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Development and Validation of a Survey-Based COPD Severity Score^*

^* From the Department of Medicine (Drs. Eisner, Balmes, and Blanc, Ms. Trupin and Ms. Earnest), University of California, San Francisco; of and Institute for Health Policy Studies (Drs. Katz and Yelin), University of California, San Francisco, San Francisco, CA.

Correspondence to: Mark D. Eisner, MD, MPH, FCCP, University of California, San Francisco, 350 Parnassus Ave, Ste 609, San Francisco, CA 94117; e-mail: eisner{at}itsa.ucsf.edu

	Abstract

TOP Abstract Introduction Materials and Methods Results Discussion References

 
Objective: To develop a comprehensive disease-specific COPD severity instrument for survey-based epidemiologic research.

Study design and setting: Using a population-based sample of 383 US adults with self-reported physician-diagnosed COPD, we developed a disease-specific COPD severity instrument. The severity score was based on structured telephone interview responses and included five overall aspects of COPD severity: respiratory symptoms, systemic corticosteroid use, other COPD medication use, previous hospitalization or intubation, and home oxygen use. We evaluated concurrent validity by examining the association between the COPD severity score and three health status domains: pulmonary function, physical health-related quality of life (HRQL), and physical disability. Pulmonary function was available for a subgroup of the sample (FEV₁, n = 49; peak expiratory flow rate [PEFR], n = 93).

Results: The COPD severity score had high internal consistency reliability (Cronbach = 0.80). Among the 49 subjects with FEV₁ data, higher COPD severity scores were associated with poorer percentage of predicted FEV₁ (r = – 0.40, p = 0.005). In the 93 subjects with available PEFR measurements, greater COPD severity was also related to worse percentage of predicted PEFR (r = – 0.35, p < 0.001). Higher COPD severity scores were strongly associated with poorer physical HRQL (r = – 0.58, p < 0.0001) and greater restricted activity attributed to a respiratory condition (r = 0.59, p < 0.0001). Higher COPD severity scores were also associated with a greater risk of difficulty with activities of daily living (odds ratio [OR], 2.3; 95% confidence interval [CI], 1.8 to 3.0) and inability to work (OR, 4.2; 95% CI, 3.0 to 5.8).

Conclusion: The COPD severity score is a reliable and valid measure of disease severity, making it a useful research tool. The severity score, which does not require pulmonary function measurement, can be used as a study outcome or to adjust for disease severity.

Key Words: chronic bronchitis • pulmonary disease, chronic obstructive • outcome assessment (health care) • pulmonary emphysema • severity of illness index

	Introduction

TOP Abstract Introduction Materials and Methods Results Discussion References

 
COPD is a major cause of impaired health status, disability, and mortality. The disease is responsible for > 2 million emergency department visits and hospitalizations annually in the United States.¹ Despite public awareness about the health risks of smoking, mortality from COPD continues to increase.¹ Consequently, there is an urgent need for epidemiologic studies that better delineate the risk factors for adverse health outcomes among adults with COPD. The conduct of such studies has been hampered by the lack of a validated disease-specific COPD severity measure beyond the basic physiologic assessment of pulmonary function. A comprehensive survey-based measure of COPD severity would be valuable for use in epidemiologic studies as a health outcome or to adjust for disease-specific severity.

Pulmonary function measurement, especially FEV₁, has traditionally been used to stage the severity of COPD in clinical practice; the recent Global Initiative for Chronic Obstructive Lung Disease has codified this practice in its guidelines.²³⁴⁵⁶ Although pulmonary function is an essential clinical tool for characterizing COPD and is a potent predictor of mortality,⁷ it is not a comprehensive disease-severity measure. FEV₁, for instance, correlates weakly with other measures of health-related quality of life (HRQL) and respiratory symptoms.⁸⁹¹⁰ Moreover, FEV₁ did not capture important treatment-related clinical improvements, manifested by fewer COPD exacerbations, in clinical trials of inhaled corticosteroids.^1112131415 In addition, spirometry can be difficult and expensive to perform in large-scale epidemiologic studies.

