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Spanevello and colleagues claim that a methacholine inhalation challenge carried out one hour before sputum induction in patients with stable asthma does not significantly alter the cellular, eosinophil cationic protein (ECP), or albumin constituents of sputum.1 These results, if correct, are important for both clinical practice and clinical trials where information regarding airway hyperresponsiveness and inflammation is needed. Being able to perform a methacholine challenge and sputum induction on the same day would be convenient.
Sixteen subjects with asthma were studied on two days within a week. Sputum induction was performed alone on one day and one hour after a methacholine challenge on the other. Cell counts and the biochemical markers of the two sputum samples were compared using the Wilcoxon signed rank test and a value of p<0.05 was considered statistically significant.
The small sample size, variability in the data, and p values near significance for neutrophils (p = 0.06) and macrophages (p = 0.08) led us to determine the power of the study. The results of a power analysis for paired continuous data showed that the study only had a 36%, 29%, 10%, 6.6%, and 19.5% chance of detecting a difference for macrophage, neutrophil, eosinophil, lymphocyte and epithelial cell counts, respectively, and 6.0% and 15.3% for ECP and albumin. Hence, while methacholine may not influence sputum cell counts, this study is too underpowered to reach this conclusion.
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authors’ reply We would like to thank Drs Labiris and Hargreave for their interesting comments on our article. They have emphasised the variability in the data and the fact that the p values for neutrophils and macrophages were near significance. Moreover, they have provided the results of a power analysis for paired continuous data showing that the study is underpowered to reach solid conclusions.
We agree that their comments are valid both for the variability of the data and that the p value was near significance for neutrophils and macrophages. However, the p value for eosinophils was far from the significance level (p = 0.49). This is important because the aim of the study was to evaluate the influence of methacholine challenge on overall sputum cellularity but, in particular, on eosinophils which are considered the most relevant inflammatory cell in bronchial asthma.
We are confident that a methacholine inhalation challenge carried out one hour before sputum induction in asthmatic subjects does not alter the eosinophil count in induced sputum, and that this result is important for both clinical practice and clinical trials involving patients with asthma.