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The Influence of Perceived Control of Asthma on Health Outcomes
Original Research ASTHMA
The Influence of Perceived Control of Asthma on Health Outcomes* Carolyn S. Calfee, MD; Patricia P. Katz, PhD; Edward H. Yelin, PhD; Carlos Iribarren, MD, MPH, PhD; and Mark D. Eisner, MD, MPH, FCCP
Background: Psychosocial factors play an important role in outcomes of asthma. Perceived control, a measure of patients’ beliefs about their ability to control their disease, has not been studied in association with asthma health outcomes Methods: We used data from a prospective cohort study of patients who were hospitalized for asthma (n ⴝ 865). After hospital discharge, we conducted structured telephone interviews to obtain demographic characteristics, asthma history, and psychological variables. Interviews included administration of the Perceived Control of Asthma Questionnaire (PCAQ). We then prospectively measured emergency department (ED) visits and hospitalizations for asthma over time, with a median follow-up time of 1.9 years Results: Greater perceived control was associated with better physical health status (mean score increment per 2-point change in PCAQ score, 1.13; 95% confidence interval [CI], 0.79 to 1.47), better asthma-related quality of life (mean score increment, ⴚ2.24; 95% CI, ⴚ2.6 to ⴚ1.83), fewer days of restricted activity due to asthma (mean increment, ⴚ1.23; 95% CI, ⴚ1.62 to ⴚ0.83), and lower asthma severity scores (mean score increment, ⴚ0.40; 95% CI, ⴚ0.53 to ⴚ0.27). In a multivariate model, greater perceived control was associated with a significantly decreased prospective risk of ED visits (hazard ratio [HR], 0.92; 95% CI, 0.86 to 0.98; p ⴝ 0.008) and hospitalizations for asthma (HR, 0.84; 95% CI, 0.78 to 0.90; p < 0.0001). There was no association found between perceived control and most aspects of preventive care or self-management Conclusions: Greater perceived control is associated with improved asthma-related health status as well as with a decreased prospective risk of severe asthma attacks resulting in emergency health-care utilization. This difference does not appear to be mediated by changes in preventive care or asthma self-management practices. (CHEST 2006; 130:1312–1318) Key words: asthma/psychology; health services; internal-external control; quality of life Abbreviations: CI ⫽ confidence interval; ED ⫽ emergency department; HR ⫽ hazard ratio; KP ⫽ Kaiser Permanente; OR ⫽ odds ratio; PCAQ ⫽ Perceived Control of Asthma Questionnaire
in many other chronic conditions, psychosocial A sfactors play a significant role in the outcomes of patients with asthma.1 Psychiatric disease and psychosocial stress have been demonstrated to contribute to asthma morbidity and mortality.2–5 Patients’ *From the Cardiovascular Research Institute (Dr. Calfee), and the Department of Medicine (Drs. Katz, Yelin, and Eisner), University of California, San Francisco, CA; and the Kaiser Permanente Division of Research (Dr. Iribarren), San Francisco, CA. Dr. Eisner was supported by grant K23 HL04201 from the National Heart, Lung and Blood Institute, National Institutes of Health, with cofunding by the Social Security Administration. The authors have reported to the ACCP that no significant conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article. 1312
coping mechanisms for dealing with chronic disease and their attitudes and beliefs about their illness may also impact outcomes. For instance, the use of an avoidant coping style and a preference for less autonomy in asthma management decisions were associated with an increased risk of hospitalization Manuscript received November 18, 2005; revision accepted April 11, 2006. Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (www.chestjournal. org/misc/reprints.shtml). Correspondence to: Mark D. Eisner, MD, MPH, FCCP, University of California, San Francisco, 350 Parnassus Ave, Suite 609, San Francisco, CA 94115; e-mail: [email protected] DOI: 10.1378/chest.130.5.1312 Original Research
and emergency department (ED) visits for asthma, even when controlling for age, gender, educational level, and income.2 In addition, psychological factors impact patients’ quality of life, as evidenced by the fact that lower degrees of self-efficacy and more depressive symptoms have been correlated with lower asthma-related quality-of-life scores.3,4,6 Perceived control of asthma, which is defined as individuals’ perceptions of their ability to deal with asthma and its exacerbations, is a psychological factor that may have an important impact on adult asthma outcomes.7 Its effect could theoretically be mediated by self-management behaviors, such as the use of peak flow monitoring or inhaled corticosteroids, that can attenuate the severity of asthma. The perceived control of asthma construct, which is related to the general theories of self-efficacy, locus of control, and learned helplessness, has been associated with asthma severity and quality of life but has not been well studied in association with asthma outcomes like health-care utilization.7 If higher perceived control were associated with improved health outcomes, programs designed to increase perceived control may be useful targets for intervention. In the present study, we examined the impact of perceived control of asthma on a broad array of health outcomes, including asthma severity, health status, and emergency health-care utilization for asthma in a large cohort of adults with severe asthma.