HRQL instruments, both generic and disease-specific, have also been used to gauge the severity of COPD.⁸¹⁶¹⁷ HRQL, however, is conceptually different from disease severity. HRQL is a patient-centered measurement of perceived satisfaction with life, as it is affected by health.¹⁸ COPD severity is a distinct construct that reflects the biological impact of disease pathophysiology on diverse aspects of physical functioning. Consequently, COPD severity is proximal to and influential of HRQL. Although HRQL instruments provide an important patient-level assessment, they cannot substitute for a specific measure of COPD severity.

We developed a comprehensive disease-specific COPD severity score for use in epidemiologic and health-outcomes research. The score, which was designed for survey administration, is an integrated measure of disease severity, based on disease status, receipt of clinical treatments, and recent hospitalization for COPD. Using a population-based sample of US adults with COPD, we assessed the reliability and validity of the COPD severity score.

	Materials and Methods

TOP Abstract Introduction Materials and Methods Results Discussion References

 
Overview
In a population-based sample of 383 US adults with COPD, we developed a disease-specific COPD severity instrument. The severity score was based on responses to a structured telephone interview. Internal consistency reliability was established using standard psychometric techniques. We evaluated concurrent validity by examining the association between the COPD severity score and three aspects of health status: pulmonary function, physical HRQL and health status, and physical disability. Pulmonary function measurements were obtained for a subset of respondents subsequent to the interviews.

Recruitment of Adults With COPD
Survey methods have been previously described in detail.¹⁹ The study was approved by the University of California, San Francisco Committee on Human Research. Briefly, 383 adults with COPD were selected from a random sample of 2,061 US adults aged 55 to 75 years identified by random-digit dialing telephone interviews. Approximately one half of the overall sample (n = 1,001) was randomly identified among residents in the 48 contiguous states of the United States. The remainder of the sample (n = 1,060) was recruited from persons who resided in geographic "hot spots" that had the highest COPD mortality rates based on the National Institute of Occupational Safety and Health Atlas of Respiratory Disease Mortality in the United States: 1982–1993.²⁰ The hot-spot sample was enriched for subjects with COPD. The overall study participation rate was 53% among households with an eligible respondent present.

During each telephone contact, one randomly selected adult per household was interviewed. Subjects were asked if they had ever received a physician’s diagnosis of any of several chronic respiratory conditions. Those who reported physician diagnoses of chronic bronchitis or emphysema were considered to have COPD, along with those who specifically reported a diagnosis of COPD. We included respondents with COPD who had concomitant asthma because they clinically resemble persons with COPD alone.²¹

Development of the COPD Severity Score
Participants underwent structured telephone interviews that elicited respiratory symptoms and medications, health status, smoking history, employment history, and sociodemographic characteristics. Survey-item responses were used to construct a disease-specific COPD severity score. The instrument and scoring instructions are available from the authors, on request.

We previously developed a disease-specific severity-of-asthma score for use in epidemiologic and outcomes research.²²²³ Using a similar process, we created a COPD severity score based on responses to survey items that comprise five overall aspects of COPD severity: respiratory symptoms, systemic corticosteroid use, other COPD medication use, previous hospitalization or intubation for respiratory disease, and home oxygen use (Table 1 ). Each item was assigned an a priori weight based on clinical aspects of the disease and its expected contribution to overall COPD severity. In order to assess the performance of the a priori weighting scheme, we derived an alternate weighting scheme by factor analysis with orthogonal rotation. Possible total scores range from 0 to 35, with higher scores reflecting more severe COPD.

