Methacholine challenge test

The metacholine challenge test is a technique use in the testing of asthma. Metacholine mimics the effects of histamine in triggering asthma attacks and impairing lung function, while at the same time breaking down quite quickly in the body and therefore presenting no long-term danger.

The test typically takes the form of an increasing dosage of inhaled metacholine, doubled at each test run. After each dose, lung function is measured. The concentration required to achieve a 20% reduction in lung function is noted. There are a number of alternative strategies in administering metacholine. If performed properly, they all work well. Some protocols use a 10 times increase in metacholine at the lower concentrations. Others vary the breathing strategy.

This is the best test for statistically distinguishing asthmatics from non-asthmatics. However it is possible to have false negatives, and false positives on this test. Asthma can also be temporary, due to an exposure to noxious stimuli (smoke inhalation, etc.). Regardless of the results of a metacholine test, anyone who appears to have asthma clinically, and who responds to asthma treatment, should not have asthma treatment with held because he passes a metacholine challenge.

The test is physically demanding, and the results can be affected by muscular weakness or exhaustion. The maneuver the patient performs is called spirometry, and can cause a sharp spike in blood pressure while being performed. Metacholine can, sometimes, stimulate the upper airway sufficiently to cause violent coughing. This can make spirometry difficult or impossible.