Spirometry, the most common of the Pulmonary Function Tests (PFTs), is the measurement of lung function, specifically the measurement of the amount (volume) and speed (flow) of air that can be inhaled and exhaled. Spirometry is an important tool used for assessing lung diseases such as asthma, cystic fibrosis, and COPD. Results are usually given in both raw data (liters, liters per second) and percent predicted - the test result as a percent of the "predicted values" for the patients of similar characteristics (height, weight, age, sex, and sometimes race). The interpretation of the results can vary depending on the physician and the source of the predicted values, but generally speaking, results nearest to 100% predicted are the most normal, and generally results over 80% are often considered normal. However, review by a doctor is necessary for accurate diagnosis of any individual situation.

The spirometry test is performed using a device called a spirometer, which comes in many different varieties. Many produce a graph called a Flow-Volume Loop, which graphically depicts the flow of air compared to the total volume inspired or expired. Flow is listed along the Y-Axis and volume along the X-Axis. Most spirometers also display a Volume-Time curve, showing volume (liters) along the Y-Axis and time (seconds) along the X-Axis (not shown).

Flow-Volume Loop. Positive values represent expiration, negative values represent inspiration. The trace moves clockwise for expiration followed by inspiration. (Note the FEV₁, FEVA_1/2 and FEV₃ values are arbitrary in this graph and just shown for illustrative purposes, they must be recorded as part of the experiment).

The basic spirometry test itself is simple, and varies slightly depending on the equipment used. Generally, the patient is asked to take the deepest breath they can, and then exhale into a machine as hard as possible, for as long as possible, followed by a rapid deep breath in. Sometimes, the test will be preceded by a period of quiet breathing in and out from the machine, or the rapid breath in will come before the hard exhalation. During the test, soft clips are used to prevent air escaping through the nose. Sometimes, filter mouthpieces are used to prevent the spread of germs, or in other cases a length of tubing is placed between the patient and the machine. The test is repeated, usually at least three times and often up to as many as eight times, to insure that the results are accurate and repeatable. Sometimes, a bronchodilator or other medication is administered, and then another round of tests taken to assess the effect on the patient.

Due to the patient cooperation required, spirometry can only be used on children old enough to comprehend and follow the instructions given (typically about 4-5 years old), and only on patients that are able to understand and follow instructions - thus, this test is not suitable for patients that are unconscious, heavily sedated, or have limitations that would interfere with vigorous respiratory efforts. Other types of PFTs are available for infants and unconscious persons.

Explanation of Common Test Values

• FVC: Forced Vital Capacity - This is the total amount of air that you can forcibly blow out after full inspiration, measured in liters.
• FEV 1: Forced Expiratory Volume in 1 Second - This is the amount of air that you can forcibly blow out in one second, measured in liters. Along with FVC it is considered one of the primary indicators of lung function.
• FEV 1 / FVC - This is the ratio of FEV 1 and FVC, which showing the amount of the FVC that can be expelled in one second. In healthy adults this should be approximately 80%.
• PEF: Peak Expiratory Flow - This is the speed of the air moving out of your lungs at the beginning of the expiration, measured in liters per second.
• FEF 25-75% or 25-50%: Forced Expiratory Flow 25-75% or 25-50% - This is the average flow (or speed) of air coming out of the lung during the middle portion of the expiration (also sometimes referred to as the MMEF, for maximal mid-expiratory flow).
• FIF 25-75% or 25-50%: Forced Inspirtory Flow 25%-75% or 25%-50% - This is similar to FEF 25%-75% or 25%-50% except the measurement is taken during inspiration.
• FET: Forced Expiratory Time - This measures the length of the expiration in seconds.

Technologies Used in Spirometers Over the Years

• Water bell
• Bellows wedge
• Fleisch-pneumotach
• Lilly (screen) pneumotach
• Turbine
• Pitot tube
• Hot-wire anemometer
• Ultrasound

See also: History of spirometers and lung function testing, Spirometry