It measures lung function, specifically the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled.

[1] Spirometry generates pneumotachographs, which are charts that plot the volume and flow of air coming in and out of the lungs from one inhalation and one exhalation.

Most spirometers display the following graphs, called spirograms: The basic forced volume vital capacity (FVC) test varies slightly depending on the equipment used.

The standard procedure ensures an accurate and objectively collected set of data, based on a common reference, to reduce incompatibility of the results when shared across differing medical groups.

When assessing possible upper airway obstruction, the technician will direct the patient to make an additional rapid inhalation to complete the round.

Clinically useful results are highly dependent on patient cooperation and effort and must be repeated for a minimum of three times to ensure reproducibility with a general limit of ten attempts.

Spirometry is indicated for the following reasons: Forced expiratory maneuvers may aggravate some medical conditions.

Multiple publications of predicted values have been published and may be calculated based on age, sex, weight and ethnicity.

Forced vital capacity (FVC) is the volume of air that can forcibly be blown out after full inspiration,[13] measured in liters.

[14] Predicted normal values for FEV1 can be calculated and depend on age, sex, height, mass and ethnicity as well as the research study that they are based on.

In restrictive diseases (such as pulmonary fibrosis) the FEV1 and FVC are both reduced proportionally and the value may be normal or even increased as a result of decreased lung compliance.

A derived value of FEV1 is FEV1% predicted (FEV1%), which is defined as FEV1 of the patient divided by the average FEV1 in the population for any person of the same age, height, gender, and race.

[14] Predicted normal values for FEF can be calculated and depend on age, sex, height, mass and ethnicity as well as the research study that they are based on.

[17][18] However, in the absence of concomitant changes in the standard markers, discrepancies in mid-range expiratory flow may not be specific enough to be useful, and current practice guidelines recommend continuing to use FEV1, VC, and FEV1/VC as indicators of obstructive disease.

[medical citation needed] Diffusing capacity (or DLCO) is the carbon monoxide uptake from a single inspiration in a standard time (usually 10 seconds).

[24] It is considered normal if it is 60% to 140% of the average value in the population for any person of similar age, sex and body composition.

[14] In those with acute respiratory failure on mechanical ventilation, "the static compliance of the total respiratory system is conventionally obtained by dividing the tidal volume by the difference between the 'plateau' pressure measured at the airway opening (PaO) during an occlusion at end-inspiration and positive end-expiratory pressure (PEEP) set by the ventilator".

It is considered normal if it is 60% to 140% of the average value in the population for any person of similar age, sex and body composition.

Maximum inspiratory pressure is an important and noninvasive index of diaphragm strength and an independent tool for diagnosing many illnesses.