Accuracy of Death Certificates for Coding Coronary Heart Disease as the Cause of Death

The death certificate is widely used to establish cause of death in epidemiologic and clinical investigations and for national statistics. Although mortality statistics are of interest to policymakers and researchers, the certification of the underlying and contributing causes of death is the responsibility of decedents' physicians, who often determine causes subjectively at the time of death. Coronary heart disease is the most common cause of death in the United States and many developed nations [1, 2]. Mortality due to coronary heart disease has been ascertained from death certificates in numerous ecological studies, in studies of secular trends in cause of death, and in therapy evaluation [3-8]. Because data on coronary heart disease mortality is used for various purposes, it is important that these data be both accurate and reliable. However, the amount of confidence that should be placed in these data depends heavily on the accuracy of the death certificate. For example, in the United States, death certificate data indicate that the rates of death from coronary heart disease increased until the mid-1960s to late 1960s and then steadily declined [8]. This trend may be the result of changes in disease rates, changes in diagnostic methods, changes in recording procedures, or a combination of these three factors. Therefore, the accuracy of death certificates must be examined, particularly with respect to chronic diseases, such as coronary heart disease, that increase in prevalence with advancing age. Data from the Framingham Heart Study allowed us to evaluate the accuracy of the death certificate diagnosis of coronary heart disease as the underlying cause of death. In this investigation, we compared cause of death obtained from the death certificate with cause of death assigned independently by a panel of trained physician-adjudicators. We also evaluated the influence of age, sex, calendar year in which death occurred, and prevalence of coronary heart disease on the accuracy of the death certificate. Methods Study Sample The Framingham Heart Study began in 1948 when 5209 residents of Framingham, Massachusetts, who were 28 to 62 years of age enrolled in a prospective epidemiologic study. The selection criteria and study design have been detailed elsewhere [9-11]. Every 2 years, members of this cohort receive follow-up evaluations that include medical histories, physical examinations, and selected laboratory tests. Ascertainment of the vital status of study participants has been essentially complete. Determination of Cause of Death Since the beginning of the study, each death has been reviewed and assigned an underlying cause by a panel of three physicians. As part of the review process, all available medical information about each death is collected. This information typically includes Framingham Heart Study records, hospitalization records, and, when available, autopsy results. In the case of an out-of-hospital, witnessed death, family members are interviewed by telephone to better ascertain the circumstances surrounding death. The death certificate is usually available to the panel, but it is not used to determine the underlying cause of death. After discussing the case, the panel jointly assigns an underlying cause of death, which is then coded into one of the following six mutually exclusive categories: 1) coronary heart disease, 2) stroke, 3) other cardiovascular disease, 4) cancer, 5) other, or 6) unknown. The panel makes a conscious effort to determine the true underlying cause of death; however, when cause cannot be reliably determined from all available data (for example, in the case of a nursing home resident with progressive inanition), the panel assigns the death to the category of unknown cause. Sudden death, defined as death occurring within 1 hour of symptom onset, is attributed to coronary heart disease unless another cause is apparent. These criteria for assigning cause of death have not changed since the beginning of the study. Diagnostic tests have improved over time, but the panel has access to the same results that are available to the physician coding the death certificate. We compared the underlying cause of death listed by each patient's physician on the death certificate with the cause assigned by the Framingham Heart Study physician panel. In 1988, a trained nosologist, blinded to the findings of the physician panel, coded each death certificate according to the International Classification of Diseases, Ninth Revision (ICD-9) [12]. For our study, we used the ICD-9 code assigned by the nosologist to determine the underlying cause of death on the death certificate. Coronary heart disease was considered the underlying cause of death if the cause of death was assigned an ICD-9 code of 410 to 414 (ischemic heart disease) by the nosologist. Although ICD-9 code 427 (cardiac dysrhythmia) has also been used to identify deaths from coronary heart