The purpose of this study is to determine the difference (if any) in the result of the methacholine challenge (a test used by physicians in diagnosing asthma) when concentrations of methacholine are quadrupled versus doubled.

The purpose of this study is to confirm that the tidal volume technique of methacholine delivery may be employed in methacholine challenges using a quadrupling dose schedule instead of the standard doubling dose schedule with similar diagnostic efficacy, with no increase in patient adverse events, and with a shorter investigational duration. This has previously been shown to be true using the dosimeter technique of methacholine challenge.

Inclusion Criteria:

• You must be over 18 years of age; and
• You must have a diagnosis of asthma; and
• Excluding asthma, you are not aware of any other lung conditions or diseases.

Exclusion Criteria:

• You have a known hypersensitivity (i.e. an overreaction of your immune system) to methacholine or other parasympathomimetic agents or cholinesterase inhibitors (i.e. agents that act in a similar way via your nervous system - to be discussed with study personnel); or
• You are a nursing mother or if you are a woman of child bearing potential who is, may be, or intends to become pregnant during testing as the effects of methacholine inhalation in these situations are not known; or
• Your baseline lung function is poor (ie. your FEV or forced expiratory volume in one second, is less than 65% of your predicted values). This will be performed and explained by study personnel prior to commencing methacholine inhalation testing; or
• You have had any respiratory infections for the last four weeks; or
• If you have allergies, and have been exposed to agents that trigger your asthma within the last four weeks; or
• If you have any significant chronic medical condition.