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The paper by Burr et al1 on the efficacy of eradicating visible indoor mould on respiratory health in patients with asthma is of great interest, but I think the authors underestimate the clinical relevance of their findings because they overestimate the lack of effect on peak expiratory flow (PEF) variability as an objective assessment of their intervention. The lack of effect on this primary end point in the presence of highly significant effects on medication use and symptoms—even after 12 months—simply illustrates once again that PEF is too insensitive to contribute meaningfully to the interpretation of our therapeutic interventions. The study by Burr et al1 and those of others2 3 are examples of investigations that demonstrate a lack of efficacy using PEF parameters as primary end points whereas the secondary end points—such as respiratory symptoms—demonstrate efficacy of the interventions. Increased PEF variability is a specific feature of unstable asthma but it is not necessarily a sensitive one. PEF mainly reflects central airway mechanics4 and is therefore not the optimal monitoring tool because asthma predominantly affects the smaller airways. Hence, PEF may severely underestimate peripheral airway patency. Clinical studies are much more convincing and powerful if sensitive and relevant end points are chosen, and I would strongly advocate using end points that are both relevant and sensitive. This will teach us more and provide more credit for all involved—doctors as well as patients.
Competing interests: None.
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