Materials and Methods Overview We used data from a prospective cohort study of adult members of an integrated health-care delivery system who were hospitalized for asthma during a 4-year period. After hospital discharge, we conducted structured telephone interviews that assessed asthma history, psychological variables, and health status. The goal was to elucidate the impact of perceived control of asthma on asthma health status and the longitudinal risk of emergency health-care utilization for asthma. The study was approved by the University of California, San Francisco Committee on Human Research and the institutional review board of the Kaiser Foundation Research Institute. Further details about the study methods can be found in the online supplement. Subject Recruitment The sampling frame for the current study was all adult members of Kaiser Permanente (KP) in northern California. To establish a cohort with more severe asthma,8 –12 we recruited adults after hospitalization for asthma using a previously established algorithm.13 Beginning in April 2000, we attempted to recruit all eligible adults who were admitted to the ICU for asthma and were therefore considered to have more severe asthma.8,14 –17 To broaden the spectrum of asthma severity, we also began recruiting a random sample of all eligible adults who www.chestjournal.org
were hospitalized for asthma in September 2000; in October 2002, we began recruiting all eligible adults who had been hospitalized for asthma. Multiple studies9,12,18,19 have demonstrated that, compared to a general sample of asthma patients, adults who were hospitalized with asthma have greater severity-of-asthma scores, poorer health status, and an increased risk of death from asthma. The complete cohort included 865 subjects who underwent structured telephone interviews (overall completion rate, 53% [of 1,632 eligible subjects]). As previously reported,20 the sociodemographic characteristics of subjects who did and did not participate were similar. Further details of subject recruitment are available in the online supplement. Perceived Control of Asthma Structured telephone interviews were conducted within 1 month of hospital discharge to collect asthma history, sociodemographic characteristics, and psychological variables. These interviews included the administration of the Perceived Control of Asthma Questionnaire (PCAQ), an 11-item instrument that has been previously validated.7 The PCAQ assesses individuals’ perceptions of their ability to manage their asthma and its exacerbations. Sample statements include “If I do all the right things, I can successfully manage my asthma” and “it seems as though fate and other factors beyond my control affect my asthma”; responses are graded on a 5-point Likert scale. The questionnaire has been published in full elsewhere.7 Similar instruments have been developed for use in patients with allergic rhinitis and arthritis.21–23 Possible scores range from 11 to 55 points, with higher scores reflecting greater perceived control of asthma. Asthma Health Status A combined approach was used to measure asthma-related health status, using both disease-specific and generic health status measurements. We measured asthma severity using a previously validated 13-item, disease-specific severity-of-asthma score based on the frequency of current asthma symptoms (daytime or nocturnal), the use of systemic corticosteroids and other asthma medications, and history of hospitalization and intubation.24,25 Higher scores reflect more severe asthma. Previous work has established the reliability, concurrent validity, and predictive validity of the severity score.24,25 Generic physical and mental health status were measured using the 12-item short form health survey (SF-12) questionnaire.26 Previous work27 has demonstrated the validity of the SF-12 instrument in adult asthma. Higher scores reflect more favorable health status. We assessed asthma-specific quality of life using the asthma quality of life questionnaire of Marks et al,28 a 20-item questionnaire that measures the physical, emotional, and social impact of asthma. Previous work26,27 demonstrates the validity and responsiveness to change in asthma status of the questionnaire by Marks et al.29,30 Higher scores represent poorer asthma-specific quality of life. Depressive symptoms were ascertained using the Center for Epidemiologic Studies depression scale, one of the best known survey instruments for identifying symptoms of depression.31 It was originally developed to identify depression in the general population.32 Scores range from 0 to 60, with higher scores indicating greater depressive symptoms. Reliability and validity have been established in both the general population and in clinical settings.31,32 Restricted activity was measured using a question from the CHEST / 130 / 5 / NOVEMBER, 2006
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National Health Interview Survey.33 Briefly, subjects were asked to indicate how many days over the past month their activities were limited due to a medical condition; the range of responses was 0 to 31 days. Longitudinal Asthma Outcomes: Emergency Health-Care Utilization for Asthma To examine the longitudinal impact of perceived control of asthma on the risk of adverse asthma health outcomes, we used the computerized utilization databases of KP to ascertain ED visits and hospitalizations for asthma that occurred after the index hospitalization and after the baseline study interview. The median duration of follow-up was 1.9 years, with a maximum of 4.3 years. Preventive Care for Asthma We measured several aspects of the recommended preventive care for asthma patients, based on the National Asthma Education and Prevention Program guidelines,34,35 during telephone interviews. These included the use of long-term controller medications over the preceding 2 weeks (eg, inhaled corticosteroids, long-acting -agonists, and leukotriene modifiers), monitoring strategies (eg, possession of a peak flowmeter and/or an action plan, defined as a set of written instructions intended to guide the patient’s actions during an asthma flare), and educational/knowledge-based efforts (eg, asthma education classes and instruction on metered-dose inhaler use). In addition, we ascertained any contacts with KP asthma care managers, who include pharmacists, nurses, and respiratory therapists. We also measured the dispensing of long-term controller medications during the past 3 months from the KP computerized pharmacy database. Statistical Analysis Statistical analysis was conducted using a statistical software package (SAS, version 9.1; SAS Institute; Cary, NC). We used Pearson product-moment correlation coefficients to examine the cross-sectional association between perceived control of asthma and asthma severity, asthma-specific quality of life, generic health status, depressive symptoms, and restricted activity days. Multivariate linear regression was used to analyze the relationship between perceived control of asthma and these health status indicators controlling for factors that could be related to both perceived control and asthma severity on conceptual grounds. These factors included age, sex, race-ethnicity, socioeconomic status (ie, educational attainment and household income), and smoking history.7 Using the regression coefficients, we expressed the change in each dependent variable (ie, severity-of-asthma score) per a one half SD in the perceived control of asthma score, which approximates the minimally important difference for most health status measures.36 We used multivariate Cox proportional hazards regression analysis to evaluate the impact of perceived control of asthma on the prospective risk of emergency health-care utilization for asthma (ie, ED visits and hospitalizations), taking variable follow-up time into account. In multivariate models, we controlled for the same covariates that were discussed above. To examine whether the impact of perceived control on health status and emergency health-care utilization was mediated by a direct effect on asthma severity, we performed additional multivariate analyses that also controlled for the severity-of-asthma score. Furthermore, we controlled for preventive care measures to elucidate whether the effect of perceived control was mediated by these factors. 1314
