On the relationship between all-cause, cardiovascular, cancer and residual mortality rates with age.

Background: The existence of a highly significant linear relationship between the natural logarithm (ln) all-cause mortality rate and age at the population level is firmly established (r>0.99). The slope and intercept of the equation, however, vary markedly between populations. Whether this relationship also applies to specific disease entities has not been established.

Methods: Use was made of mortality rates for all-cause, total cardiovascular, total cancer and residual diseases. The midpoint of 5-year age classes between the ages of 35 and 84 years, obtained for both sexes, were analysed. The mean of the three latest available years, from the period 1997-1999 were used.

Results: The relationship also applies to a slightly lesser degree to the relationship between total cardiovascular mortality rate, consisting predominantly of ischemic heart disease and stroke, and age (r>0.99). Marginally better relationships are obtained using a second-degree polynomial equation between ln all-cause mortality rate and age, age as independent variables. Total ln cancer mortality rate, however, behaves differently with a significant negative deviation of the mortality rate from linearity at older ages. Residual mortality (non-cancer, non-cardiovascular) mortality shows a mirror pattern to cancer mortality. This residual mortality expressed as a percentage of all-cause mortality varies markedly between populations. The level of some major constituents of the residual mortality rates (respiratory diseases, pneumonia, ill-defined causes and senility) also varies markedly.

Conclusions: The magnitude of the variation suggests misclassification or misdiagnosis of several important disease entities, for example, between senility and stroke or between pneumonia and lung cancer. This questions the validity of disease-specific mortality rates especially at older ages, making their comparison between countries less reliable.