What It Is:
A methacholine challenge test helps with the diagnosis of asthma. Another name for it is bronchoprovocation, which means "provoking" or challenging the bronchial airways with one of the following:- methacholine
- histamine
- cold air
- exercise
The purpose of the test is to see if the airways are what is called hyperresponsive. That means your airways narrow in response to the substance or situation used to challenge them. This hyperresponsiveness is a key feature of asthma. If the methacholine challenge does not produce hyperresponsiveness, then asthma is unlikely.
When It Is Used:
The methacholine challenge is not one of the routine tests for diagnosing asthma, as spirometry is. In fact, this test is generally only used if spirometry fails to suggest a definite diagnosis (or not) of asthma, but asthma is considered a serious possibility.According to the American Thoracic Society guidelines for methacholine challenge testing, the test is most useful in ruling out a diagnosis of asthma, since its negative predictive power is generally more accurate than its positive predictive power.
Methacholine challenge testing is also used to diagnose occupational asthma. In general, this test can only be used in adults and children old enough to perform acceptable spirometry maneuvers.Other Uses for Bronchoprovocation:
In addition, methacholine challenge testing is sometimes used for the following purposes:- to assess the relative risk of developing asthma
- to measure the severity of asthma
- to check response to treatment
Where It's Done:
This kind of testing should be performed by a skilled clinician. So, your best place to have it done is by an allergist or pulmonologist, as they are specialists trained in safe bronchoprovocation testing.How to Prepare:
Once a methacholine challenge is scheduled, your allergist will give you a list of medications and other things to avoid in the hours before the test. This can include most asthma medicines, as well a caffeine drinks. It's important to understand, also, that you may experience mild asthma symptoms during the course of the test.What Happens During the Test:
Methacholine is a dry, crystalline powder that is mixed with saline. Methacholine is derived from acetylcholine, a naturally occurring substance in the body, and can cause the airways to tighten and swell, in sensitive people.The drug is then converted to a fine mist in a nebulizer of some type, for inhalation. Before beginning the test, a baseline FEV1 is obtained with spirometry.
During the test, the clinician mixes the methacholine and delivers it via a nebulizer, while the patients sits in a relaxed position, breathing normally. A noseclip is worn during the inhalations. Lung function (FEV1) is measured before and after the nebulization to assess airway hyperresponsiveness.Increasing amounts of methacholine are given in an attempt to trigger the airway changes, with measurements taken after each dose via spirometry. If necessary, quick-relief medicine such as albuterol will be given to treat any symptoms that may arise, such as wheezing.
Interpreting the Results:
Results are reported in terms of a percent decrease in FEV1 from baseline. As a rule of thumb, if the reading drops 20% or more, it is suggestive of asthma. But, actually analyzing the results and determining a diagnosis is a rather complex process involving a number of factors and requires special skills. Your allergist can explain your results to you.Sources:
"Expert Panel Report 3: Guidelines for the Diagnosis and Management of Asthma." NHLBI Guidelines for the Diagnosis and Treatment of Asthma. 28 Aug 2007. National Heart Lung and Blood Institute. 18 Dec. 2007. <http://www.nhlbi.nih.gov/guidelines/asthma/asthgdln.pdf> "Guidelines for Methacholine and Exercise Challenge Testing—1999." Am J Respir Crit Care Med. Vol 161.