Results Patient Characteristics The baseline characteristics of the study population (n ⫽ 865) are summarized in Table 1. The mean age of the patients studied was 60 years; the majority were women, had completed at least some college, and were either current or former smokers. The mean perceived control of asthma score was 38 points (SD, 4.2 points; range, 18 to 55 points). Correlation of Perceived Control With Asthma Severity and Health Status Greater perceived control, as measured by a higher score on the PCAQ, was associated with better scores on the SF-12 for both mental health (r ⫽ 0.16; p ⬍ 0.0001) and physical health (r ⫽ 0.24; p ⬍ 0.0001), indicating more favorable health status. Similarly, greater perceived control was associated with lower asthma severity scores (r ⫽ ⫺0.22; p ⬍ 0.0001). Greater perceived control was also associated with better asthma-specific quality of life (r ⫽ ⫺0.38; p ⬍ 0.0001), fewer depressive symptoms (r ⫽ ⫺0.28; p ⬍ 0.0001), and fewer days of restricted activity due to asthma during the past month (r ⫽ ⫺0.22; p ⬍ 0.0001). Using multivariate linear regression analysis, we examined the change in health status or asthma severity per a one half SD-sized PCAQ increment (ie, 2 points) [Table 2]. The analysis controlled for
Table 1—Baseline Subject Characteristics of Adults With Severe Asthma (n ⴝ 865)* Characteristics Age,† yr Female gender Educational Level High school only or less Some college Graduated college White, non-Hispanic race/ethnicity Smoking history Never smoker Current smoker Former smoker ICU admission (baseline hospitalization) Childhood diagnosis of asthma Married or cohabitating Self-reported household income ⬍ $20,000 $20–60,000 ⬎ $60,000 Declined to answer PCAQ score‡
Value 60.4 (16.3) 70 (608) 20 (170) 57 (496) 23 (199) 59 (514) 35 (301) 10 (85) 55 (479) 22 (193) 32 (278) 61 (527) 15 (137) 66 (569) 12 (105) 6 (54) 38 (18–55; 4.2)
*Values are given as % (No.), unless otherwise indicated. †Values are given as the mean (SD). ‡Values are given as the mean (range; SD). Original Research
Table 2—Perceived Control of Asthma and Health Status* Mean Difference Asthma Status Measure
Unadjusted (95% CI)
Multivariable Model† (95% CI)
Multivariable Model Adjusted for Asthma Severity (95% CI)
Asthma severity Physical health status Mental health status Asthma-specific QOL Depressive symptoms Restricted activity days
⫺ 0.43 (⫺ 0.56 to ⫺ 0.30) 1.32 (0.96 to 1.68) 0.79 (0.45 to 1.12) ⫺ 2.50 (⫺ 2.9 to ⫺ 2.08) ⫺ 1.32 (⫺ 1.62 to ⫺ 1.02) ⫺ 1.4 (⫺ 1.80 to ⫺ 0.99)
⫺ 0.40 (⫺ 0.53 to ⫺ 0.27) 1.13 (0.79 to 1.47) 0.65 (0.32 to 0.97) ⫺ 2.24 (⫺ 2.64 to ⫺ 1.83) ⫺ 1.12 (⫺ 1.41 to ⫺ 0.82) ⫺ 1.23 (⫺ 1.62 to ⫺ 0.83)
NA 0.93 (0.59 to 1.27) 0.54 (0.21 to 0.87) ⫺ 1.72 (⫺ 2.09 to ⫺ 1.36) ⫺ 0.99 (⫺ 1.28 to ⫺ 0.69) ⫺ 0.88 (⫺ 1.27 to ⫺ 0.49)
*Values in parentheses are 95% CI. QOL ⫽ quality of life; NA ⫽ not applicable; Mean difference ⫽ change in asthma status measure for every 2-point increment (0.5 SD) in PCAQ score. †Multivariable model adjusts for age, gender, educational level, income, ethnicity, and smoking status.
age, gender, educational level, income, ethnicity, and smoking status. Greater perceived control was associated with better asthma severity, physical health status, mental health status, and asthma-specific quality of life. Higher perceived control was also associated with fewer restricted activity days during the past month and fewer depressive symptoms. The addition of asthma severity to the multivariate analysis did not substantively affect the results (Table 2). Perceived Control and Emergency Health-Care Utilization Higher perceived control of asthma was associated with a decreased likelihood of ED visits and hospitalizations for asthma (Table 3). For every 2-point increase in perceived control, the odds of ED visits declined by 8% (hazard ratio [HR], 0.92; 95% confidence interval [CI], 0.86 to 0.98; p ⫽ 0.008), and the odds of hospitalization declined by 16% (HR, 0.84; 95% CI, 0.78 to 0.90; p ⬍ 0.0001), after controlling for age, gender, educational level, income, ethnicity, and smoking status. Controlling for asthma severity had little effect on the results for hospitalization, whereas perceived control was related to the risk of ED visits somewhat less strongly after controlling for severity (Table 3). Adding an adjustment for ICU admission at study entry to the multivariate
analysis (as another marker of severe asthma) also had little effect on the risk of ED visits (HR, 0.92; 95% CI, 0.86 to 0.98) or hospitalizations (HR, 0.84; 95% CI, 0.78 to 0.91). Controlling for outpatient asthma therapy and preventive measures had little impact on the effect of perceived control on risk of ED visits (HR, 0.95; 95% CI, 0.89 to 1.01) or hospitalization (HR, 0.86; 95% CI, 0.80 to 0.93). Likewise, adding depression to the multivariate model had no substantive impact on the effect of perceived control on the risk of ED visits (HR, 0.92; 95% CI, 0.86 to 0.98) or hospitalization (HR, 0.85; 95% CI, 0.78 to 0.93). Perceived Control, Self-Management, and Preventive Therapy for Asthma There was no association found between perceived control of asthma and most aspects of recommended preventive therapy for asthma (Table 4). Specifically, greater perceived control was not associated with differential use of controller medications (eg, inhaled corticosteroids, long-acting -agonists, and leukotriene modifiers) during the 2 weeks preceding the interview. There was also no association found between perceived control and the recent dispensing of asthma controller medications (during the past 3 months) based on computerized pharmacy data.
Table 3—Prospective Impact of Perceived Control of Asthma on Emergency Health-Care Utilization for Asthma* ED Visits Unadjusted
Hospitalization Multivariate
Unadjusted
Variables
HR (95% CI)
p Value
HR (95% CI)
p Value
HR (95% CI)
p Value
Perceived control Perceived control, adjusted for baseline asthma severity
0.91 (0.85–0.97) NA
0.002
0.92 (0.86–0.98) 0.95 (0.90–1.02)
0.008 0.14
0.83 (0.77–0.88) NA
⬍0.0001
Multivariate HR (95% CI)
p Value
0.84 (0.78–0.90) ⬍0.0001 0.88 (0.82–0.95) 0.0009
*HRs are expressed per 2-point increment (0.5 SD) in perceived control. See Table 2 for abbreviation not used in the text. Multivariate model adjusts for age, gender, education level, income, ethnicity, and smoking status. www.chestjournal.org
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Table 4 —Perceived Control of Asthma and Preventive Care for Asthma* Preventive Care Measure Controller medication use (by self-report over past 2 wk)† Inhaled corticosteroid Long-acting inhaled -agonist Leukotriene modifiers Controller medication dispensing (past 3 mo)† Inhaled corticosteroid Long-acting inhaled -agonist Leukotriene modifiers Asthma monitoring Possesses peak flowmeter Uses peak flowmeter ⱖ 1 time/wk Has self-management (action) plan Asthma education (during past yr) Contact with KP care manager‡ Attended asthma education class Direct instruction on metered-dose inhaler use
Likelihood of Preventive Measure
p Value
0.99 (0.93–1.07) 0.95 (0.89–1.02) 0.92 (0.83–1.02)
0.86 0.17 0.12
1.06 (0.98–1.14) 0.95 (0.88–1.03) 0.95 (0.83–1.09)
0.17 0.18 0.47
0.99 (0.92–1.07) 0.98 (0.91–1.06) 1.02 (0.96–1.09)
0.77 0.65 0.50
0.99 (0.91–1.06) 0.99 (0.92–1.06) 1.19 (1.04–1.36)
0.73 0.71 0.01
*Values are given as OR (95% CI), unless otherwise indicated. †Medication dispensing is from KP computerized databases (688 subjects with KP pharmacy coverage); all other measures are self-reported from structured telephone interviews. ‡Telephone call or visit with asthma intervention specialist (eg, specially trained nurse, nurse practitioner, respiratory therapist, or pharmacist).
