Strengths and limitations of the study
Before drawing conclusions from the results, some issues should be considered. First, it could be argued that a panel diagnosis is a subjective tool for diagnosis of chronic respiratory disorders. However, the reproducibility of the panel diagnosis of asthma and COPD was good, and therefore it could be concluded that the subjectivity of the panel diagnosis in this study was limited.
Second, it has been recognised that the use of a fixed FEV1/FVC ratio of 70% may result in overdiagnosis of COPD in older people and in underdiagnosis in younger people. Using the lower limit of normal values for FEV1/FVC, classifying the lowest 5% of the healthy population as abnormal has been suggested to decrease misclassification.6,24 This method was not included in the panel diagnosis because it is currently not incorporated in the guidelines. Moreover, the values of the normal distribution of the Dutch population are unknown. However, it is thought that misclassification by interpreting the FEV1/FVC ratio in the study was limited because normal ageing of the lungs, as well as all other test results of the participants were taken into account when diagnosing COPD. Hence, in nine participants older than 65 years with an FEV1/FVC ratio below 70%, COPD was not diagnosed because there was no other sign of COPD except for a single episode of cough. Accordingly, COPD was diagnosed in eight participants in whom FEV1/FVC index was not below 70% but was 70% or 71%. In these participants, who were all (former) smokers and aged below 55 years, DLCO was below 80% of predicted value and/or the residual volume measured by body plethysmography was above 150% of predicted.
Third, possible selection bias by participating GPs is a shortcoming of the study. Probably, many more patients were eligible during the inclusion period. Of the 73% participating GPs, 22 GPs included more than eight participants, 40 included between two and eight, and 11 GPs included only one or two participants. The eligibility criterion ‘cough for 14 days or longer’ was used, but the duration of cough of most included participants was much longer, with interquartiles of 21, 40, and 90 days. Therefore, the results are generalisable to patients who present with persistent cough, rather than to patients who present with cough in general. It may be assumed that inclusion of participants was mainly hampered by lack of time of the GPs, considering the comprehensive protocol. Participating GPs were explicitly requested to include all eligible patients, and not only those in whom they suspected asthma or COPD. It is possible that the GPs included or excluded patients on the basis of the GPs' perception of the patients' risk of having or not having a chronic lung disorder. However, as data on eligible patients who were not included were not gathered, it is not possible to determine with certainty the direction of the possible bias caused by selection. Nevertheless, it can be assumed that overestimation of the prevalence by selection is more likely than underestimation.
Fourth, in 14 participants both asthma and COPD were diagnosed according to the Dutch guidelines for COPD and asthma.21,22 Internationally, there is no consensus on how to diagnose these people with signs of both disorders. In international guidelines, most cases of asthma with persistent obstruction are defined as COPD.5,6
Finally, in participants with recurrent complaints of cough or wheezing and normal spirometry results, asthma was not diagnosed in the present study. Possibly, some of these patients did, nevertheless, have asthma, which might have been revealed by a histamine or metacholine provocation test.5,25 However, these tests were not included for practical reasons. A strong aspect of the study is the use of extensive pulmonary function and other tests to diagnose or exclude asthma and COPD.
Comparison with existing literature
The prevalence of undetected COPD or asthma in a study population of people who consult their GP for cough was, to the authors' knowledge, reported before only once, in 1998.11 In this study by Thiadens et al, 74 of the 192 participants had asthma (39%) and 14 had COPD (7%). The lower mean age of the participants of that study was probably the main cause for the high prevalence of asthma and the low prevalence of COPD, compared to the present results. In a study by van Schayck et al, cough was found to be a strong predictor for obstructive lung disease in general practice: in 64 smokers with chronic cough, 16 (27%) had obstructive lung disease. However, no distinction was made between asthma and COPD.1 With regard to the severity of COPD, other authors also found (although in different study populations) that undetected COPD was mainly mild or moderate.4,26 Regarding asthma, it has been reported that its development is not rare at higher age and that underdiagnosis is substantial in older people.27,28
The underdiagnosis of relevant chronic respiratory morbidity that was found could have been caused by a doctor or a patient delay. Symptoms of asthma or COPD, like cough, breathlessness, wheezing, and sputum might have been attributed to smoking habits, older age, or poor fitness. Smokers might have been restrained in consulting their GP for their complaints because they felt guilty about smoking. Moreover, a lot of patients who did visit the GP with cough before had probably been diagnosed with uncomplicated respiratory tract infections, instead of with COPD or asthma.
