Pulmonary function testing is a method of determining how well the lungs and the airways are working and how efficiently they transfer oxygen into the blood. The term of pulmonary function tests is a general term includes a broad range of tests for studying the different functions of the lungs separately.Spirometry is the cornerstone of the PFTs; it is a simple computerized device that can measure how lungs exhale and it provides the results either as absolute values or percentage of normal range. Some devices are able to provide a graphic presentation of the flow of exhaled (and also inhaled) air against lung volumes in what is known by flow volume loop.Spirometry measures all air that patient can blow out of the lungs, however, lungs can never be fully empty, and the volume of air remaining in the lungs after forced expiration can be measured by other several methods like body plethysmography or gas dilution technique.