Similarly, perceived control was not significantly associated with most measures of asthma self-management (Table 4). Specifically, perceived control scores were not related to the regular use of a peak flowmeter, knowledge of an asthma action plan, or attendance at an asthma education class, or with contact with a KP asthma care manager to discuss asthma-related issues over the past 12 months. However, greater perceived control was associated with an increased likelihood of receiving direct instruction on the use of a metered-dose inhaler (odds ratio [OR], 1.19; 95% CI, 1.04 to 1.36; p ⫽ 0.01). Greater perceived control of asthma was related to a lower likelihood of using some therapies that were indicative of inadequately treated asthma. For example, increased perceived control was associated with a decreased likelihood of receiving therapy with oral corticosteroids at the time of the interview (OR, 0.88; 95% CI, 0.81 to 0.95; p ⫽ 0.001) but had no significant association with the likelihood of being corticosteroid-dependent (OR, 0.94; 95% CI, 0.88 to 1.01; p ⫽ 0.11), defined as having required therapy with oral corticosteroids for ⬎ 3 months during the prior year. Greater perceived control was likewise associated with a lower likelihood of using nebulizer therapy (OR, 0.85; 95% CI, 0.79 to 0.91; p ⬍ 0.0001). Discussion We found that, in a cohort of insured patients with asthma, greater perceived control is associated with 1316
improved measures of physical and mental health status, better quality of life, fewer depressive symptoms, and fewer days of restricted activity due to asthma. In addition, we found that greater perceived control is associated with a decreased prospective risk of ED visits and a decreased likelihood of hospitalization for asthma. Our results expand on our previous findings by showing that perceived control influences the risk of emergency health-care utilization for asthma during longitudinal follow-up.7 The impact of the perceived control of asthma on health outcomes did not seem to be completely mediated by asthma severity, selfmanagement behaviors, or the use of preventive therapies for asthma (as measured by this study). The mechanism of the effect of perceived control on health-care utilization remains unclear. Patients with low perceived control may believe that they are unable to manage even mild asthma attacks at home and thus present for emergency care; however, this mechanism would not seem to account for the increased rate of hospitalization among patients with low perceived control. It remains possible that the effects of perceived control could be mediated by better adherence to asthma therapies; a quantitative assessment of medication adherence would be necessary to evaluate this hypothesis. Alternatively, perceived control could affect the perception of dyspnea and other respiratory symptoms, thereby affecting health status, quality of life, and decisions about seeking emergency health care. The finding that greater perceived control is Original Research
linked to undergoing instruction in the proper use of metered-dose inhalers was not unexpected; in fact, the lack of correlation with other asthma self-management programs was somewhat surprising. Previous studies23 have demonstrated that increasing patients’ knowledge about how to treat their symptoms may increase their self-efficacy, a concept that is closely linked to perceived control. Similarly, perceived control has been previously shown37,38 to be increased as a result of asthma self-management educational programs. Although most studies have suggested that similar self-management programs impact health outcomes via changes in behaviors such as adherence to treatment, others39 have not confirmed this hypothesis and have suggested that self-management courses impact health outcomes via some other, as yet undefined pathway. Our study has several limitations. First, findings from this study population may not be generalizable to all patients with asthma. Patients were recruited to the study after hospitalization for asthma, which is a marker of severe disease8; thus, these results may not be applicable to those patients with mild asthma. Also, participants were members of a large healthcare delivery system and therefore had access to health care, although members of KP of northern California have been found to have sociodemographic characteristics similar to those of other members of the regional population.40,41 There was a greater prevalence of women in our study, but this imbalance is consistent with the higher prevalence of asthma among women nationwide. The use of International Classification of Diseases, ninth revision, coding to determine study eligibility could potentially lead to inaccurate classification; we sought to avoid this issue by an in-depth review of randomly selected charts to confirm that the conditions of patients were accurately coded. In addition, although we were able to obtain data from pharmacy dispensing records for asthma medications, we do not have specific objective measures of treatment adherence. Another significant limitation of this study is the lack of a separate objective measure of asthma control. The finding that greater perceived control was not associated with increased use of asthma self-management strategies could reflect patients’ inaccurate perceptions of their actual asthma control. For example, if patients with poorly controlled disease perceive their asthma to be well controlled, they might not engage in salutary self-management strategies, such as use of controller medications and peak flow meters. In our analysis, we believe that the bias would be conservative if patients’ perceived control “overestimated” their actual control (eg, the protective association between perceived control and www.chestjournal.org
the risk of rehospitalization would be even more favorable than that we actually observed). We also did not include a measure of pulmonary function in our cohort. However, multiple studies24,42,43 have demonstrated that pulmonary function correlates only modestly with other measures of asthma severity and health status. We have previously shown that the severity-of-asthma score is a reliable and valid survey-based measure of asthma severity.25 Moreover, it strongly predicts longitudinal hospitalization for asthma.44 In conclusion, perceived control of asthma appears to be linked to asthma outcomes, including health status measures and health-care utilization, in patients with severe asthma, even after controlling for asthma severity. Further studies are needed to determine whether interventions designed to increase perceived control will improve health outcomes and, if so, by what mechanism.
