According to the USA FDA package insert
Methacholine Chloride is a parasympathomimetic (cholinergic) bronchoconstrictor agent to be administered ... by inhalation, for diagnostic purposes.
it is
FOR DIAGNOSTIC PURPOSES ONLY AND SHOULD NOT BE USED AS A THERAPEUTIC AGENT.
(their caps). On the question Elizabeth raised
Pediatric Use: The safety and efficacy of Methacholine Chloride inhalation challenge have not been established in children below the age of 5 years.
(meaning, I suppose, it has been, at least to the FDA's standards, for age 5 and above).
From the AARC (American Association for Respiratory Care) guidelines
2.1 The methacholine challenge test is one method of assessing airway responsiveness. In this test, the patient inhales an aerosol of one or more concentrations of methacholine. Results of pulmonary function tests (eg, spirometry, specific conductance) performed before and after the inhalations are used to quantitate response. This guideline applies to adults and children capable of adequately performing spirometry or body plethysmography and of cooperating during the course of the challenge.
2.2 A positive test is defined as a decrease from the baseline forced expiratory volume in the first second (FEV1) or of the postdiluent FEV1 value of 20%, or of a decrease in specific conductance of 35-45% from the baseline or post-diluent value.
So our Australians didn't "invent" this test, just the specific protocol they used.
The thing I couldn't find on-line -- I would have to look at a real medicine book (and maybe wouldn't even get it there) -- is how much this is actually used in diagnosis of asthma in real practice by real M. D.'s. I know from a previous life (as an editor as W. B. Saunders, a big medical publisher), that a procedure can have all of these FDA approvals and professional society guidelines and still not be widely used. It looks like methacholine is used some, but more than that I cannot say (and I'm not even sure of that).
Charles Geyer
Professor, School of Statistics
University of Minnesota
on sabbatical 2004-2005 at
Department of Statistics
University of Washington