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.