Missing values for COPD severity score components were rare, ranging from 0 to 2% per item. We used previously established methods to impute missing values.²³ For dyspnea during the past 14 days or nights, we assigned missing values the maximum of either the day or night response (n = 9 subjects and n = 5 subjects, respectively; no subjects were missing both values). Respondents who indicated antibiotic use during the past 12 months but who did not indicate the number of courses were assigned to the most common "one to two courses" category (n = 9 subjects). Missing values for medication use and other questions were defined as zero (ie, not used or not present) [range, 0 to 2% missing per item].

COPD Severity Score Reliability
Internal consistency reliability was evaluated by principal component analysis to examine whether the COPD severity score appeared to measure a single construct. Cronbach was also calculated as a measure of internal consistency.²⁵ As noted above, we compared the a priori weighting scheme with an alternate weighting scheme derived from factor analysis.

Concurrent Validity
We assessed concurrent validity of the COPD severity score by evaluating its association with three aspects of health status defined a priori: pulmonary function, physical HRQL and health status, and physical disability. The variables selected to represent these aspects of health status were based on our theoretical model of COPD severity. We anticipated that greater COPD severity would correlate with impairment in these three dimensions of health status.

We used two methods for obtaining pulmonary function measurements among a subset of 264 subjects (70%) who we recontacted in follow-up to the initial interview. Of the 264 subjects, 170 respondents indicated that they had undergone pulmonary function testing during the past 5 years. These subjects were contacted by mail to request permission to retrieve their pulmonary function testing results from their outpatient medical record. The health-care providers for 71 participating subjects were contacted, and 49 pulmonary function reports were obtained. From these reports, we ascertained FEV₁ for 49 subjects and peak expiratory flow rate (PEFR) for 44 subjects. In the remaining 24 instances, no record of pulmonary function testing was located by the health-care provider. The mean time difference between the pulmonary function test and the initial interview date, which generated the survey responses for calculation of the COPD severity score, was 0.4±1.6 years [± SD]). We used the predicted values provided by the pulmonary function testing laboratory where the tests were conducted.

Respondents who indicated no recent pulmonary function testing at follow-up were contacted by letter to recruit them for a telephone-based PEFR measurement. Of the 94 eligible subjects, 68 subjects were sent peak flowmeters by mail, and 49 participated in the peak expiratory flowmeter program. Participating subjects were mailed a peak flowmeter (Astech; Dey Laboratories; Napa, CA). One research assistant, who was trained by an investigator with expertise in asthma care, carried out a structured telephone-based program with subjects that lasted approximately 10 min per session. Each subject performed three peak expiratory flow maneuvers according to American Thoracic Society guidelines and similar to other telephone-based peak flow monitoring programs.²⁶²⁷ The mean time difference between the initial interview and the peak flow measurement was 1.4 ± 0.2 years. Predicted values were based on reference standards developed for the third National Health and Nutrition Examination Survey.²⁸ In total, we had peak expiratory flow data for 93 subjects (44 subjects who underwent pulmonary function testing and 49 subjects who completed the home peak flowmeter program).

Physical HRQL was assessed using the Short-Form (SF)-12 physical component summary score. The SF-12 is derived from the Medical Outcomes Study SF-36 instrument, which is the most widely used measure of generic HRQL. The SF-36 has been extensively validated in the general population²⁹ and among adults with COPD.³⁰ Defined from the eight SF-36 subscales by factor analysis, the physical component summary score reflects an underlying physical dimension of physical HRQL.³¹ Higher scores reflect more favorable health states.

As another measure of physical health status, daily activity restriction was ascertained using a question adapted from the National Health Interview Survey (NHIS).³² Specifically, respondents were asked to indicate how many days their activity was limited due to a respiratory condition during the past month. Respondents were also asked to rate the severity of their fatigue or tiredness on a scale of 0 to 10, in which 0 reflected no fatigue and 10 indicated very severe fatigue.

