Ischemic heart disease and hypertension: effect of disease coding on epidemiologic assessment.

During the changeover from the eighth to the ninth revision of the "International Classification of Diseases: Manual of the International Statistical Classification of Diseases, Injuries, and Causes of Death" (ICD), there were several major alterations of coding for the rubrics Ischemic Heart Disease (IHD) and Hypertension (HBP). As expected, these changes caused major discontinuities for IHD and HPB. These discontinuities were not, however, uniform over sex-race groups. When examined by component ICD codes, the discontinuities were found to vary in both magnitude and direction among the groups. In addition to discontinuity, there was a change in the rate of decline for IHD and HBP after the changeover. This rate of decline varied as well by sex-race group. In general, the decline among blacks was slower than among whites. Earlier studies that assessed IHD mortality have used different groupings of ICD codes to obviate the discontinuity, and researchers have observed a similar differential decline. These results should be viewed with caution because of the potential impact of differential coding on sex-race groups. As preparations are made for ICD-10, special attention should be given to the preservation of epidemiologic continuity to provide better assessment of trends in population subgroups.