disease [13], none of the deaths in our sample was assigned this code by the nosologist. Other cardiovascular disease was considered to be the underlying cause of death for ICD-9 codes of 390 to 404, 415 to 425, 428, 429, and 440 to 459. Death certificates of persons who died after 1988 have not been nosologically coded. Study Design By the end of 1988, 2888 participants from the original cohort had died. Death certificates were available and nosologically coded for 2719 of these participants (94.1%). Because few deaths occurred at young ages, we excluded participants whose age at death was younger than 45 years (n = 36). We examined the utility of the death certificate for coding coronary heart disease (as opposed to any other cause) as the underlying cause of death. With the cause of death determined by the physician panel serving as the reference standard, the sensitivity, positive predictive value, specificity, and negative predictive value of the death certificate for coding coronary heart disease as the underlying cause of death were calculated according to their usual definitions [14]. In the context of vital statistics, sensitivity has also been called the detection rate and positive predictive value has been called the confirmation rate [15]. To assess the effect of such cases on the overall utility of the death certificate, these variables were calculated with inclusion and exclusion of cases with panel-assigned unknown cause of death to assess the effect of such cases on the overall utility of the death certificate. For comparative purposes, the sensitivity, positive predictive value, specificity, and negative predictive value of the death certificate for cancer and for stroke as the underlying cause of death were also calculated. Cancer was considered the underlying cause of death if the ICD-9 code was 140 to 239; stroke was considered the underlying cause of death if the ICD-9 code was 430 to 438. To determine whether there was a time trend in the accuracy of the death certificate during the study, we compared the sensitivity, positive predictive value, specificity, and negative predictive value of the death certificate for coronary heart disease during three consecutive decades from 1955 through 1984. These decades were chosen because too few deaths occurred before 1955 to allow meaningful comparisons with later periods. To account for aging of the cohort, we further restricted this time-trend analysis to decedents whose age at death was 50 to 84 years (n = 2033). Statistical Analysis All analyses were performed by using SAS software [16]. Ninety-five percent CIs for the estimates of sensitivity, specificity, and positive and negative predictive values were calculated by using asymptotic calculations of the normal distribution. The chi-square test statistic with five degrees of freedom was used to assess equality of the marginal rates of diagnosis of underlying cause of death between the death certificate and the physician panel [17, 18]. Statistics were adjusted for age, sex, calendar year in which death occurred, or prevalence of coronary heart disease by using logistic regression [19] where such adjustment was indicated. For analysis of time trends in these statistics, we did not use the aggregate sample method described by Coughlin and colleagues [20]. Instead, logistic models were fitted to various subsets of participants. We let X = 1 if the panel assigned coronary heart disease as the cause of death and let X = 0 otherwise; similarly, we let Y = 1 if the death certificate recorded coronary heart disease as the cause of death and let Y = 0 otherwise. For sensitivity, we modeled Pr(Y = 1 X = 1); for specificity, Pr(Y = 0 X = 0); for positive predictive value, Pr(X = 1 Y = 1); and for negative predictive value, Pr(X = 0 Y = 0). For example, only patients with panel-assigned coronary heart disease death were used for analysis of sensitivity. Co-variates included in the models to test for time trend in the diagnosis of death from coronary heart disease were sex of the decedent, indicators for age group at time of death (50 to 64 years of age, 65 to 74 years of age, and 75 to 84 years of age), and decade of death (0 = 1955 to 1964, 1 = 1965 to 1974, and 2 = 1975 to 1984). Hosmer-Lemeshow statistics were computed to assess goodness of fit for the trend model. Further checks were made by using models with unique coefficients for each period and by using models without any time-period variable. A P value less than 0.05 was considered statistically significant. Results We analyzed a total of 2683 decedents whose underlying causes of death were coded by the death certificate and determined by the physician panel. The distribution of this sample by age and sex is given in Table 1. Overall, 52.6% of decedents were male and 41.9% were at least 75 years of age. Table 1. Decedents by Age at Death and Sex Table 2 presents a cross-classification of the 2683 deaths by underlying cause as assigned by the physician panel and as taken from the death ce