The Impact on Relative Risk Estimates of Inconsistencies Between ICD-9 and ICD-10

David B Richardson ^1*

¹ University of North Carolina-Chapel Hill, United States

^* To whom correspondence should be addressed. E-mail: david_richardson{at}unc.edu.

Accepted 19 May 2006

Abstract

Background: The 10th revision of the International Classification of Diseases (ICD) represents a major change in the ICD system. This paper investigates the impact on relative risk estimates of inconsistencies in outcome classification between ICD-9 and ICD-10, including scenarios in which occupational exposure levels are correlated with year of death (and therefore with the ICD revision in effect at death). The setting of interest is a cohort mortality study in which follow-up spans the periods during which ICD-9 and ICD-10 were in effect. The relative risk estimate obtained when death certificates are coded to the ICD revision in effect at time of death is compared to the relative risk estimate that would be obtained if all death certificates were coded to a consistent ICD revision (i.e., ICD-10). The ratio of these relative risks is referred to as the coefficient of bias.

Methods: Simple equations relate the coefficient of bias to the sensitivity and specificity of the classification of decedents into categories of cause of death via ICD-9 (treating classifications based upon ICD- 10 as the standard). Bridge-coded mortality data for 2,296,922 decedents (i.e., death certificates coded to ICD-9 and ICD-10) are used to derive estimates of sensitivity and specificity by category of cause of death. Numerical examples illustrate the application of these equations.

Results: Estimates of the sensitivity of classification of decedents into categories of death defined by ICD-9 ranged from 0.26 - 1.00. Specificity was above 0.98 for all categories of cause of death. Numerical examples illustrate that inconsistencies in outcome classification between ICD-9 and ICD-10 may have substantial impact on relative risk estimates if there is a strong relationship between exposure status and the proportion of deaths coded to a given ICD revision.

Conclusions: For analyses of mortality outcomes that exhibit poor comparability between ICD-9 and -10, it may be prudent to recode cause of death information to a standard ICD revision in order to avoid bias that can occur when exposures are correlated with the proportion of deaths coded to a given ICD revision.

Keywords: cause of death, death certificates, epidemiologic methods, occupational mortality

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