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nation of responsiveness to change. J Clin Epidemiol 1999; 52:667– 675 McDowell I, Newell C. Measuring health: a guide to rating scales and questionnaires. New York, NY: Oxford University Press, 1996 Radloff LS. The CES-D Scale: a self-report depression scale for research in the general population. Appl Psychol Meas 1977; 1:385– 401 National Center for Health Statistics. 1997 National Health Interview Survey. Hyattsville, MD: National Center for Health Statistics, 1997 National Asthma Education Program. Guidelines for the diagnosis and management of asthma: expert panel report; National Heart, Lung, and Blood Institute. J Allergy Clin Immunol 1991; 88:425–534 National Asthma Education and Prevention Program. Expert panel report: guidelines for the diagnosis and management of asthma; update on selected topics—2002. J Allergy Clin Immunol 2002; 110(suppl):S141–219 Norman GR, Sloan JA, Wyrwich KW. Interpretation of changes in health-related quality of life: the remarkable universality of half a standard deviation. Med Care 2003; 41:582–592 Janson SL, Fahy JV, Covington JK, et al. Effects of individual self-management education on clinical, biological, and adherence outcomes in asthma. Am J Med 2003; 115:620 – 626 Saini B, Krass I, Armour C. Development, implementation, and evaluation of a community pharmacy-based asthma care model. Ann Pharmacother 2004; 38:1954 –1960 Lorig K, Seleznick M, Lubeck D, et al. The beneficial outcomes of the arthritis self-management course are not adequately explained by behavior change. Arthritis Rheum 1989; 32:91–95 Karter AJ, Ferrara A, Liu JY, et al. Ethnic disparities in diabetic complications in an insured population. JAMA 2002; 287:2519 –2527 Krieger N. Overcoming the absence of socioeconomic data in medical records: validation and application of a census-based methodology. Am J Public Health 1992; 82:703–710 Carranza Rosenzweig JR, Edwards L, Lincourt W, et al. The relationship between health-related quality of life, lung function and daily symptoms in patients with persistent asthma. Respir Med 2004; 98:1157–1165 Juniper EF, Svensson K, O’Byrne PM, et al. Asthma quality of life during 1 year of treatment with budesonide with or without formoterol. Eur Respir J 1999; 14:1038 –1043 Eisner MD, Katz PP, Yelin EH, et al. Risk factors for hospitalization among adults with asthma: the influence of sociodemographic factors and asthma severity. Respir Res 2001; 2:53– 60
Original Research
The Influence of Perceived Control of Asthma on Health Outcomes* Carolyn S. Calfee, MD; Patricia P. Katz, PhD; Edward H. Yelin, PhD; Carlos Iribarren, MD, MPH, PhD; and Mark D. Eisner, MD, MPH, FCCP
Background: Psychosocial factors play an important role in outcomes of asthma. Perceived control, a measure of patients’ beliefs about their ability to control their disease, has not been studied in association with asthma health outcomes Methods: We used data from a prospective cohort study of patients who were hospitalized for asthma (n ⴝ 865). After hospital discharge, we conducted structured telephone interviews to obtain demographic characteristics, asthma history, and psychological variables. Interviews included administration of the Perceived Control of Asthma Questionnaire (PCAQ). We then prospectively measured emergency department (ED) visits and hospitalizations for asthma over time, with a median follow-up time of 1.9 years Results: Greater perceived control was associated with better physical health status (mean score increment per 2-point change in PCAQ score, 1.13; 95% confidence interval [CI], 0.79 to 1.47), better asthma-related quality of life (mean score increment, ⴚ2.24; 95% CI, ⴚ2.6 to ⴚ1.83), fewer days of restricted activity due to asthma (mean increment, ⴚ1.23; 95% CI, ⴚ1.62 to ⴚ0.83), and lower asthma severity scores (mean score increment, ⴚ0.40; 95% CI, ⴚ0.53 to ⴚ0.27). In a multivariate model, greater perceived control was associated with a significantly decreased prospective risk of ED visits (hazard ratio [HR], 0.92; 95% CI, 0.86 to 0.98; p ⴝ 0.008) and hospitalizations for asthma (HR, 0.84; 95% CI, 0.78 to 0.90; p < 0.0001). There was no association found between perceived control and most aspects of preventive care or self-management Conclusions: Greater perceived control is associated with improved asthma-related health status as well as with a decreased prospective risk of severe asthma attacks resulting in emergency health-care utilization. This difference does not appear to be mediated by changes in preventive care or asthma self-management practices. (CHEST 2006; 130:1312–1318) Key words: asthma/psychology; health services; internal-external control; quality of life Abbreviations: CI ⫽ confidence interval; ED ⫽ emergency department; HR ⫽ hazard ratio; KP ⫽ Kaiser Permanente; OR ⫽ odds ratio; PCAQ ⫽ Perceived Control of Asthma Questionnaire
in many other chronic conditions, psychosocial A sfactors play a significant role in the outcomes of patients with asthma.1 Psychiatric disease and psychosocial stress have been demonstrated to contribute to asthma morbidity and mortality.2–5 Patients’ *From the Cardiovascular Research Institute (Dr. Calfee), and the Department of Medicine (Drs. Katz, Yelin, and Eisner), University of California, San Francisco, CA; and the Kaiser Permanente Division of Research (Dr. Iribarren), San Francisco, CA. Dr. Eisner was supported by grant K23 HL04201 from the National Heart, Lung and Blood Institute, National Institutes of Health, with cofunding by the Social Security Administration. The authors have reported to the ACCP that no significant conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article. 1312
coping mechanisms for dealing with chronic disease and their attitudes and beliefs about their illness may also impact outcomes. For instance, the use of an avoidant coping style and a preference for less autonomy in asthma management decisions were associated with an increased risk of hospitalization Manuscript received November 18, 2005; revision accepted April 11, 2006. Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (www.chestjournal. org/misc/reprints.shtml). Correspondence to: Mark D. Eisner, MD, MPH, FCCP, University of California, San Francisco, 350 Parnassus Ave, Suite 609, San Francisco, CA 94115; e-mail: [email protected] DOI: 10.1378/chest.130.5.1312 Original Research
and emergency department (ED) visits for asthma, even when controlling for age, gender, educational level, and income.2 In addition, psychological factors impact patients’ quality of life, as evidenced by the fact that lower degrees of self-efficacy and more depressive symptoms have been correlated with lower asthma-related quality-of-life scores.3,4,6 Perceived control of asthma, which is defined as individuals’ perceptions of their ability to deal with asthma and its exacerbations, is a psychological factor that may have an important impact on adult asthma outcomes.7 Its effect could theoretically be mediated by self-management behaviors, such as the use of peak flow monitoring or inhaled corticosteroids, that can attenuate the severity of asthma. The perceived control of asthma construct, which is related to the general theories of self-efficacy, locus of control, and learned helplessness, has been associated with asthma severity and quality of life but has not been well studied in association with asthma outcomes like health-care utilization.7 If higher perceived control were associated with improved health outcomes, programs designed to increase perceived control may be useful targets for intervention. In the present study, we examined the impact of perceived control of asthma on a broad array of health outcomes, including asthma severity, health status, and emergency health-care utilization for asthma in a large cohort of adults with severe asthma.
