Spirometry is the process of measuring the volume and airflow within the lungs. This Pulmonary Function Test or PFT helps to indicate the health of the lungs by measuring the breath. The functioning of the lung is measured in terms of volume or amount of air that can be inhaled or exhaled, and the maximum speed or airflow possible. The test results are used to plot a graph that can be used to identify diseases of the lungs, such as pulmonary fibrosis, chronic obstructive pulmonary disease or asthma.

The pulmonary function testing is carried out using a device called spirometer. The spirometer is a hand held device that has to be blown into for measuring the forced vital capacity (FVC) and forced expiratory volume (FEV) numbers. These numbers are used be the physician to detect cases of lung diseases, and monitor the condition of lungs over the course of therapy. The FEV numbers indicate the air flow with the lungs, and the FVC numbers indicate the air volume.

The procedure for performing a spirometry test is quite simple in nature, but requires the cooperation of the patient. The patient puts the spirometer tube in his mouth, takes a deep breath in, and exhales as hard and long as possible into the spirometer. Usually, the patient exhales forcefully into the sensor for more than six seconds, giving a more accurate reading. The six-second test for acquiring FEV6 can be used instead of FVC test, since it is more convenient for the tester as well as the patient.

The procedure for the test might vary depending on the type of spirometer used. The patient might take a rapid deep breath in immediately after the long exhalation, which can be used to detect obstruction in the air canal. Some tests might use quiet breathing or rapid breathing before the actual test is performed.

Pulmonary function testing includes the use of nose clips, which are lightly placed on the nose to obstruct air escaping from nose. Mouthpieces are also used to avoid microorganisms from spreading in the environment. The common guidelines used for pulmonary function testing and analysis are established by the European Respiratory Society or ERS and the American Thoracic Society or ATS.

The primary limitation of the pulmonary function test is that it depends on the cooperation of the patient. The spirometric maneuver is repeated several times to ensure accuracy and consistency of data. The patient needs to understand the procedure detailed by the physician and follow all instructions meticulously. Due to these limitations, the numbers generated using the spirometric tests are conservative and underestimated. Hence, the test cannot be performed on infants and children who are unable to comprehend and follow the directions. The test is unsuitable for patients who are not in a conscious state of mind, due to sedation, under the influence of drugs or medication, or have issues comprehending instructions. The spirometric test cannot be performed on patients who are unconscious, or are unable to exhale forcefully into the spirometer. For such cases, a different set of lung function tests are used for lung assessment.

References: http://en.wikipedia.org/wiki/Spirometry