Asthma is a disease of the windpipes characterized by their episodic narrowing and widening due to over sensitivity to environmental substances. Doctors test for this sensitive nature to diagnose asthma. Like your blood is collected when you need to be tested for fever, in the same way, the air in your lungs is collected and the amount is measured to test if you have asthma. By breathing into a machine, the amount of air that you can move into and out of your lungs can be measured.
This amount is then compared with a person who does not have asthma and whose wind pipes are wide open. Since narrowed wind pipes will allow less air to move through them, the measured amount of air will be less in a person with asthma compared to a person without asthma. This test is called spirometry. If this test is normal, then the doctor will go on to test for oversensitivity of the wind pipes. This is done by asking you to take puff from an inhaler and then measuring the amount of air that you can now move into and out of your lungs. This inhaler contains medicine that widens the wind pipes.
If your windpipes are narrowed, they will open up by the medicine inside the inhaler and you will be able move much more air through your wind pipes than you did before using the inhaler. However, if your wind pipes are already fully open prior to taking the inhaler, no effect will be seen on your wind pipes and they will remain unchanged. If the amount of air you can move through your wind pipes increases tremendously after the inhaler, the test is considered positive and it means that you have asthma. This test is called “before” and “after” the inhaler test or “pre” and “post” spirometry.
Now there are instances when both the tests mentioned above are normal but the person still has asthma. This happens in cases of mild asthma which goes undetected by the abovementioned tests. In such cases, a more advanced test is done which is called a Methacholine challenge test. This test is also a “before” and “after” test and measures the amount of air you can move through your windpipes before Methacholine inhalation and after Methacholine inhalation. The only difference is that Methacholine does not widen the wind pipe but further narrows it. For this reason it is never used as a first line test.
If your windpipes get narrowed after using the Methacholine inhaler you will be able move a much lesser amount of air through your wind pipes than you did before using Methacholine. However, if your wind pipes are not over sensitive, no effect will be seen on your wind pipes and they will remain unchanged.
If the amount of air you can move through your wind pipes reduces tremendously after the Methacholine inhaler, the test is considered positive and it means that you have asthma. This test is called the Methacholine challenge test because we are in a way challenging the wind pipes. Occasionally, people who do have asthma may get difficulty breathing or an asthma attack after this test. Hence doctors will take your consent before proceeding with this test.