© 2002 American Thoracic Society
The Burden of Asthma in the United StatesLevel and Distribution Are Dependent on Interpretation of the National Asthma Education and Prevention Program GuidelinesChanning Laboratory, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts; Department of Medicine, The University of Adelaide, The Queen Elizabeth Hospital Campus, Woodville, South Australia, Australia; University of Arizona, Respiratory Sciences Center, Tucson, Arizona; Center for Healthcare Studies, Northwestern University, Chicago, Illinois; Center for Health Studies, Group Health Cooperative of Puget Sound, and Department of Pediatrics, University of Washington, Seattle, Washington; and Departments of Community Health Systems and Medicine, University of California, San Francisco, California Correspondence and requests for reprints should be addressed to Anne Fuhlbrigge, M.D., M.S., Channing Laboratory, Brigham and Women's Hospital, 181 Longwood Avenue, Boston, MA 02115. E-mail: anne.fuhlbrigge{at}channing.harvard.edu ABSTRACT Asthma imposes a growing burden on society in terms of morbidity, quality of life, and healthcare costs. Although federally sponsored national surveys provide estimates of asthma prevalence, these surveys are not designed to characterize the burden of asthma by self-reported disease activity. We sought to characterize asthma burden in the United States. This study was based on a cross-sectional random-digit-dial household telephone survey designed to identify adult patients and parents of children with current asthma. Global asthma burden was comprised of three components: short-term symptom burden (4-week recall), long-term symptom burden (past year), and functional impact (activity limitation). Using this construct, only 10.7% of individuals were classified as having a global asthma burden consistent with mild intermittent disease, and 77.3% had moderate to severe persistent disease. These results suggest that a majority of the United States population with asthma experiences persistent rather than intermittent asthma burden. In addition, the discordance in type and distribution of asthma symptoms reported by individual subjects suggests that the exact estimate of the burden of asthma is related to how the National Asthma Education and Prevention Program classification is operationalized. Inquiry into recent day or nighttime symptoms alone underestimates the burden of asthma and may lead to inadequate treatment of asthma based on national guideline recommendations.
Key Words: asthma • epidemiology • burden of illness • severity of illness index Asthma imposes a growing burden on society in terms of morbidity, quality of life, and healthcare costs. An increasing body of evidence describes the rising prevalence of asthma within the United States and around the world (1–3). Data from the National Health Interview Study suggest that in the United States alone, the number of cases of asthma reported since 1980 has increased by 75% and that the rate among children under the age of 5 has increased 160% (4). Although national data obtained by the National Health Interview Study provide prevalence estimates, the study was not designed to estimate the social burden of asthma for individuals of differing disease severity. The distribution of asthma severity among persons with asthma in the United States population is not well understood. Previous studies have estimated that between two-thirds to three-fourths of those with asthma have mild disease (5). More recent studies have suggested that the proportion of persons with mild asthma is significantly less than previously reported (6, 7); however, these studies are limited to either health plan–based populations or persons enrolled in clinical trials. Such studies have based their classification scheme on the 1997 Expert Panel Reports on the Diagnosis and Treatment of Asthma of the National Institutes of Health. The guidelines propose that asthma severity is based on symptomatic and functional assessments, including the frequency and severity of asthma symptoms, the frequency of rescue medication use, and objective measures of lung function. The guidelines also suggest that physicians should inquire about the impact of asthma on the patient and family, with the goal that therapy should allow patients to have few symptoms and little interference with activities (8). Therefore, a more global concept of severity would assess both the recent and chronic symptoms of asthma as well as its impact on a patient's functional status. In this study, we sought to characterize accurately the distribution of asthma burden using a National Asthma Education and Prevention Program (NAEPP)–based classification scheme within the U.S. population using data from a national probability sample of persons with current asthma. METHODS The data for this study come from a national probability sample of adult patients and parents of children with current asthma. Interviews were conducted by trained interviewers in 42,022 households with telephones in the United States and were refused or terminated in another 4,153 households, providing a participation rate of 91%. Persons were classified as having current asthma if they (1) had ever been diagnosed by a physician as having asthma and (2) either had taken medication for their asthma or had asthma symptoms in the past year. One or more persons who met the criteria for current asthma were identified in 3,273 of the 42,022 households (7.8%) in which a screening interview was completed. Interviews were completed with 2,509 of the 3,273 selected asthma patients or parents (76.7%). The vast majority (94.6%) of eligible nonrespondents were awaiting callback for interviews when the field period for the survey ended. This analysis was limited to those individuals who were 16 years old or older, yielding a national sample of older adolescents and adults with asthma (n = 1,788).
