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(Chest. 2000;117:354S-359S.)
© 2000 American College of Chest Physicians

The Prevalence of COPD*

Using Smoking Rates to Estimate Disease Frequency in the General Population

Paul Stang, PhD; Eva Lydick, PhD; Cheryl Silberman, PhD; Angela Kempel, PhD and Elizabeth T. Keating, BSN

* From Galt Associates (Dr. Stang), Sterling, VA; SmithKline Beecham Pharmaceuticals (Drs. Lydick and Silberman and Ms. Keating), Collegeville, PA; and Pharmametrics GmbH (Dr. Kempel), Freiburg, Germany.

Correspondence to: Paul Stang, PhD, 1744 Dekalb Pike, Suite 175, Blue Bell, PA 19422; e-mail: pstang{at}galt-assoc.com


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
 References
 
Study objectives: To develop and validate a model based on smoking rates that will provide reliable estimates of the true prevalence of COPD that include both clinically detected and undetected patients.

Design: Model based on literature review. Age- and gender-specific rates of lung impairment by smoking status were applied to US smoking data. Resultant estimates were compared to the actual prevalence of obstructive airway disease as estimated by US national surveys. The model then was applied to estimate the prevalence of COPD in several European countries, where national data on undiagnosed lung disease do not exist.

Setting: The model was adapted from both a literature review and health-care data, and the analysis was applied to the United States and Europe.

Results: Using smoking rates, we estimate from our model that 15.3 million people who are > 40 years of age in the United States have COPD. The prevalence estimate, based on spirometric definitions for COPD in the same age group using the Third National Health and Nutrition Examination Survey (NHANES III), is 17.1 million people. NHANES III and other US national health-care surveys further suggest that only between 2.4 and 7 million people actually have COPD diagnosed; thus, the proportion of COPD that is currently being diagnosed in the United States is between 14% and 46% of all cases. Using smoking rates and our model, which was developed and validated for the United States, we calculated the prevalence of COPD for Germany (2.7 million people), the United Kingdom (3.0 million people), Spain (1.5 million people), Italy (2.6 million people), and France (2.6 million people) in those people > 45 years of age.

Conclusions: Smoking rates appear to provide a useful method of estimating current COPD prevalence in those countries where more objective data are unavailable. These results are important because recognition of the true burden of disease and corresponding efforts to increase early identification of COPD can help to reduce the morbidity and mortality associated with COPD in populations at risk.

Key Words: COPD • prevalence • smoking


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Conclusion
 References
 
COPD is a leading cause of death and disability worldwide. It is largely preventable but is expensive to treat.1 The World Bank estimates that COPD is responsible for > 29 million disability-adjusted life-years and 1 million years of life lost per annum around the world. These figures place COPD as the fifth most significant global health problem, and COPD is expected to become the third leading cause of death in the first quarter of the next century.2 Furthermore, COPD is currently the 12th leading cause of disability worldwide and is expected to be the fifth leading cause of disability by 2020.2 However, even these figures may not reflect the true scope of the disease, as there is a relative lack of data available to determine the prevalence and burden of COPD across countries.

COPD is very difficult to diagnose and detect clinically in its milder forms without the use of spirometry. Unfortunately, spirometry is not routinely employed in patients at risk for developing COPD (ie, smokers), and spirometric test results are not routinely recorded or consistently interpreted. This is due, in part, to the gradual and insidious development of airflow obstruction in COPD and, in part, to absent or nonspecific symptoms that may occur in patients with mild forms of the disease.3 4 5 Along with the difficulty of early detection, the clinician is challenged by the need to distinguish COPD from the persistent airflow limitation of chronic asthma in older subjects. Recent studies appear to indicate that most older adults with asthma have a chronic irreversible component to their disease.6 The following factors favor the diagnosis of COPD and may help clinicians distinguish between these two conditions: a history of smoking of > 10 pack-years; evidence of emphysema on imaging; decreased diffusing capacity; and chronic hypoxemia.