Materials and Methods Overview We used data from a prospective cohort study of adult members of an integrated health-care delivery system who were hospitalized for asthma during a 4-year period. After hospital discharge, we conducted structured telephone interviews that assessed asthma history, psychological variables, and health status. The goal was to elucidate the impact of perceived control of asthma on asthma health status and the longitudinal risk of emergency health-care utilization for asthma. The study was approved by the University of California, San Francisco Committee on Human Research and the institutional review board of the Kaiser Foundation Research Institute. Further details about the study methods can be found in the online supplement. Subject Recruitment The sampling frame for the current study was all adult members of Kaiser Permanente (KP) in northern California. To establish a cohort with more severe asthma,8 –12 we recruited adults after hospitalization for asthma using a previously established algorithm.13 Beginning in April 2000, we attempted to recruit all eligible adults who were admitted to the ICU for asthma and were therefore considered to have more severe asthma.8,14 –17 To broaden the spectrum of asthma severity, we also began recruiting a random sample of all eligible adults who www.chestjournal.org
were hospitalized for asthma in September 2000; in October 2002, we began recruiting all eligible adults who had been hospitalized for asthma. Multiple studies9,12,18,19 have demonstrated that, compared to a general sample of asthma patients, adults who were hospitalized with asthma have greater severity-of-asthma scores, poorer health status, and an increased risk of death from asthma. The complete cohort included 865 subjects who underwent structured telephone interviews (overall completion rate, 53% [of 1,632 eligible subjects]). As previously reported,20 the sociodemographic characteristics of subjects who did and did not participate were similar. Further details of subject recruitment are available in the online supplement. Perceived Control of Asthma Structured telephone interviews were conducted within 1 month of hospital discharge to collect asthma history, sociodemographic characteristics, and psychological variables. These interviews included the administration of the Perceived Control of Asthma Questionnaire (PCAQ), an 11-item instrument that has been previously validated.7 The PCAQ assesses individuals’ perceptions of their ability to manage their asthma and its exacerbations. Sample statements include “If I do all the right things, I can successfully manage my asthma” and “it seems as though fate and other factors beyond my control affect my asthma”; responses are graded on a 5-point Likert scale. The questionnaire has been published in full elsewhere.7 Similar instruments have been developed for use in patients with allergic rhinitis and arthritis.21–23 Possible scores range from 11 to 55 points, with higher scores reflecting greater perceived control of asthma. Asthma Health Status A combined approach was used to measure asthma-related health status, using both disease-specific and generic health status measurements. We measured asthma severity using a previously validated 13-item, disease-specific severity-of-asthma score based on the frequency of current asthma symptoms (daytime or nocturnal), the use of systemic corticosteroids and other asthma medications, and history of hospitalization and intubation.24,25 Higher scores reflect more severe asthma. Previous work has established the reliability, concurrent validity, and predictive validity of the severity score.24,25 Generic physical and mental health status were measured using the 12-item short form health survey (SF-12) questionnaire.26 Previous work27 has demonstrated the validity of the SF-12 instrument in adult asthma. Higher scores reflect more favorable health status. We assessed asthma-specific quality of life using the asthma quality of life questionnaire of Marks et al,28 a 20-item questionnaire that measures the physical, emotional, and social impact of asthma. Previous work26,27 demonstrates the validity and responsiveness to change in asthma status of the questionnaire by Marks et al.29,30 Higher scores represent poorer asthma-specific quality of life. Depressive symptoms were ascertained using the Center for Epidemiologic Studies depression scale, one of the best known survey instruments for identifying symptoms of depression.31 It was originally developed to identify depression in the general population.32 Scores range from 0 to 60, with higher scores indicating greater depressive symptoms. Reliability and validity have been established in both the general population and in clinical settings.31,32 Restricted activity was measured using a question from the CHEST / 130 / 5 / NOVEMBER, 2006
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National Health Interview Survey.33 Briefly, subjects were asked to indicate how many days over the past month their activities were limited due to a medical condition; the range of responses was 0 to 31 days. Longitudinal Asthma Outcomes: Emergency Health-Care Utilization for Asthma To examine the longitudinal impact of perceived control of asthma on the risk of adverse asthma health outcomes, we used the computerized utilization databases of KP to ascertain ED visits and hospitalizations for asthma that occurred after the index hospitalization and after the baseline study interview. The median duration of follow-up was 1.9 years, with a maximum of 4.3 years. Preventive Care for Asthma We measured several aspects of the recommended preventive care for asthma patients, based on the National Asthma Education and Prevention Program guidelines,34,35 during telephone interviews. These included the use of long-term controller medications over the preceding 2 weeks (eg, inhaled corticosteroids, long-acting -agonists, and leukotriene modifiers), monitoring strategies (eg, possession of a peak flowmeter and/or an action plan, defined as a set of written instructions intended to guide the patient’s actions during an asthma flare), and educational/knowledge-based efforts (eg, asthma education classes and instruction on metered-dose inhaler use). In addition, we ascertained any contacts with KP asthma care managers, who include pharmacists, nurses, and respiratory therapists. We also measured the dispensing of long-term controller medications during the past 3 months from the KP computerized pharmacy database. Statistical Analysis Statistical analysis was conducted using a statistical software package (SAS, version 9.1; SAS Institute; Cary, NC). We used Pearson product-moment correlation coefficients to examine the cross-sectional association between perceived control of asthma and asthma severity, asthma-specific quality of life, generic health status, depressive symptoms, and restricted activity days. Multivariate linear regression was used to analyze the