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The International Classification of Diseases (ICD) is a coding system maintained by the World Health Organization (WHO) and the National Center for Health Statistics and is used to classify causes of death on death certificates and diagnoses, injury causes, and medical procedures for hospital and emergency department visits. These codes are updated every decade or so to account for advances in medical technology. Currently, the U.S. is using the 10th revision of the ICD (ICD-10).
ICD codes are used on the death certificate primarily for surveillance purposes. There is a strong emphasis in the U.S. Vital Events protocols on correct classification of underlying cause of death and related causes of death on the death certificate, and the process is well-defined and regularly audited. The validity is excellent to the extent that the persons completing the death certificate record the causes of death accurately and legibly. (Sometimes death certificates are completed by the Office of the Medical Examiner, but they are also completed by physicians, funeral directors, and law enforcement.)

ICD codes that are used in the hospital discharge query system are the codes that were recorded on the UB92, a standard electronic billing form used across the country. The primary purpose of supplying codes to this form is to bill for hospital services. In general, it is believed that the ICD codes on the UB92 do an adequate job of accurately recording the reason (diagnosis) for each hospital visit.
The following material on this page was excerpted directly from A Guide to State Implementation of ICD-10 for Mortality Part II: Applying Comparability Ratios. (December 2000) National Center for Health Statistics, pp. 6-7.

ICD Revisions

Since the beginning of the century the ICD for mortality has been modified about once every ten years, except for the twenty year interval between the last two revisions, ICD-9 and ICD-10, as shown below:

ICD Implementation Dates in the U.S.


Years in Effect

First (ICD-1)

1900 - 1909

Second (ICD-2)

1910 - 1920

Third (ICD-3)

1921 - 1929

Fourth (ICD-4)

1930 - 1938

Fifth (ICD-5)

1939 - 1948

Sixth (ICD-6)

1949 - 1957

Seventh (ICD-7)

1958 - 1967

Eighth, Adapted (ICDA-8)

1968 - 1978

Ninth (ICD-9)

1979 - 1998

Tenth (ICD-10)

1999 -

The rationale for the periodic revisions has been to reflect advances in medical science and changes in diagnostic terminology. Historically, the U.S. accepted the WHO versions of the ICD, except for the Eighth Revision, when the U.S. produced its own "adapted" version, which is symbolized by the "A" in ICDA-8. The U.S.'s rejection of the WHO version reflected principally disagreements on the content of the circulatory chapter. That changes in the ICD for mortality have been made only every ten to twenty years rather than annually promotes comparability over time in mortality trend data.

Differences between ICD-10 and ICD-9

ICD-10 differs from ICD-9 in a number of respects: 1) ICD-10 is far more detailed than ICD-9; about 8,000 categories compared with 4,000 categories. The expansion was mainly to provide more clinical detail for morbidity applications; 2) ICD-10 uses 4-digit alphanumeric codes compared with 4-digit numeric codes in ICD-9; 3) three additional chapters have been added and some chapters rearranged; 4) cause-of-death titles have been changed, and conditions have been regrouped; 5) some coding rules have been changed. 6) Finally, ICD-10 is published in three volumes compared with two volumes in ICD-9.

Statistical Impact and Comparability

The introduction of a new revision of the ICD can create major discontinuities in trend data, as shown in Figure 1. Figure 1 shows trends in leading causes of death in the United States from 1950 to 1997 in terms of age-adjusted death rates. The lines on the chart are not continuous, but rather are broken by vertical lines that represent the introduction of a new revision of the ICD. Thus, ICD-9 was introduced in 1979. Further, the level of the rates is sometimes discontinuous between revisions. For example, a large discontinuity occurred between 1978 and 1979 in mortality for the 11th leading cause of death, "Nephritis, nephrotic syndrome, and nephrosis." The rate for this cause in 1979 was over 70 percent higher than in the previous year, because of the introduction of ICD-9.

The extent of the discontinuity is measured using a "comparability ratio," which results from double-coding a large sample of the national mortality file, once by the old revision (ICDA-8), and again by the new revision (ICD-9), and expressing the results of the comparison as a ratio of deaths for a cause of death by the later revision divided by the number of that cause of death coded and classified by the earlier revision. The national Comparability Study for ICD-9 was carried out using a sample of 137,000 deaths (and a special sample of 13,000 deaths for infants) occurring in 1976 (3). The ratios for 1976 were considered applicable to deaths occurring in 1978, and represent the break in trend resulting from introducing the new coding and classification system. The ratio for Nephritis is 1.74 indicating that 74 percent more deaths occurred from this cause in 1979 compared with 1978 only because of the introduction of ICD-9. The comparability ratio for Septicemia of 0.85 indicates that about 15 percent fewer deaths occurred in 1979 compared with 1978, because of the introduction of the new revision of the ICD.

For ICD-10, two sets of comparability ratios are being prepared: a preliminary set scheduled for early release based on a very large sample of deaths (in excess of 1.8 million records) occurring in 1996 that will accompany the publication of preliminary national mortality data for 1999, and a final set of comparability ratios based on the entire national mortality file of 1996 (over 2.3 million records) that will be published one year later. The final comparability ratios are expected to differ little from the preliminary ratios, but will permit calculation of state-specific ratios, cross tabulation by age and sex, and the use of more detailed tabulation lists than the standard ICD-10 tabulation list of 113 Selected Causes of Death (see Appendix I). Preliminary comparability ratios for ICD-10 are shown in a companion document to this Guide.

ICD Code Links

ICD-10 and ICD-10-CM
*ICD-10-CM was implemented nationwide on October 1, 2015. New Jersey hospital records for October 1, 2015 through December 31, 2015 were back-coded to ICD-9-CM so the entire data year would be coded the same way.

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