The terminology in the field of obstructive lung diseases is also complex and often confusing, despite several attempts during the last 30 years to define asthma, chronic bronchitis, and emphysema. Diagnostic labels vary by individual physician, by country, and by time period.7 8 9 This makes estimation of the incidence and prevalence of COPD through a review of the biomedical literature extremely difficult, if not impossible, as a number of reports in the literature cite their own definitions of COPD that vary across time and investigation.

There are several possible ways to estimate the prevalence and, consequently, the burden of COPD. Health-care resource use attributed to COPD diagnoses, ie, outpatient visits or hospital discharges, may provide some estimate of the prevalence of COPD. An estimate based on resource utilization will underestimate true prevalence, however, as only those patients seeking medical attention or recognized by the clinician are captured. Most commonly, patients seeking medical attention have acute exacerbations and, in most cases, present with later stage disease.10 Thus, estimates of the prevalence of COPD are probably biased toward the more severe and symptomatic cases. Furthermore, those with comorbid conditions may elude detection as treatment focuses on the more immediate and visible chronic diseases, leaving COPD relatively undetected for some period of time. COPD estimates also are hindered by inaccuracies in coding as well as inconsistent physician recognition of early disease. Additionally, health-care resource utilization data can be difficult to obtain or may be unavailable in some countries. Therefore, some more reliable means is needed to estimate the true prevalence of COPD in the general population.

Studies have repeatedly found that cigarette smoking is the most important and consistent determinant of the development and progression of COPD.3 11 12 13 14 15 16 Most publications cite that 10 to 15% of smokers are expected to develop COPD; however, this figure appears to have been derived from the report of the Surgeon General11 and the occupational cohort work by Fletcher et al3 and has not been supported consistently across ages or cohorts. The work described in this article pursues an alternative, albeit simplified, approach with a goal of estimating true COPD prevalence based on smoking rates. We sought to provide more precise estimates by including in our analysis only those studies that report rates of COPD based on systematic spirometric examination of a general population. This increases the validity of our model in more precisely estimating the extent of COPD in the general population.

There are several diagnostic schemes accepted in various parts of the world relating COPD diagnosis to spirometric test values. However, as we were undertaking the initial development and validation effort using data from the United States, we used the American Thoracic Society guidelines.17 This approach, if successful, could simplify the estimation of actual COPD prevalence and could facilitate estimates of the social and economic costs of COPD in the near term, despite the absence of high-quality longitudinal or cross-sectional assessments of pulmonary function and smoking status.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Conclusion
 References
 
Estimates of Diagnosed COPD
Various estimates of prevalence were examined. As an initial effort in determining the value of utilization-based estimates of COPD prevalence, outpatient (1995 National Ambulatory Medical Care Survey) and inpatient (1994 National Hospital Discharge Survey) data were used to determine the total number of encounters that were attributable to COPD in each setting. These estimates then were divided by the mean number of visits per diagnosed COPD subject reported in the Third National Health and Nutrition Examination Survey (NHANES III) to derive an estimate of COPD prevalence. These approximations represent the number of recognized cases of COPD but fail to identify those COPD sufferers who had not yet come to medical attention or who had been overlooked during their consultation (eg, undiagnosed or misdiagnosed cases).

Estimates of Total COPD Prevalence
Using a computerized MEDLINE search, we identified studies in the biomedical literature that met the following criteria: population-based; presented information about smoking status (never, current, and former smokers), age, and race (analyses were confined to whites to facilitate extrapolation to European countries); and used a spirometric definition of COPD. We also restricted our estimates in the United States to those persons aged >= 40 years to reduce the likelihood of including cases of asthma. We thought that these criteria would provide us with the best estimates of the true rate of COPD by age, gender, and smoking status. Estimates from the literature were then smoothed using a simple linear regression to facilitate extrapolation with available country-specific smoking data.

The proportion of individuals in each smoking category (never, current, or former) within specific age groups was multiplied by the proportion of individuals with COPD as estimated by regression lines. Age groups were estimated by their midpoint values.