relationship between perceived control of asthma and these health status indicators controlling for factors that could be related to both perceived control and asthma severity on conceptual grounds. These factors included age, sex, race-ethnicity, socioeconomic status (ie, educational attainment and household income), and smoking history.7 Using the regression coefficients, we expressed the change in each dependent variable (ie, severity-of-asthma score) per a one half SD in the perceived control of asthma score, which approximates the minimally important difference for most health status measures.36 We used multivariate Cox proportional hazards regression analysis to evaluate the impact of perceived control of asthma on the prospective risk of emergency health-care utilization for asthma (ie, ED visits and hospitalizations), taking variable follow-up time into account. In multivariate models, we controlled for the same covariates that were discussed above. To examine whether the impact of perceived control on health status and emergency health-care utilization was mediated by a direct effect on asthma severity, we performed additional multivariate analyses that also controlled for the severity-of-asthma score. Furthermore, we controlled for preventive care measures to elucidate whether the effect of perceived control was mediated by these factors. 1314
Results Patient Characteristics The baseline characteristics of the study population (n ⫽ 865) are summarized in Table 1. The mean age of the patients studied was 60 years; the majority were women, had completed at least some college, and were either current or former smokers. The mean perceived control of asthma score was 38 points (SD, 4.2 points; range, 18 to 55 points). Correlation of Perceived Control With Asthma Severity and Health Status Greater perceived control, as measured by a higher score on the PCAQ, was associated with better scores on the SF-12 for both mental health (r ⫽ 0.16; p ⬍ 0.0001) and physical health (r ⫽ 0.24; p ⬍ 0.0001), indicating more favorable health status. Similarly, greater perceived control was associated with lower asthma severity scores (r ⫽ ⫺0.22; p ⬍ 0.0001). Greater perceived control was also associated with better asthma-specific quality of life (r ⫽ ⫺0.38; p ⬍ 0.0001), fewer depressive symptoms (r ⫽ ⫺0.28; p ⬍ 0.0001), and fewer days of restricted activity due to asthma during the past month (r ⫽ ⫺0.22; p ⬍ 0.0001). Using multivariate linear regression analysis, we examined the change in health status or asthma severity per a one half SD-sized PCAQ increment (ie, 2 points) [Table 2]. The analysis controlled for
Table 1—Baseline Subject Characteristics of Adults With Severe Asthma (n ⴝ 865)* Characteristics Age,† yr Female gender Educational Level High school only or less Some college Graduated college White, non-Hispanic race/ethnicity Smoking history Never smoker Current smoker Former smoker ICU admission (baseline hospitalization) Childhood diagnosis of asthma Married or cohabitating Self-reported household income ⬍ $20,000 $20–60,000 ⬎ $60,000 Declined to answer PCAQ score‡
Value 60.4 (16.3) 70 (608) 20 (170) 57 (496) 23 (199) 59 (514) 35 (301) 10 (85) 55 (479) 22 (193) 32 (278) 61 (527) 15 (137) 66 (569) 12 (105) 6 (54) 38 (18–55; 4.2)
*Values are given as % (No.), unless otherwise indicated. †Values are given as the mean (SD). ‡Values are given as the mean (range; SD). Original Research
Table 2—Perceived Control of Asthma and Health Status* Mean Difference Asthma Status Measure
Unadjusted (95% CI)
Multivariable Model† (95% CI)
Multivariable Model Adjusted for Asthma Severity (95% CI)
Asthma severity Physical health status Mental health status Asthma-specific QOL Depressive symptoms Restricted activity days
⫺ 0.43 (⫺ 0.56 to ⫺ 0.30) 1.32 (0.96 to 1.68) 0.79 (0.45 to 1.12) ⫺ 2.50 (⫺ 2.9 to ⫺ 2.08) ⫺ 1.32 (⫺ 1.62 to ⫺ 1.02) ⫺ 1.4 (⫺ 1.80 to ⫺ 0.99)
⫺ 0.40 (⫺ 0.53 to ⫺ 0.27) 1.13 (0.79 to 1.47) 0.65 (0.32 to 0.97) ⫺ 2.24 (⫺ 2.64 to ⫺ 1.83) ⫺ 1.12 (⫺ 1.41 to ⫺ 0.82) ⫺ 1.23 (⫺ 1.62 to ⫺ 0.83)
NA 0.93 (0.59 to 1.27) 0.54 (0.21 to 0.87) ⫺ 1.72 (⫺ 2.09 to ⫺ 1.36) ⫺ 0.99 (⫺ 1.28 to ⫺ 0.69) ⫺ 0.88 (⫺ 1.27 to ⫺ 0.49)
*Values in parentheses are 95% CI. QOL ⫽ quality of life; NA ⫽ not applicable; Mean difference ⫽ change in asthma status measure for every 2-point increment (0.5 SD) in PCAQ score. †Multivariable model adjusts for age, gender, educational level, income, ethnicity, and smoking status.
age, gender, educational level, income, ethnicity, and smoking status. Greater perceived control was associated with better asthma severity, physical health status, mental health status, and asthma-specific quality of life. Higher perceived control was also associated with fewer restricted activity days during the past month and fewer depressive symptoms. The addition of asthma severity to the multivariate analysis did not substantively affect the results (Table 2). Perceived Control and Emergency Health-Care Utilization Higher perceived control of asthma was associated with a decreased likelihood of ED visits and hospitalizations for asthma (Table 3). For every 2-point increase in perceived control, the odds of ED visits declined by 8% (hazard ratio [HR], 0.92; 95% confidence interval [CI], 0.86 to 0.98; p ⫽ 0.008), and the odds of hospitalization declined by 16% (HR, 0.84; 95% CI, 0.78 to 0.90; p ⬍ 0.0001), after controlling for age, gender, educational level, income, ethnicity, and smoking status. Controlling for asthma severity had little effect on the results for hospitalization, whereas perceived control was related to the risk of ED visits somewhat less strongly after controlling for severity (Table 3). Adding an adjustment for ICU admission at study entry to the multivariate
analysis (as another marker of severe asthma) also had little effect on the risk of ED visits (HR, 0.92; 95% CI, 0.86 to 0.98) or hospitalizations (HR, 0.84; 95% CI, 0.78 to 0.91). Controlling for outpatient asthma therapy and preventive measures had little impact on the effect of perceived control on risk of ED visits (HR, 0.95; 95% CI, 0.89 to 1.01) or hospitalization (HR, 0.86; 95% CI, 0.80 to 0.93). Likewise, adding depression to the multivariate model had no substantive impact on the effect of perceived control on the risk of ED visits (HR, 0.92; 95% CI, 0.86 to 0.98) or hospitalization (HR, 0.85; 95% CI, 0.78 to 0.93). Perceived Control, Self-Management, and Preventive Therapy for Asthma There was no association found between perceived control of asthma and most aspects of recommended preventive therapy for asthma (Table 4). Specifically, greater perceived control was not associated with differential use of controller medications (eg, inhaled corticosteroids, long-acting -agonists, and leukotriene modifiers) during the 2 weeks preceding the interview. There was also no association found between perceived control and the recent dispensing of asthma controller medications (during the past 3 months) based on computerized pharmacy data.
