Any data on the prevalence of COPD must depend on the definition that is adopted. Early stages of COPD are not associated with symptoms, or only with 'smoker's cough' that is accepted as 'inevitable'. These individuals are unknown to the medical profession. Once symptoms develop, the COPD has typically become fairly advanced, with an FEV1 that has already fallen to less than 60% of the predicted value or worse. Most studies will therefore underestimate the true prevalence and potential impact of the disease.
Historical variations in the terminology and International Classification of Diseases (ICD) codes used for COPD also create difficulties in compiling data on COPD. Until the late 1960s, the terms 'chronic bronchitis' and 'emphysema' were commonly used. Following the eighth revision of ICD codes, 'COPD' was used increasingly frequently in the United States, but often not in other countries, making comparison difficult. The current tenth revision of the ICD recognizes a broad band of 'COPD and allied conditions' (ICD-10 codes J42-46).
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