Background: The prevalence of asthma has increased over recent decades and the reasons for this are poorly\r\nunderstood. A sensitive tool that can evaluate potential risk factors for asthma is bronchial hyperresponsiveness\r\n(BHR), a key physiological characteristic of asthma. However, although the minimum clinically important difference\r\nin BHR for an individual is accepted to be around one doubling dose, the minimum important change in a\r\npopulation is not defined. As with surrogate measures of cardiovascular disease risk such as blood pressure and\r\ncholesterol, a change that is not clinically important in an individual may be extremely important in public health\r\nterms.\r\nFindings: To assess the potential impact of a small absolute change in BHR across a population, we modelled the\r\neffect of different changes in BHR on the prevalence rates of moderate and severe BHR in an asthmatic population.\r\nWe calculate that a one half doubling dose increase in BHR increases the prevalence of moderate and severe BHR\r\nby 30%. If this was accompanied by an equivalent increase in the population prevalence of moderate and severe\r\nasthma, this would be highly significant in public health terms.\r\nConclusions: We propose that a one half doubling dose worsening in BHR across a population may represent an\r\nimportant change
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