What causes false negative results in a methacholine test?

True or False Test?: A "false negative" test means the presence of a "confounding variable" that invalidates the test because it nullifies a test that would probably be positive if the variable were absent, like a brochodilator medicine (e.g.various inhalers and pills for asthma). So the test should be done in the absence of those medications, if possible-or-used as a validation as to the medication's effectiveness.