However, the decline in other respiratory deaths occurred chiefly in the infective categories rather than in diseases characterized by airways obstruction which are the more likely to be misclassified. Since asthma has received much attention in both the lay and the medical press over the last decade, it is possible that it has become a more popular death certificate diagnosis. With the increased availability and use of effective bronchodilators, reversible airway obstruction may be demonstrated in more cases of obstructive respiratory disease which may then be diagnosed as asthma. Therefore, asthma deaths which may have previously been diagnosed as due to other obstructive respiratory diseases (ie, false negatives) may now be diagnosed correctly. However, this effect could not account for the recent increase in asthma mortality in the United States. The combined death rate from all obstructive airways diseases has been increasing since the late 1970s, while deaths attributed to bronchitis or emphysema have been stable (Fig 3). There could be, however, a reduction in false negative reporting of asthma deaths previously coded, for example, as cardiac deaths.
The lack of reciprocal trends in mortality from asthma and from all other respiratory diseases in the New Zealand data indicate that the epidemic there in the late 1970s was likely to have been real, and explanations other than changes in diagnostic fashions must be sought. The epidemic appears to be explained by several factors. A striking increase in the sales of asthma drugs in New Zealand in 1980 which did not occur to the same extent in the United Kingdom or Australia may indicate that the management of asthma has changed in New Zealand, but may also reflect, in part, an increase in asthma severity or prevalence. Prescribed pharmaceuticals were available free of charge in New Zealand during the period of the epidemic, while the cost of primary care consultations was increasing, and this may have encouraged excessive self medication.