Table 3—Prospective Impact of Perceived Control of Asthma on Emergency Health-Care Utilization for Asthma* ED Visits Unadjusted
Hospitalization Multivariate
Unadjusted
Variables
HR (95% CI)
p Value
HR (95% CI)
p Value
HR (95% CI)
p Value
Perceived control Perceived control, adjusted for baseline asthma severity
0.91 (0.85–0.97) NA
0.002
0.92 (0.86–0.98) 0.95 (0.90–1.02)
0.008 0.14
0.83 (0.77–0.88) NA
⬍0.0001
Multivariate HR (95% CI)
p Value
0.84 (0.78–0.90) ⬍0.0001 0.88 (0.82–0.95) 0.0009
*HRs are expressed per 2-point increment (0.5 SD) in perceived control. See Table 2 for abbreviation not used in the text. Multivariate model adjusts for age, gender, education level, income, ethnicity, and smoking status. www.chestjournal.org
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Table 4 —Perceived Control of Asthma and Preventive Care for Asthma* Preventive Care Measure Controller medication use (by self-report over past 2 wk)† Inhaled corticosteroid Long-acting inhaled -agonist Leukotriene modifiers Controller medication dispensing (past 3 mo)† Inhaled corticosteroid Long-acting inhaled -agonist Leukotriene modifiers Asthma monitoring Possesses peak flowmeter Uses peak flowmeter ⱖ 1 time/wk Has self-management (action) plan Asthma education (during past yr) Contact with KP care manager‡ Attended asthma education class Direct instruction on metered-dose inhaler use
Likelihood of Preventive Measure
p Value
0.99 (0.93–1.07) 0.95 (0.89–1.02) 0.92 (0.83–1.02)
0.86 0.17 0.12
1.06 (0.98–1.14) 0.95 (0.88–1.03) 0.95 (0.83–1.09)
0.17 0.18 0.47
0.99 (0.92–1.07) 0.98 (0.91–1.06) 1.02 (0.96–1.09)
0.77 0.65 0.50
0.99 (0.91–1.06) 0.99 (0.92–1.06) 1.19 (1.04–1.36)
0.73 0.71 0.01
*Values are given as OR (95% CI), unless otherwise indicated. †Medication dispensing is from KP computerized databases (688 subjects with KP pharmacy coverage); all other measures are self-reported from structured telephone interviews. ‡Telephone call or visit with asthma intervention specialist (eg, specially trained nurse, nurse practitioner, respiratory therapist, or pharmacist).
Similarly, perceived control was not significantly associated with most measures of asthma self-management (Table 4). Specifically, perceived control scores were not related to the regular use of a peak flowmeter, knowledge of an asthma action plan, or attendance at an asthma education class, or with contact with a KP asthma care manager to discuss asthma-related issues over the past 12 months. However, greater perceived control was associated with an increased likelihood of receiving direct instruction on the use of a metered-dose inhaler (odds ratio [OR], 1.19; 95% CI, 1.04 to 1.36; p ⫽ 0.01). Greater perceived control of asthma was related to a lower likelihood of using some therapies that were indicative of inadequately treated asthma. For example, increased perceived control was associated with a decreased likelihood of receiving therapy with oral corticosteroids at the time of the interview (OR, 0.88; 95% CI, 0.81 to 0.95; p ⫽ 0.001) but had no significant association with the likelihood of being corticosteroid-dependent (OR, 0.94; 95% CI, 0.88 to 1.01; p ⫽ 0.11), defined as having required therapy with oral corticosteroids for ⬎ 3 months during the prior year. Greater perceived control was likewise associated with a lower likelihood of using nebulizer therapy (OR, 0.85; 95% CI, 0.79 to 0.91; p ⬍ 0.0001). Discussion We found that, in a cohort of insured patients with asthma, greater perceived control is associated with 1316
improved measures of physical and mental health status, better quality of life, fewer depressive symptoms, and fewer days of restricted activity due to asthma. In addition, we found that greater perceived control is associated with a decreased prospective risk of ED visits and a decreased likelihood of hospitalization for asthma. Our results expand on our previous findings by showing that perceived control influences the risk of emergency health-care utilization for asthma during longitudinal follow-up.7 The impact of the perceived control of asthma on health outcomes did not seem to be completely mediated by asthma severity, selfmanagement behaviors, or the use of preventive therapies for asthma (as measured by this study). The mechanism of the effect of perceived control on health-care utilization remains unclear. Patients with low perceived control may believe that they are unable to manage even mild asthma attacks at home and thus present for emergency care; however, this mechanism would not seem to account for the increased rate of hospitalization among patients with low perceived control. It remains possible that the effects of perceived control could be mediated by better adherence to asthma therapies; a quantitative assessment of medication adherence would be necessary to evaluate this hypothesis. Alternatively, perceived control could affect the perception of dyspnea and other respiratory symptoms, thereby affecting health status, quality of life, and decisions about seeking emergency health care. The finding that greater perceived control is Original Research
linked to undergoing instruction in the proper use of metered-dose inhalers was not unexpected; in fact, the lack of correlation with other asthma self-management programs was somewhat surprising. Previous studies23 have demonstrated that increasing patients’ knowledge about how to treat their symptoms may increase their self-efficacy, a concept that is closely linked to perceived control. Similarly, perceived control has been previously shown37,38 to be increased as a result of asthma self-management educational programs. Although most studies have suggested that similar self-management programs impact health outcomes via changes in behaviors such as adherence to treatment, others39 have not confirmed this hypothesis and have suggested that self-management courses impact health outcomes via some other, as yet undefined pathway. Our study has several limitations. First, findings from this study population may not be generalizable to all patients with asthma. Patients were recruited to the study after hospitalization for asthma, which is a marker of severe disease8; thus, these results may not be applicable to those patients with mild asthma. Also, participants were members of a large healthcare delivery system and therefore had access to health care, although members of KP of northern California have been found to have sociodemographic characteristics similar to those of other members of the regional population.40,41 There was a greater prevalence of women in our study, but this imbalance is consistent with the higher prevalence of asthma among women nationwide. The use of International Classification of Diseases, ninth revision, coding to determine study eligibility could potentially lead to inaccurate classification; we sought to avoid this issue by an in-depth review of randomly selected charts to confirm that the conditions of patients were accurately coded. In addition, although we were able to obtain data from pharmacy dispensing records for asthma medications, we do not have specific objective measures of treatment adherence. Another significant limitation of this study is the lack of a separate objective measure of asthma control. The finding that greater perceived control was not associated with increased use of asthma self-management strategies could reflect patients’ inaccurate perceptions of their actual asthma control. For example, if patients with poorly controlled disease perceive their asthma to be well controlled, they might not engage in salutary self-management strategies, such as use of controller medications and peak flow meters. In our analysis, we believe that the bias would be conservative if patients’ perceived control “overestimated” their actual control (eg, the protective association between perceived control and www.chestjournal.org
the risk of rehospitalization would be even more favorable than that we actually observed). We also did not include a measure of pulmonary function in our cohort. However, multiple studies24,42,43 have demonstrated that pulmonary function correlates only modestly with other measures of asthma severity and health status. We have previously shown that the severity-of-asthma score is a reliable and valid survey-based measure of asthma severity.25 Moreover, it strongly predicts longitudinal hospitalization for asthma.44 In conclusion, perceived control of asthma appears to be linked to asthma outcomes, including health status measures and health-care utilization, in patients with severe asthma, even after controlling for asthma severity. Further studies are needed to determine whether interventions designed to increase perceived control will improve health outcomes and, if so, by what mechanism.
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