • bladder (N33.0*)
    • cervix (N74.0*)
    • kidney (N29.1*)
    • male genital organs (N51.-*)
    • ureter (N29.1*)
      Tuberculous female pelvic inflammatory disease (N74.1*)

(iii)    If neither the symbol nor the alternative code appear in the title, the rubric as a whole is not subject to dual classification but individual inclusion terms may be; if so, these terms will be marked with the symbol and their alternative codes given, e.g.

     A54.8      Other gonococcal infections
    • peritonitis+ (K67.1*)
    • pneumonia+ (J17.0*)
    • septicaemia
    • skin lesions

Other optional dual coding in ICD10

There are certain situations, other than in the dagger and asterisk system, that permit two ICD codes to be used to describe fully a person's condition. The note in the tabular list, 'Use additional code, if desired ..', identifies many of these situations. The additional codes would be used only in special tabulations.

These are:

(i)    for local infections, classifiable to the 'body systems' chapters, codes from Chapter I may be added to identify the infecting organism, where this information does not appear in the title of the rubric. A block of categories, B95-B97, is provided for this purpose in Chapter I.

(ii)    for neoplasms with functional activity. To the code from Chapter II may be added the appropriate code from Chapter IV to indicate the type of functional activity.

(iii)    for neoplasms, the morphology code on pp. 1181-1204 of Volume 1, although not part of the main ICD, may be added to the Chapter II code to identify the morphological type of the tumour.

(iv)    for conditions classifiable to F00-F09 (Organic, including symptomatic, mental disorders) in Chapter V, where a code from another chapter may be added to indicate the cause, i.e. the underlying disease, injury or other insult to the brain.

(v)    where a condition is caused by a toxic agent, a code from Chapter XX may be added to identify that agent.

(vi)    where two codes can be used to describe an injury, poisoning or other adverse effect: a code from Chapter XIX, which describes the nature of the injury, and a code from Chapter XX, which describes the cause. The choice as to which code should be the additional code depends upon the purpose for which the data are being collected. (See introduction to Chapter XX, p. 1011 of Volume 1.)

Lookup ICD codes?

If you wish to look up general use ICD codes, then see:

World Health Organization (WHO) web site
Public Health Information System (PHIN) Vocabulary Standards and Specifications web site

You can look up diseases from the ICD 9 and 10 code books in WONDER, however, only those codes that have been classified as causes of death in the corresponding WONDER online database are available here. For example, diseases classified as underlying causes of death in the Compressed Mortality datasets are available for look up inside the Compressed Mortality online database query screens. The groups for selected causes of infant death are available in the Linked Birth / Infant Death online database query screens. Refer to the following table:

Data Description  Data Set Name   ICD availability
Underlying Cause of Death   Compressed Mortality 1999 and later   ICD10 codes
      113 selected causes of death
Underlying Cause of Death   Compressed Mortality 1979 -1998   ICD9 codes
      72 selected causes of death
Infant Deaths   Linked Birth / Infant Death records 1999 and later   ICD10 codes
      130 selected causes of infant death
Infant Deaths   Linked Birth / Infant Death records 1995 - 1998   ICD9 codes

  1. Go to the data request screen for the data set of interest.
  2. Locate the section labeled "Select cause of death" on the request screen.
  3. Select the code set by clicking the radio button with that label, such as "113 selected causes of death."
  4. Click on the Finder button for Search capability.
  5. More help is available at Finder Help.

This page last reviewed: Friday, June 26, 2009
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