Concurrent validity was also assessed using three measures of physical disability and health impacts. Self-rated general health was assessed with a question developed for the NHIS and also used in the SF-36 questionnaire.²⁹ Based on this item, we defined adverse physical health status as self-reported fair or poor general health (as opposed to good, very good, or excellent). Although this question comprises one aspect of the SF-12 physical component summary score, it has independent value as a measure of health status.²¹ Current employment status was ascertained by questions that were patterned after the Bureau of Labor Statistics Current Population Survey.³³ Using these items, we ascertained whether respondents were unable to work secondary to a respiratory condition. Activities of daily living were assessed with the "body care" section of the Functional Performance Inventory SF.³⁴ These activities included dressing or undressing, showering or bathing, caring for feet, washing hair, shaving or applying makeup, and preparing meals. We defined difficulty with activities of daily living as being unable to do the activity or able to do the activity with much difficulty.

Statistical Analysis
The analysis was conducted using statistical software (Version 8.1; SAS Institute; Cary, NC). Internal consistency reliability was evaluated using principal component analysis and Cronbach . We evaluated concurrent validity by examining the association between the COPD severity score and pulmonary function, physical HRQL/health status, and physical disability. The Spearman rank correlation was used to evaluate the association between the COPD severity score and pulmonary function measures (percentage of predicted FEV₁ and percentage of predicted PEFR). Using the Spearman rank correlation, we examined the association between COPD severity score and physical health status in three separate analyses: among subjects with available FEV₁ data, among subjects with PEFR data, and among the total cohort. In the first two analyses, we controlled for the level of physiologic impairment using percentage of predicted FEV₁ or percentage of predicted PEFR, respectively. This was accomplished by calculating the partial correlation between COPD severity score and each physical health status measure, after adjusting for pulmonary function.³⁵ Using an analogous strategy, we used logistic regression analysis to examine the association between COPD severity score and physical disability. Because sociodemographic factors may be on the causal pathway between COPD severity and health status, we did not control for these variables in multivariate analysis. As a sensitivity analysis, we also repeated the analysis using a narrower definition of COPD that only included subjects reporting emphysema or COPD, excluding those with chronic bronchitis alone.

	Results

TOP Abstract Introduction Materials and Methods Results Discussion References

 
Subject Characteristics
The mean age of subjects with COPD was 64 ± 6 years (Table 2 ). The majority of subjects had a lifetime history of cigarette smoking (81%). Other sociodemographic characteristics are shown in Table 2.

Reliability
Internal consistency was high (Cronbach = 0.80). In addition, the principal component analysis revealed loading on a single factor (Eigenvalue 3.3) that contributed 71% of the variance of the model. There was a rapid fall-off in the scree plot, with a second component Eigenvalue of 0.55.

Concurrent Validity: Pulmonary Function
Measurement of pulmonary function was available for 93 subjects. In the subset of 49 subjects with available FEV₁ data, higher COPD severity scores were associated with poorer FEV₁ percentage of predicted (r = –0.40, p = 0.005) [Table 3 ]. Among the 93 persons with PEFR measurements, greater COPD severity was also related to worse PEFR percentage of predicted (r = – 0.35, p < 0.001).

	Discussion

TOP Abstract Introduction Materials and Methods Results Discussion References

 
In this population-based study, we developed a survey-based COPD severity score that incorporates clinical aspects of the disease, including respiratory symptoms, oral corticosteroid use, other COPD medication use, previous hospitalization and intubation, and home oxygen therapy. The COPD severity score is internally consistent, reliable, and appears to capture a broad range of disease severity. We have demonstrated the concurrent validity of the COPD severity score using diverse measures of physical health status, disability, and pulmonary function.

This severity instrument could prove to be a valuable tool for survey-based research investigating the epidemiology and health outcomes of COPD. The COPD severity score can function as a study outcome variable in studies of risk factors for adverse health impacts, such as environmental exposures or health-care processes. The severity score can also be used to adjust for differences in baseline disease severity in cohort studies. This may be particularly valuable in effectiveness studies of therapeutic interventions for COPD. Importantly, the COPD severity score does not require measurement of pulmonary function, which may be major logistical advantage for population-based studies.