Last, we adapted our approach to use available data (including information obtained through government surveys and information in the biomedical literature) on smoking rates in Germany, the United Kingdom, Spain, and Italy to derive estimates of COPD prevalence in those countries. In adapting our approach for France, smoking rates were obtained from a population-based study.18

Validation of Model Estimates
We validated our estimates by comparing them with estimates calculated using NHANES III data, which includes self-reported diagnosis, the results of pulmonary function tests, and smoking status.19 20 An estimate of the number of persons diagnosed with COPD was determined from NHANES III based on the number of individuals who reported having been told by a physician that they had either chronic bronchitis or emphysema.19 An estimate of all cases of obstructive lung disease was based on spirometry using American Thoracic Society criteria. These estimates from NHANES III for the total number of cases of COPD then were compared to estimates obtained from our regression model based on reports in the literature.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Conclusion
 References
 
Diagnosed COPD in the United States
The results of our utilization analysis are shown in Tables 1 and 2 . Based on self-reported annual outpatient visits for individuals reported in NHANES III who self-report a diagnosis of chronic bronchitis or emphysema, the estimate is that there are 2.8 million persons with COPD in the United States aged 17 to 75 years, and 1.7 million persons aged > 45 years. Similarly, based on hospitalizations recorded in the 1994 National Hospital Discharge Survey, and self-reported frequency of hospitalization in the previous year among individuals in NHANES III with a self-reported diagnosis of chronic bronchitis and emphysema, the number of people affected with COPD would appear to be substantially higher: 4.5 million people in the United States aged 17 to 75 years, and 3.9 million people aged > 45 years. These figures provided reasonable estimates of the number of clinically detected COPD cases in the United States and, thus, of the minimum number of people actually having COPD in the United States. A review of NHANES III self-reported data suggests that there are only 2,450,000 cases of diagnosed COPD in the United States (a figure that is notably close to the estimates given above that are based on utilization), while a review of the 1994 National Health Interview Survey self-report data suggests that this figure is 7,011,000.


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  		Table 1. Prevalence of COPD by Outpatient Visits*

 

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  		Table 2. Prevalence of COPD by Hospitalizations*

 
Total COPD Prevalence in the United States
Four studies met our criteria of population-based estimates of COPD determined by spirometry within age, gender, and smoking stratifications.21 22 23 24 These four reports were used to estimate the age-specific and smoking-specific rates of COPD in the United States using linear smoothing across the point estimates (Fig 1 ; described in Table 3 ). Table 4 shows the smoking rates for the United States by age. Although there were numerous studies that collected population data with spirometry, only these four studies reported the key criteria necessary to identify people with or without obstructive lung disease.



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  		Figure 1. COPD rates by smoking status in men (top) and women (bottom).

 

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  		Table 3. Sources Used to Evaluate the Relationship Between Smoking and COPD

 

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  		Table 4. US Smoking Rates*

 
Based on our application of the smoking rates model, we estimate the prevalence of COPD to be 15,337,000 patients aged > 40 years compared to the "gold standard" NHANES III prevalence estimate of 17,110,000 for the same age range.19 20 Thus, the proportion of cases of COPD that is currently diagnosed in the United States is between 14% (2.45 million of 17.1 million cases) and 46% (7.0 million of 15.337 million cases) of the actual number of cases of COPD in the population.

Total COPD Prevalence in Europe
Smoking rates for the five European countries that were studied are presented in Table 5 , and the corresponding estimates of COPD prevalence based on our model are presented in Table 6 . Our estimates of COPD prevalence in these countries range from 1.5 million people with COPD in Spain to 3.0 million people in the United Kingdom, with estimates of 2.7 million people with COPD in Germany, 2.6 million people in Italy, and 2.6 million people in France. In comparison with the number of hospitalizations in these same countries (Table 7 , which does not include France), a striking difference in frequency can be seen even without adjusting the number of hospitalizations to represent the actual numbers of patients.


