A Methacholine Challenge Test is a lung function test that specifically tests for asthma. It may be performed if your symptoms or previous spirometry do not clearly establish a diagnosis of asthma. When Methacholine is inhaled it will cause the airways to spasm and narrow if asthma is present. The patient does not feel the spasm, but the lung function test will show a 20% decrease if asthma is present. A Bronchodilator is always given at the end of testing to reverse any effects of the methacholine.