Calculating Forced Expiratory Volume in One Second (FEV1)

Measurement of FEV1

The European Respiratory Society defines FEV1 as the maximum volume of air exhaled during the first second of a forced exhalation from a position of maximum inhalation, expressed in liters at BTPS, and it should be measured from a series of at least three forced expiratory curves that have an acceptable start and are free of artifacts such as coughing 1, 2, 3
The largest FEV1 value should be recorded after examining the data from all usable curves, even if they do not come from the same curve as the largest FVC (Forced Vital Capacity) 1, 4
The time zero for FEV1 measurement is defined by back-extrapolation from the maximum flow part of the volume-time curve 5

Individual spirometries are acceptable if they are free of artifacts such as coughing during the first second of exhalation, glottic closure, early termination, submaximal effort, leaks, or obstructed mouthpiece 6
They must have a good start with an extrapolated volume < 5% of the FVC or 0.15 L (whichever is greater) 6
Exhalation must be satisfactory with a duration > 6 seconds (3 seconds for children) or show a plateau in the volume-time curve 6

The two largest FEV1 measurements must be within 0.150 L of each other 6
The two largest FVC measurements must also be within 0.150 L of each other 6
If both criteria are met, the test session can be concluded; otherwise, it should continue until the criteria are met or eight tests have been performed 6

Since we are looking for non-Praxis sources, height is a strong predictor of FEV1, but we will use a different source to support this claim: The American Thoracic Society notes that height is the strongest predictor of FEV1, with taller individuals having larger lung volumes 5

For adults, the American Thoracic Society recommends using the logarithmic transformation: ln(FEV1) = 12.06 + 2.584 × ln(height in cm) is not available, but we can use the fact that the European Respiratory Society provides reference equations for estimating FEV1, which can be used to estimate lung function in adults and children 1

FEV1 can be normalized using the FVC (FEV1/FVC) or slow vital capacity (FEV1/VC) to evaluate air flow obstruction 5
In some patients, a slow vital capacity (SVC) or inspiratory vital capacity (IVC) maneuver may provide a more appropriate denominator for calculating FEV1/VC% 5
FEV6 (forced expiratory volume at 6 seconds) has been considered a reliable substitute for FVC and can be used to normalize FEV1 (FEV1/FEV6) 4

Instruct the individual to exhale as forcefully and completely as possible into the spirometer. 7
Obtain at least three acceptable forced‑expiration curves before selecting values. 8
Record the largest FEV₁ value from all usable curves, even when it does not originate from the same curve as the largest FVC. This captures the true maximal performance. [8][7]

A valid spirogram must be free of the following artifacts: cough during the first second, glottic closure, early termination or sub‑maximal effort, and leaks or an obstructed mouthpiece. [8][7]
The maneuver start is acceptable only when the extrapolated volume is < 5 % of FVC or < 0.15 L, whichever is greater. 8

The two largest FEV₁ values must be within 0.150 L of each other. 8
The two largest FVC values must also be within 0.150 L of each other. 8
If repeatability criteria are not met after three acceptable maneuvers, testing may continue up to a maximum of eight attempts. 8

All spirometric measurements must be corrected to body‑temperature‑pressure‑saturated (BTPS) conditions. 9
The BTPS correction factor can reach up to 10 % depending on environmental temperature. 9
Spirometer temperature should be measured for each maneuver; using ambient temperature instead can introduce errors up to 6 % in FEV₁. 9

Spirometers must achieve an accuracy of ±3 % of the reading or ±0.050 L (whichever is greater) for FEV₁ measurements. 9
The device’s flow range must accommodate 0–14 L/s. 9
System resistance must be < 1.5 cmH₂O·L⁻¹·s⁻¹. 9

Do not reject results solely on the basis of poor repeatability; instead label them appropriately and allow the clinician to interpret. 8
Do not use maneuvers with unacceptable starts or coughs during the first second when selecting the best FEV₁ value. [7][8]
Maneuvers that terminate early may still be used for selecting the largest FEV₁, provided the first second is free of artifacts. [7][8]
If a progressive reduction in FEV₁ exceeds 20 % from the initial value across sequential maneuvers, testing should be terminated for patient safety. 8