What parameters are measured during spirometry testing?

What Spirometry Testing Measures

Spirometry is a physiological test that measures how an individual inhales or exhales volumes of air as a function of time, with the primary signal measured being either volume or flow. 1

Primary Parameters Measured

The two fundamental measurements obtained during spirometry testing are:

Forced Vital Capacity (FVC)

• FVC is the maximal volume of air exhaled with maximally forced effort from a maximal inspiration, expressed in liters at body temperature and ambient pressure saturated with water vapor (BTPS). 1
• This represents the total volume delivered during an expiration made as forcefully and completely as possible starting from full inspiration. 1

Forced Expiratory Volume in One Second (FEV₁)

• FEV₁ is the maximal volume of air exhaled in the first second of a forced expiration from a position of full inspiration, expressed in liters at BTPS. 1
• This is the volume delivered in the first second of an FVC maneuver. 1

FEV₁/FVC Ratio

• The ratio of FEV₁ to FVC should be reported as a decimal fraction (not as a percentage) to minimize miscommunications. 1
• This ratio is critical for identifying obstructive airflow patterns. 2, 3

Additional Spirometric Parameters

Beyond the core measurements, spirometry can derive several other clinically useful parameters:

Flow Measurements

• Peak expiratory flow (PEF) - the maximum flow rate achieved during forced expiration, correlated with pleural pressure and effort during the initial portion of the maneuver. 1
• Mean forced expiratory flow between 25% and 75% of FVC (FEF₂₅₋₇₅%) - provides information about small airway function. 1

Volume Measurements

• Slow vital capacity (VC) - the volume change at the mouth between full inspiration and complete expiration performed slowly without forced effort. 1
• Inspiratory capacity (IC) - the maximal volume that can be inspired from the resting expiratory level. 1
• FEV₆ - forced expiratory volume at 6 seconds, which has been considered a reliable substitute for FVC and may be more reproducible and less physically demanding. 1, 4

Timing Parameters

• Forced expiratory time (FET) - reported to aid quality assessment of the maneuver. 1

Technical Measurement Details

Spirometry measures volumes and flows with specific accuracy requirements: the spirometer must achieve accuracy of ±3% of the reading or ±0.050 L (whichever is greater) over a volume range of at least 8 liters at flows between 0 and 14 L/s. 1, 4

All measurements must be corrected to BTPS conditions (body temperature, ambient pressure, saturated with water vapor), as this correction factor can reach up to 10% depending on environmental temperature. 4

Display Formats

Spirometry results are conventionally displayed in two complementary formats:

• Volume-time curve (spirogram) - shows the volume exhaled over time, providing detail for the latter part of the maneuver and allowing assessment of end-of-test criteria. 1
• Flow-volume loop - displays flow versus volume, providing more detail for the initial portion (first second) of the FVC maneuver and helping assess effort and detect upper airway obstruction. 1, 5

Clinical Context

Spirometry serves as a screening test of general respiratory health, analogous to how blood pressure provides information about cardiovascular health, though it does not directly lead to an etiological diagnosis on its own. 1

The test requires cooperation between the subject and examiner, and results depend on both technical and personal factors, making standardization of technique and equipment calibration essential for accurate measurements. 1