What is the international classification of diseases (ICD)?

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Development of an international classification of diseases, beginning in the early twentieth century, provided a systematic method of organizing a statistical summary of organic disorders. Many of these illnesses have an impact on the biological basis for behavioral disorders.
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An understanding of the mathematical principles that underlay the existence and spread of disease in populations had its origins within the Royal Society of London during the seventeenth century. Founded in 1662, the Royal Society included among its members John Graunt, a local tradesman. Graunt collected and organized bills of mortality from local parishes, which represented the first complete listing of causes of morbidity and mortality in local populations. Descriptions were simplistic compared with data collected now; nevertheless, the principle that information about disease could be statistically compiled would lead to further refinements and increasing accuracy. In 1836 the establishment of the Registrar-General’s Office in London provided a central clearinghouse for compilation of such statistics. In particular, under the leadership of William Farr, compiler of statistical abstracts and finally superintendent, the office represented the first complete centralized bureau for analysis of disease in a population.

Farr initially divided diseases into five classes, three of the major groups being zymotic or infectious diseases; developmental diseases, such as those related to age or nutrition; and violent diseases. While some of Farr’s conclusions are obviously outdated, the separation of behavioral disease from those with clearly contagious characteristics represented an early attempt to distinguish the two.

The major impetus to categorizing morbidity or mortality statistics was the increasing level of information gathering within individual European countries. The development of the germ theory of disease provided a means of diagnosis for individual illnesses; as noted in several studies of the history of information technology, such growing medical statistics were a part of the larger quantification of everyday life in many of these Western countries.

As noted by information technologist Geoffrey Bowker, the International Statistical Institute (IST) during its 1891 meeting in Vienna established a committee under the auspices of Jacques Bertillon, chief of statistical works in Paris, to develop a system for the categorization of illnesses. At its meeting in Chicago two years later, the committee presented a system that was immediately adopted by the larger institute and that was implemented by most countries. The classification became known as the International Classification of Diseases, or ICD; the first system became known as ICD-1. The initial listing included two hundred categories, the number of lines present on the paper used by Bertillon’s committee during its deliberations.

Periodic Revision

As further refinements in research into diagnosis or understanding of disease came about, it was quickly clear that the original categories of illness would be insufficient as a universal classification system. Meetings at approximately ten-year intervals addressed such changes and resulted in significant revisions. The first major revision occurred in 1909 (ICD-2), the second in 1920 (ICD-3), and so on. Following World War I, the League of Nations became the governing body that dealt with the classification system.

At the International Health Conference that met in New York in 1946, the World Health Organization was charged with supervision of the system, including any necessary revisions; the result was ICD-6, which included nonfatal diseases such as those found in psychiatric disorders. In the years since, there have been periodic changes and revisions in that classification system; the ICD-10 was published in 1992, with the next revision slated for publication in 2017. The full name of the publication is now The International Statistical Classification of Diseases and Related Health Problems, although the common acronym of ICD is still used. The number of classifications has ballooned to more than 14,400, with national Clinical Modifications sometimes including even more; the US clinical modification for the ICD-10, for example, has over 68,000.

With development of computer technology, the use of numeric codes became standard in ICD classification. Among other changes, such a numeric system allowed for the encoding of more than just a single underlying cause of death on death certificates; contributing causes could also now be included. The result was a more accurate rendering of disorders affecting an individual.

ICD Classification and Behavioral Disorders

ICD classification represents to a significant degree a classification system for causes of death. Its primary function is to track the changes in diagnosis and spread of disease in populations for epidemiological purposes. However, among the illnesses that have been included in the revisions since World War II are those that represent psychiatric and behavioral disorders.

The changes in the coding scheme in ICD-10 represents the most significant revision in the area of mental illnesses. In ICD-9, numeric codes numbered 001-999 were utilized. For the ICD-10 system, an alphanumeric scheme was adopted, which used a letter followed by a two-numeral character (A00-Z99). For example, Alzheimer’s disease as a cause of death has been classified as G30 in the ICD-10 coding system. The coding of mental disorders increased from thirty categories in ICD-9 (290-319) to one hundred categories in ICD-10 (F00-F98). Each “family” of disorders represents a particular form or cause. For example, F00-F09 includes only disorders with an organic basis. F10-F19 includes “Mental and behavioral disorders due to psychoactive substance abuse,” and so on.

Some of these categories are further subdivided to allow for divisions within the form of the illness. For example, the category F60 represents “Specific personality disorders.” The category is subdivided into ten levels on the basis of specific forms or diagnoses of such disorders: F60.0 represents “Paranoid personality disorder,” F60.1 represents “Schizoid personality disorder,” and so on.


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