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From the authors

Medical Research Institute of New Zealand, Wellington, New Zealand.

M.R. Sears is correct in his assumption that a proportion of subjects determined by spirometry to have "chronic obstructive pulmonary disease (COPD)" in our Wellington, New Zealand study have asthma-related impairment of lung function 1. This comment also applies to all other prevalence surveys using purely spirometric criteria, especially those which base their diagnosis of COPD on pre-bronchodilator values only. We are not aware that other criteria, such as self-reporting of a doctor's diagnosis, a diagnosis based on the presence of respiratory symptoms, or a diagnosis based on other definitions of airflow obstruction, have any greater claim to accuracy in determining prevalence rates. In fact, our study showed that only 17 (15%) out of 116 subjects who met the Global Initiative for Chronic Obstructive Lung Disease (GOLD) criteria had a doctor's diagnosis of COPD. The GOLD definition of COPD is the internationally accepted criteria 2 and is the definition we used for estimating COPD prevalence. It is also the criteria chosen in the recently reported Burden of Obstructive Lung Disease (BOLD) initiative 3.

We calculated the proportion of subjects in the study with asthma. A subject was considered to have asthma if: there was a post-bronchodilator increase in forced expiratory volume in one second (FEV₁) 15% predicted 4; subjects had peak flow variability 20% during 1 week of testing 4; subjects had doctor-diagnosed asthma in conjunction with current symptoms (wheeze, or nocturnal shortness of breath and wheeze, or nocturnal chest tightness in the preceding 12 months); or there was doctor-diagnosed asthma in conjunction with inhaler use in the preceding 12 months. Subjects could belong to more than one of these categories.

Out of the 749 study participants, 179 (24%) met the criteria for a diagnosis of asthma. Of these, 135 (75%) out of 179 had a doctor's diagnosis, 61 (34%) met the criteria for peak flow variability and 43 (24%) met the criteria of 15% reversibility in FEV₁. Of the participants in the study with GOLD-defined COPD, 65 (56%) out of 116 also met the definition of asthma. Of these, 45 (69%) out of 65 had a doctor's diagnosis of asthma, 32 (49%) had peak flow variability and 31 (47.6%) had 15% reversibility (these were not mutually exclusive).

The relationship between asthma and COPD is complex. Historically, COPD and asthma have become increasingly differentiated over time 5 and in the "classic" forms of presentation may be easy to separate by the means of structural and physiological findings 6. However, these differences may not be so clear-cut in the clinical setting and there are significant overlaps in clinical, physiological and pathological features of the two diseases, including the mechanisms of bronchial hyperresponsiveness and atopy that drive obstructive phenotypes 7. Although asthma may progress to chronic irreversible airflow obstruction, changes in reversibility over time are not widely reported 8, 9. It may be that we require a new taxonomy to better define the various disorders of airway obstruction.

So in the end perhaps it all comes back to the title of our paper 1: COPD prevalence...a matter of definition.

REFERENCES

1. Shirtcliffe P, Weatherall M, Marsh S, et al. COPD prevalence in a random population survey: a matter of definition. Eur Respir J 2007;30:232–239.[Abstract/Free Full Text]
2. Rabe KF, Hurd S, Anzueto A, et al. Global strategy for the diagnosis, management, and prevention of chronic obstructive pulmonary disease: GOLD executive summary. Am J Respir Crit Care Med 2007;176:532–555.[Abstract/Free Full Text]
3. Buist AS, McBurnie MA, Vollmer WM, et al. International variation in the prevalence of COPD (the BOLD Study): a population-based prevalence study. Lancet 2007;370:741–750.[CrossRef][Web of Science][Medline] [Order article via Infotrieve]
4. Welcome to GINA the Global Initiative For Asthma. www.ginasthma.com Date last accessed: January 30, 2007
5. Guerra S. Overlap of asthma and COPD. Curr Opin Pulm Med 2005;11:7–13.[CrossRef][Web of Science][Medline] [Order article via Infotrieve]
6. Sciurba FC. Physiologic similarities and differences between COPD and asthma. Chest 2004;126: Suppl. 2 117S–124S.[CrossRef][Medline] [Order article via Infotrieve]
7. Kraft M. Asthma and chronic obstructive pulmonary disease exhibit common origins in any country!. Am J Respir Crit Care Med 2006;174:238–244.[Free Full Text]
8. Vonk JM, Jongepier H, Panhuysen CI, Schouten JP, Bleecker ER, Postma DS. Risk factors associated with the presence of irreversible airflow limitation and reduced transfer coefficient in patients with asthma after 26 years of follow up. Thorax 2003;58:322–327.[Abstract/Free Full Text]
9. Ulrik CS, Backer V. Nonreversible airflow obstruction in life-long nonsmokers with moderate to severe asthma. Eur Respir J 1999;14:892–896.[Abstract/Free Full Text]

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