WISH: Caution on Assessing Trends in Causes of Death - Accounting for the Change in 1999 from ICD-9 to ICD-10

The International Classification of Diseases (ICD) is a system designed to promote international comparability in the classification of disease, including reporting causes of death on the death certificate. 

Titles for each cause of death and the exact diseases included in each cause are revised periodically to incorporate advances in medical knowledge. The most recent change in ICD codes, from ICD Ninth Edition (ICD-9) to ICD Tenth Edition (ICD-10), was implemented nationwide beginning with 1999 deaths.

It is important for analysts to realize that misleading differences in mortality trends may appear for some causes of death because of this change in the classification system. 

Part of the difference in numbers and rates of deaths for some conditions between 1998 and 1999 will be due to this change. The coding change may also mask real changes in mortality rates.

Research by the National Center for Health Statistics provides comparability ratios between the ICD-9 and ICD-10 results for the cause-of-death groups presented in this module. The comparability ratio for a given cause of death is measured by the net effect of the change in coding systems on the numbers of deaths from that cause. Those ratios and notes on how to use them are available in a table of comparability ratios (PDF).

Analysts examining mortality trends for any cause of death should check the table to determine how much that cause is affected by the change in coding system. 

For each cause of death, the table shows the actual ICD-9 count for 1998 deaths, the estimated number of 1998 deaths if ICD-10 coding was used, and the actual ICD-10 count for 1999 deaths.