Questionnaire
Asthma Burden Classification The criteria for categorizing asthma burden were based on the NAEPP Expert Panel II recommendations for assessing asthma severity (8). Moderate and severe persistent asthma were combined, creating three categories of increasing severity (mild intermittent, mild persistent, and moderate/severe persistent). The NAEPP Expert Panel II criteria assign individuals to the highest grade in which any feature occurs (a detailed outline of the questions incorporated and algorithm used to categorize asthma burden can be found in the online data supplement). No lung function data were available.
Statistical Analysis RESULTS
Demographic Characteristics of the Asthma Population
Distribution of Disease Stratified by Measures of Burden The distribution of asthma burden is influenced by how asthma symptoms are measured. Persons with asthma report varying patterns of asthma symptoms; individual subjects were discordant in their report of type of short-term symptoms (data not shown). Only slightly more than half (58%) of the population was classified similarly on the basis of their report of both day and nighttime symptoms. A higher proportion of individuals was classified as having moderate to severe disease on the basis of nocturnal symptoms than on the basis of daytime symptoms (28.2% and 19.1%, respectively) (Table 2) . When day and nighttime symptoms were combined into the short-term symptom burden measure, the proportion of individuals classified as having moderate-to-severe persistent symptoms increased considerably (35.9%) (Table 2).
Similar to the discordance seen with nighttime versus daytime symptoms, a difference in the classification of individuals was seen according to whether short- or long-term symptoms were used; 39% of persons were classified differently by the two measures, with 11.4% of persons differing by two levels of burden (data not shown). The distribution of burden by activity limitation (functional impact) also demonstrated a large difference compared with the distribution of burden assessed by short-term symptoms, with a higher proportion of individuals classified as having moderate to severe persistent disease (70% versus 35.8%, respectively). The impact of asthma on the daily activities was considerable. Two-thirds of individuals reported that asthma had "some" or "a lot" of impact on physical activity. The assessment of global asthma burden was influenced by the discordance observed in how individual subjects report asthma symptoms. On the basis of the severity criteria derived from the NAEPP guidelines, only a minority (7.3%) of individuals were classified as having a global asthma burden consistent with mild intermittent disease; the large majority (77.3%) were classified as having moderate to severe disease (Figure 1) .
Association Between Measures of Disease Burden and Asthma Morbidity We next examined the association of asthma burden with patient characteristics and measures of asthma morbidity. There was an association between burden level and age, sex, education, income, and insurance status, but no association with race was observed. Persons reporting a higher short-term symptom burden also reported a lower level of education, lower income, and less private health insurance (Table 3) . In addition, persons with moderate-to-severe persistent asthma were more likely to be exposed to tobacco smoke in the home than were persons with mild intermittent disease (44.9% and 27.3%, respectively).
We observed a strong relationship between the level of short-term symptom burden and the report of medication use. Persons reporting a higher level of asthma burden reported increasing use of reliever medication over the last 4 weeks (42.1%, 77.4%, and 81.2% among subjects with mild intermittent, mild persistent, and moderate/severe persistent, respectively). Similarly, an association between short-term symptom burden and report of controller medication use was observed; however, the report of controller use in the last 4 weeks was low (16.5%, 29.7%, and 31.1% among subjects with mild intermittent, mild persistent, and moderate/severe persistent, respectively) (Table 4) .
There was a strong association between the level of short-term symptom burden and healthcare use (Table 4). Persons with moderate to severe persistent asthma had an almost fourfold increase in hospitalization rates in the past year (13.9% versus 3.6%), had a greater than twofold increase in urgent care visits (33.6% versus 16.1%), and were more than four times as likely to miss over 5 days of work related to their asthma than were persons with mild intermittent asthma (19.2% versus 4.4%, respectively). In addition, report of satisfaction with health was also strongly associated with short-term asthma burden; 59% of persons with mild intermittent asthma reported "very good to excellent" health, whereas only 31.6% of patients with moderate to severe asthma reported a similar level of satisfaction (  2 = 80, p < 0.001). The pattern and strength of the relationships between long-term symptom burden and socioeconomic status, medication use, healthcare use, and satisfaction with care were similar (data not shown). DISCUSSION Among a national probability sample of persons with current asthma in the United States, the majority of subjects report a symptom burden consistent with moderate-to-severe persistent disease (based on a NAEPP classification). Our findings, in association with other recent reports, suggest the distribution of symptom burden may be significantly higher than previously estimated. Interestingly, the impact of asthma on activity level is substantial. Assessment of activity limitation dramatically influenced the distribution of burden of asthma. Of importance is the large discordance between the level of symptom burden described by short-term symptoms, versus long-term symptoms, or activity limitation (functional impact). Asthma is an episodic disease, and individual variability in clinical presentation is not surprising. A classification system based only on symptoms over the last 4 weeks should be expected to give a different estimate of the overall burden of disease than one that incorporates long-term symptoms or activity limitation. Colice and colleagues (7) also observed a significant discordance between types of asthma