The methods that we used to recruit adults with COPD had both strengths and limitations. The population-based methods used to identify subjects should ensure the generalizability of the test characteristics of the COPD severity score. The use of self-reported physician diagnosis, however, may result in some misclassification of disease status. This methodology is a standard epidemiologic approach for identifying COPD using survey-based techniques.¹ The high prevalence of lifetime smoking, which was > 80%, supports the diagnosis of COPD. Moreover, the prevalence of COPD in our general population sample (13.5%) was similar to that reported in two other population-based studies conducted in the United States, The third National Health and Examination Survey and the NHIS.¹ In addition, the majority of subjects indicated recent respiratory symptoms or COPD medication use (89%), consistent with a diagnosis of COPD. Finally, reanalysis using a more restrictive definition of COPD that excludes chronic bronchitis did not appreciably affect the results. Although the enrolled sample appears similar to COPD patients in general, we cannot exclude some degree of misdiagnosis and misclassification of disease status.

Another limitation is the availability of pulmonary function testing on a subset of the overall sample, the use of such data from several pulmonary function laboratories, and the time lag between interview and pulmonary function measurement. When we examined the subgroups with available pulmonary function measurements, we found strong evidence of reliability and validity. This indicates that estimates of reliability and validity are robust and not subject to selection bias. If the time lag between interview and pulmonary function measurement introduced bias, it would be expected to be a conservative one (ie, toward the null). The consistency of findings across different study outcomes, ranging from pulmonary function to physical health status and disability, also supports the psychometric soundness of the COPD severity score.

The COPD severity score was a reliable and valid tool in our cohort study. It is based on survey data, which could be affected by subject misreporting, but has the significant advantage of deployment in large-scale epidemiologic studies in which pulmonary function measurement and physical examination are not logistically feasible. The score, however, should be validated in a separate, larger cohort of COPD patients, which we will carry out in a different ongoing cohort study of COPD. In addition, we did not have additional objective measures, such as the 6-min walk test, which will also provide additional validation in our ongoing COPD cohort study.

Although the COPD severity score was designed for epidemiologic and health-outcomes research, it could have applications in clinical settings. Before clinical use can be recommended, further validation and comparison with the recently published multidimensional BODE score need to be undertaken,³⁶ which we are planning in an ongoing COPD cohort study. It will be instructive to compare our COPD severity score, which takes into account both current symptom status and the therapy necessary to achieve this status, with the BODE score, which grades patients based on dyspnea, airflow obstruction, exercise capacity, and body mass index. One key difference is that the BODE score requires in-person physical testing, which could reduce its applicability in large-scale population-based epidemiologic studies based on feasibility concerns.

The COPD severity score is a simple, reliable, and valid survey-based instrument that can be used in epidemiologic and health-outcomes studies. Developed for telephone interview administration, the COPD severity score may be adapted to other research settings, such as personal interview or written questionnaire. Future studies will evaluate other psychometric properties, such as longitudinal responsiveness to change in COPD status.

	Footnotes

 
Abbreviations: CI = confidence interval; HRQL = health-related quality of life; NHIS = National Health Interview Survey; OR = odds ratio; PEFR = peak expiratory flow rate; SF = Short-Form

Support was provided by grant R01 HL607438 from the National Heart, Lung, and Blood Institute, National Institutes of Health, and Flight Attendants Medical Research Institute grant CoE2001. Dr. Eisner was also supported by grant K23 HL04201 from the National Heart, Lung, and Blood Institute.

Received for publication August 20, 2004. Accepted for publication December 21, 2004.

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This Article Abstract Full Text (PDF) Free Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Add to My Personal Article Archive Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (4) Citing Articles via Google Scholar Google Scholar Articles by Eisner, M. D. Articles by Blanc, P. D. Search for Related Content PubMed PubMed Citation Articles by Eisner, M. D. Articles by Blanc, P. D.