Also known as bronchoprovocation test, the methacholine challenge test is done to evaluate how reactive the lungs are to things in the environment. The test can help doctors diagnose asthma in someone who is experiencing shortness of breath, cough, and chest tightness.
During the test, you will inhale progressively increasing doses of a chemical called methacholine, a drug that causes narrowing of the airways, similar to the one experienced by patients with asthma. You will be asked to blow into a machine 4-5 times to collect and measure air in your lungs after each dose of methacholine.
The dose of methacholine will be progressively increased with each blow to see at what dose do your airways (windpipes) get narrowed significantly. If your airways get narrowed more than 20 percent from the time you started the test prior to any methacholine, the test will be stopped and considered completed.
Test will also be stopped and considered completed if no change in your airways is observed despite maximum dose of methacholine.
Your doctor will review the medications you are currently taking and will require you to stop using some of your inhalers at least several hours or days before the procedure. This is done so it does not interfere with the test for if you are taking an inhaler to open up your windpipes, the methacholine test may become falsely negative due to the effect of the inhalers on your windpipes.
You will also be asked not to consume any products with caffeine (i.e., tea, soda, or coffee) at least four hours before the test. A methacholine challenge test can be done in the doctor’s office or in a pulmonary function lab.
However, there are specific conditions where the test should not be performed: nursing or pregnancy, brain or aortic aneurysm, uncontrolled blood pressure, or if the lung function is very low on the baseline spirometry test.
Before the methacholine challenge, you will be asked to undergo a breathing test called spirometry. This test is done to establish a baseline. Following the procedure, you will be given methacholine through a nebulizer at a very small dose. Some patients may experience chest tightness or cough. Others on the other hand, won’t exhibit any symptoms at all.
The methacholine challenge test is considered positive if the methacholine causes at least a 20% (or greater) decrease in the patient’s ability to breath when compared to the baseline. A positive test will suggest that the airways are reactive and an asthma diagnosis may be considered. A negative test on the other hand, means an asthma diagnosis is unlikely.
While the methacholine test is considered very safe, it can result in tightening of the airways or bronchoconstriction. In rare cases, some will experience symptoms of an asthma attack like chest tightness, wheezing, coughing, and shortness of breath. The test may also cause some to feel dizzy or uncomfortable.
If the chest result indicates that there is a constriction of the airways, patients are given a bronchodilator to relieve the symptoms. Prior to the test, your medical history is reviewed first. If you have a cough or cold or if you feel unwell on the day of the test, let your doctor know.