Asthma can be difficult to diagnose for sure, because there is no one test that will prove that you have asthma. However, a test that is used frequently to help with asthma diagnosis is the metacholine challenge test.
This test checks to see if your airways are “hyperresponsive.” That means that your airways overreact to certain stimuli, such as allergens like pollen or pet dander. If the test is negative, then a doctor can be relatively certain that you do NOT have asthma. Typically, if you have asthma, then you will have hyperresponsive airways, at least sometimes.
However, airway hyperresponsiveness can ebb and flow, usually being worst during asthma flareups and receding if you are on anti-inflammatory medications. So, that can make getting an accurate result with a metacholine challenge test somewhat unpredictable.
What a Metacholine Challenge Test Is
Metacholine is a chemical very similar to the substance histamine, which is found in your body. Histamine is a naturally-occurring chemical that is believed to stimulate the allergic response in sensitive people.
Doctors have found that giving a small dose of metacholine will produce allergy and asthma symptoms similar to those caused by histamine when a sensitive person is exposed to an allergen. But metacholine has few side effects, which makes it a better choice for a test like this than histamine.
The allergic response is measured by taking spirometry readings both before and after the metacholine is administered via nebulizer. Depending on how the patient reacts to the metacholine, increasingly higher doses are given every few breaths/minutes.
When It Is Used
Doctors prefer to diagnose asthma by looking at symptoms and medical history. They can also use a spirometer to measure airway effectiveness. But these measures alone are not always conclusive enough to make a definite diagnosis of asthma.
So, in certain cases, doctors will take the next step and do a metacholine challenge. However, because spirometry readings are an important part of the process, it is essential that the patient be able to effectively use a spirometer.
When It Should NOT Be Used
As mentioned above, this test cannot be used in patients who are not able to use a spirometer, such as very young children. But people who have any of the following conditions should also not be subjected to this testing:
- Pregnancy (and also nursing moms)
- Severe breathing problems
- Heart attack or stroke within the last 3 months
- Aortic aneurysm
- Uncontrolled high blood pressure
- Recent respiratory infection
Possible Issues With the Testing
Several factors can interfere with the accuracy of the metacholine challenge test. First off, if you are already taking asthma medication to control your symptoms while the doctor makes a definite diagnosis, you will need to not be on them on the day of the test. Your doctor will tell you how/when to hold your medication.
Other things that can interfere with this test include:
- Caffeine the day of the test
- Respiratory infections
The results of this test are said to be positive if you have a 20 percent or greater fall in the FEV1 spirometry measurment (a measure of how forcefully you exhale). But a positive result does not necessarily mean you have asthma. There are other conditions that can also produce a positive result, so the doctor will consider metacholine challenge results in conjunction with your entire medical history.
Because this test can cause airway constriction and symptoms of asthma, it should only be conducted by a physician under close observation.
Registered nurse & healthcare writer, living with allergies & asthma.