How is forced expiratory volume in one second (FEV1) calculated?

How FEV1 is Calculated

FEV1 is the maximum volume of air exhaled during the first second of a forced expiration from full inspiration, measured in liters at body temperature and pressure saturated with water vapor (BTPS), with timing initiated from time zero defined by back-extrapolation. 1

Measurement Procedure

Patient Preparation and Maneuver Execution

• The patient inhales to total lung capacity (full inspiration) 1
• The patient then exhales as forcefully and completely as possible into the spirometer 2
• At least three acceptable forced expiratory curves must be obtained 2, 1

Determining Time Zero

• Time zero is established by back-extrapolation from the steepest part of the volume-time curve, not from when the patient begins exhaling 1
• This back-extrapolation method is the gold standard, as alternative methods using flow or volume thresholds show too much variability to provide uniformly accurate equivalents 3

Selecting the Final FEV1 Value

• Record the largest FEV1 value from all usable curves, even if it does not come from the same curve as the largest FVC 2, 1
• This approach differs from selecting a single "best" curve and ensures you capture the patient's true maximum performance 2

Quality Control Criteria

Within-Maneuver Acceptability

Each spirogram must be free from artifacts including: 2, 1

• Cough during the first second of exhalation
• Glottic closure affecting measurement
• Early termination or submaximal effort
• Leaks or obstructed mouthpiece

The maneuver must have a good start with extrapolated volume <5% of FVC or <0.15 L, whichever is greater 2, 1

Between-Maneuver Repeatability

• The two largest FEV1 values must be within 0.150 L of each other 2, 1
• The two largest FVC values must also be within 0.150 L of each other 2
• If these criteria are not met after three acceptable maneuvers, continue testing up to a maximum of eight attempts 2

Technical Considerations

BTPS Correction

• All measurements must be corrected to BTPS conditions 2
• The BTPS correction factor can be as large as 10% depending on environmental temperature 2
• Spirometer temperature should be measured for each maneuver, not assumed constant, as errors up to 6% in FEV1 can occur if ambient temperature is used instead of actual spirometer temperature 2

Equipment Accuracy Requirements

• Spirometers must have accuracy of ±3% of reading or ±0.050 L (whichever is greater) for FEV1 measurement 2
• Flow range must accommodate 0-14 L/s 2
• Resistance must be <1.5 cmH₂O·L⁻¹·s⁻¹ 2

Common Pitfalls to Avoid

• Do not reject results solely based on poor repeatability—label them appropriately and leave interpretation to the clinician's discretion 2
• Do not use maneuvers with unacceptable starts or coughs during the first second when determining the best FEV1 value 2
• Maneuvers with early termination may still be used for selecting the largest FEV1, as long as the first second is artifact-free 2
• If a patient shows progressive reduction in FEV1 exceeding 20% from start value across sequential maneuvers, terminate testing for patient safety 2