To the Editor:
Official mortality statistics are based on the underlying cause of death selected according to coding rules developed by the World Health Organization as part of the International Classification of Diseases.1,2 An understanding of cause-of-death certification practices and coding rules is important for the analysis and interpretation of mortality statistics. Some diseases or conditions are less likely than others to be selected as the underlying cause of death for mortality tabulation depending on how they are reported on the death certificate. Hypertension is a good example, as it is selected as the underlying cause of death for mortality tabulation in less than one fourth of cases in which hypertension is mentioned anywhere on the death certificate.3
In cases where a generalized disease such as hypertension is reported as the underlying cause of death or selected as a tentative underlying cause of death and other significant diseases such as ischemic heart disease or cerebrovascular disease are mentioned, the coding rules require reselection of the underlying cause of death preferring ischemic heart disease or cerebrovascular disease over hypertension. The eAppendix 1 (http://links.lww.com/EDE/A638) presents the coding rules pertaining to hypertension and eAppendix 2 (http://links.lww.com/EDE/A638) illustrates some examples.
The percentage of U.S. cases with mention of hypertension in which hypertension was selected as the underlying cause of death decreased from 25% in 1980 to 19% in 1998.4 However, it is not known whether this represents a decrease in the likelihood that certifying physicians reported hypertension as the underlying cause of death, or an increasing likelihood that the application of coding rules resulted in the selection of another cause of death.
We analyzed U.S. multiple-cause mortality files from 1980 to 2005 to examine how application of the coding rules affected the level and trend of hypertension mortality in the United States.5
Hypertension mortality based on any mention of hypertension on the death certificate increased substantially during the study period. In contrast, there was no increase in hypertension mortality based on the underlying cause of death, either as reported by certifying physicians (ie, the first diagnosis on the lowest used line in Part I of the death certificate) or selected according to coding rules (eAppendix 3, http://links.lww.com/EDE/A638).
Of 59,413 deaths in which hypertension was reported by the certifying physician as the underlying cause of death in 1980, hypertension was replaced by another cause of death as the underlying cause of death in 56% of cases due to application of the coding rules. The percent increased to 61% in 2005 (Table). Among cases in which the coding rules were applied, 6% in 1980 were due to reporting of incorrect causal cause-of-death sequences by the certifying physician; this increased to 22% in 2005.
An increase in the application of the coding rules since 1990 was noted for deaths in which hypertension was reported as the underlying cause of death. This is consistent with a previous study that indicated an increase in the reporting of incorrect causal sequences.6 In addition, hypertension was less likely to be replaced by ischemic heart disease as the underlying cause of death, which may be due in part to the long-term decline in ischemic heart disease mortality.7
In conclusion, the estimation of the level and trend of hypertension mortality is complicated by the coding rules for selecting the underlying cause of death for statistical tabulation. The result is that the burden of hypertension mortality calculated based on the underlying cause of death is underestimated. To properly interpret the level and trend of hypertension mortality, multiple-cause-of-death data should be used to examine hypertension mortality, taking into account all deaths with any mention of hypertension as a causal or contributing factor, hypertension reported as the underlying cause of death by certifying physicians, and hypertension selected as the underlying cause of death according to coding rules.
Institute of Public Health College of Medicine National Cheng Kung University Tainan, Taiwan
Robert N. Anderson
Mortality Statistics Branch Division of Vital Statistics National Center for Health Statistics Hyattsville, MD firstname.lastname@example.org
Department of Society, Human Development, and Health, Harvard School of Public Health, Harvard University, Boston, MA
1. Anderson RNRogers RG, Crimmins EM. Coding and classifying causes of death: trends and international differences. In: International Handbook of Adult Mortality. 2011 New York, NY Springer:467–489
2. World Health Organization. International Statistical Classification of Diseases and Related Health Problems (10th Revision, Volume 2). Instruction Manual. 20042nd ed Geneva, Switzerland World Health Organization
3. Redelings MD. A comparison of underlying cause and multiple causes of death: US vital statistics, 2000–2001. Epidemiology. 2006;17:100–103
4. Ayala C, Croft JB, Wattigney WA, Mensah GA. Trends in hypertension-related death in the United States: 1980–1998. J Clin Hypertens (Greenwich). 2004;6:675–681
6. Lu TH, Anderson RN, Kawachi I. Trends in frequency of reporting improper diabetes-related cause-of-death statements on death certificates, 1985–2005: an algorithm to identify incorrect causal sequences. Am J Epidemiol. 2010;171:1069–1078
7. Ford ES, Ajani UA, Croft JB, et al. Explaining the decrease in U.S. deaths from coronary disease, 1980–2000. N Engl J Med. 2007;356:2388–2398