Methacholine challenge test
From Free net encyclopedia
The methacholine challenge test is a technique use in the testing of asthma. It 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 methacholine, 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 methacholine. If performed properly, they all work well. Some protocols use a 10 times increase in methacholine at the lower concentrations. Others vary the breathing strategy.
This is the best test for statistically distinguishing asthmatics from non-asthmatics. However, since it is a statistical measure, and asthma is a condition that exists on a continium, defining whether someone has asthma becomes an exercise in philosophy and logic, the closer your test results are to the 20% "cut-off". A 20% drop from the baseline (measured before administering methacholine) is considered by most people to be diagnostic of asthma, regardless of where in the test this 20% drop occurs. It is possible to have false negatives, and false positives on this test. Asthma can also be temporary, due to an exposure to noxious stimulii (smoke inhalation, etc.). Regardless of the results of a methacholine 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 methacholine challenge.
The test is physically demanding, and the results can be affected by muscular weakness or exhaustion. The maneuver the patient performs is called a forced vital capacity, and can cause a sharp spike in blood pressure while being performed. Methacholine can, sometimes, stimulate the upper airway sufficient to cause violent coughing. This can make performing the forced vital capacity correctly, difficult or impossible.Template:Med-stub