How to Prepare for a Methacholine Challenge

A methacholine challenge (MCT) is a breathing test that, if positive, aids a diagnosis of asthma or, if negative, leads a doctor to consider further investigation of other medical conditions. The MCT test measures how airways react to an irritant called methacholine. A respiratory therapist will ask patients to breathe in and out of a tube while measuring airway response to increasing doses of methacholine. The methacholine challenge is simple and painless, but some advance preparation is required.

Instructions

• 1

  Discontinue asthma medication as instructed by a physician prior to the methacholine challenge. Per the American Association for Respiratory Care, most asthma medications should be stopped 24 to 48 hours before the test.

• 2

  Request instructions on how to handle any asthma symptoms that occur when medication is stopped. Short acting bronchodilators like albuterol or other medications may be recommended depending on individual patient circumstances.

• 3

  Recognize that the methacholine challenge is just part of the picture. The test, by itself, is not diagnostic. Physicians must take into account patient symptoms, history and the results of the MCT when determining a diagnosis of asthma.

• 4

  Know what a positive methacholine challenge is. A positive MCT result, indicative of asthma, is defined as a drop in FEV1 (Forced Expiratory Volume at 1 second) of 20% or more.

• 5

  Avoid false negatives. The American Journal of Respiratory and Critical Care Medicine reports a methacholine challenge can result in a false negative when medication is not properly discontinued, if patients used anti-inflammatory medications, if patients are sensitive to only one allergen or irritant, and if the equipment or test procedure is faulty.

• 6

  Be aware that the MCT actively works to irritate airways and it is not uncommon to experience airway constriction or other asthma symptoms during the test. A nebulizer treatment or albuterol inhaler will be used once the test is finished to reverse these effects.

• 7

  Plan to continue with daily activities after the test. In most cases, the MCT is well tolerated and patients should be able to return to work or other commitments with no complications.