In this prospective cohort study of adults with severe asthma and comparable access to care, asthma severity and perceived asthma control were both risk factors for death. Thus, gradations of asthma severity, even within a cohort characterized by disease severe enough to have led to recent hospitalization, confers substantive all-cause mortality risk. Moreover, in examining death from a respiratory cause, asthma severity exhibits even more predictive power. This demonstrates internal consistency for this measure, since we would expect that a respiratory-specific measure would be a stronger predictor of a respiratory-related outcome.
Even taking asthma severity into account, the subjective perception of disease control independently conferred mortality risk. Improved perceived asthma control has previously been demonstrated to be associated with decreased prospective risk of severe asthma attacks and health care utilization.27, 35
Perceived asthma control, which is closely related to self-efficacy, is a patient’s self-perception of his or her ability to manage asthma and its exacerbations.35
Higher perceived control may result in improved disease control, either through improved self-management behaviors, improved medication adherence, or through unknown mechanisms.27
Our data showing that higher perceived asthma control predicts lower mortality even after adjustment for asthma severity suggests that perceived control is playing an independent and an important role. Prior research in other chronic diseases models has shown that clinical outcomes can be improved by intervention programs focused on improving self-efficacy,46, 47
which is confidence in one’s ability to carry out a behavior necessary to reach a desired goal.47
Further studies should investigate whether improving perceived asthma control can also reduce mortality risk.
Our observation that African American race was not a risk factor for mortality is all the more striking in this context, given that multiple prior studies have demonstrated a markedly higher risk of death from asthma among African Americans. There are several possible explanations for our findings. First, our study prospectively identified patients with asthma rather than relying on potentially less accurate cause-of-death information to define our cohort.8-12
Second, within the closed panel managed care organization in which the cohort was studied, asthma patients likely had equivalent access to care, thus controlling for this potential confounding variable. One key aspect to such health care access may be asthma-specific disease management programs. During the last decade, KP Northern California has made a major effort to standardize and improve asthma care through disease management programs, including clinical guidelines, patient self management education, disease registries, risk stratification, proactive outreach, reminders, multidisciplinary care teams, and performance feedback to providers.48
We found that adults with severe asthma had a markedly higher mortality than the age- and gender-matched California population (SMR 367%). However, cause-of-death was specifically attributed to asthma in relatively few subjects. There are several explanations for these contrasting findings. Asthma may play an indirect role in death from other causes, such as cardiovascular disease, either via inflammatory pathways or as a result of the treatment of the asthma with such medications as bronchodilators.37, 38
Alternatively, cause-of-death information on death certificates may be inaccurate, as numerous studies have concluded generally.49, 50
Some authors have suggested that physicians may not consider cause-of-death accuracy to be a priority and that declining autopsy rates have exacerbated death certificate unreliability.49, 50
Other studies have shown that deaths from asthma specifically are often mistakenly attributed to COPD.8-11
It is therefore possible that some of the deaths attributed to COPD in our study were in fact due to asthma. Finally, although we cannot say whether asthma was responsible for the entire increase in mortality suggested by the standardized mortality ratio, the fact that the severity-of-asthma score was related to increased mortality suggests that patients’ asthma did play a role.
Many more deaths were attributed to COPD than to asthma, which raises the question of whether some subjects may have had COPD rather than asthma on enrollment. However, the diagnosis of asthma was previously validated at baseline by an in-depth medical record review of a stratified random sample of 100 patients.26
In addition, all subjects reported a physician diagnosis of asthma during the survey. Finally, in a sensitivity analysis in which we attempted to more strictly ensure that included subjects had asthma rather than COPD as their primary underlying diagnosis, the relationships of the severity-of-asthma score and perceived asthma control score to mortality were virtually unchanged. Although the confidence intervals were slightly wider, this is expected when an analysis is performed on a smaller subset of a study population. Consequently, misclassification of COPD as asthma on enrollment does not appear to explain the results.
Despite its strengths, our study has limitations. Although we recruited a cohort with severe asthma, follow-up was relatively brief and death still remained an uncommon event. It is possible that the lack of associations, as seen with gender, were due to a lack of statistical power. In particular, the small number of Asian and Latino participants make it difficult to draw clear conclusions about their risk. This is unfortunate, because little has been published about the risk of death among asthmatic Asians, and our estimate of risk for Asians was the highest of any group. Despite the limitations of study size for the other groups, African Americans comprised nearly 1 in 5 of our cohort and, given the observed point estimate and 95% confidence intervals, African American race appears very unlikely to confer a higher risk of death. We also recruited a cohort with relatively severe asthma after hospitalization for this condition. As a consequence, our results will likely not apply to populations with mild asthma.
Overall, these results demonstrate an elevated risk of death among adults with severe asthma and show that the potentially modifiable factors of asthma severity and perceived asthma control are prospectively associated with death. This provides justification for interventional research to investigate whether reducing asthma severity and improving perceived control can improve asthma outcomes. The severity score’s application in a clinical context requires further validation, but ultimately it may help to identify patients at highest risk of death. Finally, it is noteworthy that our study did not find higher rates of death for African American asthmatics. Establishing access to care, as all patients in our cohort had done, stands out as one factor which may improve racial disparities in asthma mortality.