Pulmonary function tests are usually performed as follows: i. Pulmonary ventilation function, which is the most basic item of pulmonary function tests, can determine whether the patient has ventilation dysfunction and determine its degree and type. Bronchial excitation test, if the ventilation function is normal, bronchial excitation test will be performed, bronchial excitation test is used to diagnose and identify asthma. Bronchodilator test, if the ventilation function is abnormal, a bronchodilator test is performed. If the result of bronchodilator test is positive, the patient may have asthma. IV. Residual air volume, which is usually used to determine the degree of emphysema. V. Diffusion function, which is usually used for interstitial lung fibrosis. Pulmonary function tests should be performed by keeping the breath through the mouth while holding the mouth as tightly as possible to avoid air leakage during the test.