symptom burden. Our results emphasize that the distribution of severity is greatly influenced by the type of symptom reports used in a classification system. The NAEPP guidelines do not outline clear cutpoints between all levels of symptoms within the severity classification. Therefore, how the NAEPP guidelines are operationalized can influence the distribution of severity reported. The shift in the distribution of symptom burden by inclusion of specific questions on activity limitation illustrates that evaluation of specific day and nighttime symptoms alone may underestimate the impact of asthma on patient's lives. In addition, patients' failure to communicate the impact of the disease on their daily activities may contribute to the discordance observed in symptom reports. Osborne and colleagues (9) compared physician-assessed severity and components of NAEPP-based severity score and found no correlation between physician-assessed severity and the level of current asthma symptoms. The lack of correlation persisted when asthma symptoms were separated into daytime and nocturnal symptoms. Thus, difficulties experienced by both patients and their physicians in recognizing asthma severity and subsequent undertreatment have been previously recognized and may be a reason for the high-level asthma burden observed (10–14). The role of the indoor environment on asthma prevalence and severity has been increasingly recognized (15, 16). Among adults with asthma, personal and passive tobacco exposures have also been associated with increasing severity and healthcare use (17). Given the high prevalence of tobacco exposure reported among our population, tobacco exposure may be a factor in the high level of symptom burden observed. Information regarding exposure to other indoor allergens was not available in this survey. Asthma symptoms have been shown to be strongly associated with increased risk of emergency room visits, hospitalization, and sick absences (9, 18). Similarly, we observed a strong association between symptom burden and other measures of asthma morbidity, including healthcare use, rescue bronchodilator use, and health status. Healthcare use was more than twofold greater in subjects with mild intermittent asthma than in persons with moderate to severe persistent asthma. The association persisted independent of the measure of symptom burden examined. Although the report of asthma symptom burden was associated with the report of controller medication use, the report of use over the last 4 weeks was low. These results are consistent with previous reports that suggest that the overall rate of use of controllers among subjects with asthma is low (19–21). Self-reported symptom burden correlates well with important measures of asthma morbidity. Asthma burden was also associated with demographic measures. We observed that a higher level of asthma-symptom burden was associated with lower income and education levels. However, we did not see a relationship between symptom burden and race. Previous reports indicate that socioeconomic status is an important contributing factor to asthma morbidity and mortality (22, 23); however, the separate effects of race and socioeconomic status are difficult to discern, and whether race has an independent effect on asthma morbidity has not been established. We must consider limitations of our classification of asthma burden. The NAEPP guidelines list activity limitation as a component of their severity classification. However, the criteria are not specifically defined, and the breakdown of severity levels for our analysis could be based only loosely on specific criteria outlined within the guidelines. Independent of our categorization of activity limitation, more than one-third of subjects reported "a lot" of activity limitation, and two-thirds reported at least "some" limitation in activity secondary to asthma. This level of activity limitation among people with asthma clearly indicates that the goals of asthma therapy to "maintain normal activity levels (including exercise and other physical activity)" are not being met (8). Additional limitations of our classification scheme must be discussed. Unlike the NAEPP scheme, our classification was based on self-reported information. No spirometry data were available. However, the Expert Panel II criteria assign individuals to the highest grade in which any feature occurs; the addition of lung function measurements would serve only to increase the number of people classified into higher burden categories. Furthermore, Colice and colleagues (7) recently observed that inclusion of FEV1 in their severity classification had little effect in shifting patients into a higher severity classification. Second, the NAEPP classification scheme categorizes subjects based on characteristics present before the institution of therapy. A proportion of our population was receiving medication at the time that they were surveyed; this could cause the reported symptom burden to underestimate the true symptom burden of this population. Third, our survey was limited to households with phones. However, estimates suggest 94.1% of occupied housing units in the United States had telephone service in 1999 (24). Our study population is representative of the United States population. Recent United States census figures for the general population report a distribution of race and education level similar to our asthma population. The study population did have a higher proportion of females compared with the general population, yet female predominance is reported in adult asthma populations (25).
Conclusion Acknowledgments The authors acknowledge the assistance of Patricia Vanderwolf and John Boyle of Schulman, Ronca, and Bucuvalas for statistical analysis in this study. FOOTNOTES Supported by a Mentored Clinical Scientist Development Award (1 KO8 HL03919–01) from the National Heart, Lung, and Blood Institute (A.L.F.), the Thoracic Society of Australia and New Zealand/Allen and Hanbury's Respiratory Research Fellowship (R.J.A.), and GlaxoSmithKline. This article has an online data supplement, which is accessible from this issue's table of contents online at www.atsjournals.org Received in original form July 12, 2001; accepted in final form May 29, 2002 REFERENCES
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