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Children with cough, unassociated with wheeze or other evidence of systemic disease, are commonly seen in paediatric practice. In the distant past, the association between cough and wheeze was underrecognised but in recent years, as highlighted by McKenzie, children with cough alone have increasingly been diagnosed as having asthma.1
Problems relating to clinical studies of cough
Studies that include cough as an outcome need to be interpreted in light of inherent methodological problems. First, although the history of cough is a commonly used variable in epidemiology,2 3the reporting of cough has poor repeatability. The chance corrected agreement (Cohen’s κ) between two occasions when cough was reported, is unacceptably low (0.14−0.19).4 5 In contrast, wheeze related questions have excellent κ values of 0.76−0.88.4 5 Second, cough, especially nocturnal cough, is unreliably reported compared with objective measures.6 7 In adults, the reporting and scoring of cough is dependent on psychological factors.8 Third, although cough scores using diary cards are widely used as an outcome measure, until recently no validated subjective cough scoring system was available.9 Compared to objective measurements, diary cards have been shown to be inaccurate for nocturnal cough6 and for metered dose inhaler use. Moreover, diary cards in studies can be parent completed6 or child completed, and these are not interchangeable.9 The accuracy of findings based entirely on diary cards is questionable.
Finally, a difficulty in the evaluation of the benefits of a therapeutic trial for cough is that cough usually resolves spontaneously (period effect). The widely quoted studies on “cough variant asthma (CVA)” were based on anecdotal evidence or were not placebo controlled.10-13 In trials that included a placebo arm, cough medications were no more effective than placebo.14 15 Guidelines that recommend a therapeutic trial of asthma medications for cough (and thus diagnose …