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  		Table 5. Smoking Rates for People > 45 Years of Age in Europe

 

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  		Table 6. COPD Prevalence in Europe Using Smoking Rates*

 

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  		Table 7. Hospitalizations for COPD (With Obstruction) in Europe

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Conclusion
 References
 
Smoking rates appear to provide a valuable method of estimating current COPD prevalence. Although many studies have relied on the 10 to 15% rate of COPD among smokers, they often have failed to account for variations across age and gender, which would have caused them to miss a significant proportion of cases. Even with the restricted age range of >= 40 years, we found that the rate of COPD increased from 2 to 10% among never-smokers and from 17 to 43% among current smokers as the age range increased. The results obtained using a smoking-rates model for estimating COPD prevalence are similar to those obtained directly from the NHANES III "gold standard" (15.3 people with COPD vs 17.1 million people with COPD, respectively). Both the model and the rates from NHANES III were estimates of COPD based on the prevalence of obstructive lung disease as assessed by spirometry rather than by a clinical diagnosis, which may have been overinclusive. However, because of the close agreement of the results from the two methods and the need for better data in other countries, we believe that there is value in our application of the regression model to smoking data from European countries. Since smoking prevalence data are readily available in most countries, our model can provide a practical means of estimating more accurately the true burden of COPD in a population. Although it is based on a spirometric definition of COPD and is anchored to the principal etiologic factor for the disease, the limitations of taking a simplified approach to a very complex physiologic process should be borne in mind.

The key data informing our model were taken from a handful of population-based studies that had complete spirometry data and smoking status information. Although we have no reason to believe that these data are misleading, even minor variations could lead to overestimation or underestimation. Furthermore, we limited our US analysis to those people > 40 years of age, as there are very limited data available on COPD rates in younger populations and the data that do exist are probably contaminated by poorly differentiated asthma cases. However, the bulk of the disease occurs in the sector of the population aged > 40 years of age,3 15 17 and it is unlikely that we are losing a large fraction of the population with even mild cases of COPD.

Tobacco exposure continues to change, especially with the introduction of new therapies and public health measures to curb tobacco use. Our implicit assumption is that age and smoking status (current, former, or never) serve as surrogates for total exposure to cigarette smoke (for example, pack-years). Other countries could have shorter or longer tobacco exposure times than are currently applicable in the United States. Additionally, the spirometry definitions used in our analysis are not equivalent to clinical COPD definitions, and the definitions are not identical within the four studies. Given the difficulty of accurately diagnosing COPD, particularly in its early stages, this is clearly a study limitation. In addition, the precision of estimates from the model will vary with the quality of available smoking data. Last, the estimates make no adjustment for secular trends in smoking rates, competing causes of morbidity and mortality in a given population, or ethnic differences in smoking rates, which may influence the generalized estimates of COPD prevalence in some European communities.

Despite its prominent impact on the health of a nation, COPD receives disproportionately less funding from the National Institutes of Health when compared with other illnesses based on mortality, disability, and years of life lost.25 Only recognition of the true size of the population affected by COPD is likely to lead to efforts to increase diagnosis and to improve treatment, including greater efforts for smoking cessation. When compared to several measures of clinically recognized COPD, it is clear that people with diagnosed COPD make up only a small proportion of the number of people with COPD who actually are present in the community.


   Conclusion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
References
 
As a result of the large reserves in human lung function and the fact that COPD is a slowly progressive disease, the clinical diagnosis of COPD is often delayed until extensive and irreparable damage has occurred. Smoking rates appear to provide a useful method of estimating total COPD prevalence, not just the total number of people with diagnosed disease. The results of the application of our model and the analysis of available data indicate that people with diagnosed COPD make up only a small proportion of the people with COPD who actually are present in the community. The recognition of the true size of the problem and corresponding efforts to increase early identification of COPD can help to reduce the morbidity and mortality that is associated with COPD in at-risk populations.


   Acknowledgements
 
The contributions of Robert Gagnon in the statistical analysis of NHANES III and of Lynn Hoffman in the preparation of this article are gratefully acknowledged.


   Footnotes
 
Abbreviations: NHANES III = Third National Health and Nutrition Examination Survey

This study was supported by SmithKline Beecham Pharmaceuticals.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
 References
 

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