Guide — Pulmonary Function Testing
Spirometry Basics
The foundation of pulmonary function testing - what FEV₁, FVC, and the FEV₁/FVC ratio measure, how a good effort is coached and judged (ATS/ERS acceptability and repeatability), and how the ratio separates an obstructive from a restrictive pattern.
9 min read · Pulmonary Function Testing
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Spirometry is the most common pulmonary function test. It measures how much air a patient can exhale and how fast, producing the values that define obstruction and screen for restriction. A valid test depends on patient effort and technique as much as on the equipment.
Key Concepts — The Core Values
- FVC (forced vital capacity) — the total volume forcibly exhaled after a maximal inhalation.
- FEV₁ (forced expiratory volume in 1 second) — the volume exhaled in the first second, the flow workhorse.
- FEV₁/FVC ratio — the fraction of the vital capacity exhaled in the first second, and the single most important spirometry value. A low ratio defines obstruction.
- A normal FEV₁/FVC ratio is roughly 0.75–0.80 in adults and falls with age. The fixed cutoff of below 0.70 defines obstruction (GOLD); the lower limit of normal (LLN, about the 5th percentile) is more accurate, especially in the elderly (it avoids over-diagnosis) and the young.
- Other values: FEF25-75 (mid-expiratory flow, an early small-airway marker), PEF (peak expiratory flow), and FET (forced expiratory time, which should be at least 6 seconds).
| Value | What It Measures |
|---|---|
| FVC | Total volume forcibly exhaled after a maximal inhalation |
| FEV₁ | Volume exhaled in the first second — the flow workhorse |
| FEV₁/FVC | Fraction of the vital capacity exhaled in the first second; a low ratio defines obstruction |
| FEF25-75 | Mid-expiratory flow — an early small-airway marker |
| PEF | Peak expiratory flow |
| FET | Forced expiratory time — should be at least 6 seconds |
Assessment & Findings — Test Quality (ATS/ERS)
| Criterion | What to Look For |
|---|---|
| Sharp start | Small back-extrapolated volume — no hesitation off the top |
| No early cough | No cough in the first second of the maneuver |
| No early termination | Effort sustained, not cut short |
| Adequate end | A plateau, or at least 6 seconds of exhalation |
| No leak | Seal around the mouthpiece maintained throughout |
| Repeatability | Two largest FVC within 150 mL and two largest FEV₁ within 150 mL |
Obtain at least 3 acceptable efforts (up to 8 attempts), and report the largest FVC and the largest FEV₁ — which may come from different curves. Coaching matters: a poor effort mimics restriction or under-reads obstruction.
RT Priorities & Interventions
- Coach a maximal, sustained effort. A full inhalation, a fast hard blast, and continued exhalation to the plateau.
- Recognize and reject unacceptable efforts — cough, submaximal, early stop, or leak.
- Withhold bronchodilators before testing per protocol when reversibility is being assessed.
- Interpret in order: the ratio first, then the FVC, then severity by percent predicted.
Common Pitfalls
- Accepting a submaximal effort — it falsely lowers the FVC and can mimic restriction.
- Using the fixed 0.70 ratio uncritically in the elderly (it over-diagnoses obstruction) or the young (it under-diagnoses).
- Confusing a low FEV₁ alone with obstruction — it is the ratio that defines obstruction.
Board Exam Pearls
- A low FEV₁/FVC ratio defines obstruction; spirometry alone cannot confirm restriction (which needs a low TLC).
- Fixed cutoff: FEV₁/FVC below 0.70 (GOLD); the LLN is more accurate.
- Repeatability: the top two FVC and top two FEV₁ values within 150 mL.
- Report the largest FVC and largest FEV₁ — they can be from different efforts.
- FEF25-75 is an early small-airway marker but is effort- and volume-dependent.
FAQ
What does the FEV₁/FVC ratio tell me?
It is the proportion of the vital capacity exhaled in the first second, and it is the key to obstruction. A low ratio (below about 0.70, or below the lower limit of normal) means airflow is obstructed. A normal or high ratio with low volumes suggests restriction, which spirometry can only suggest - confirming it needs lung volumes.
Why can't spirometry diagnose restriction by itself?
Restriction is defined by a reduced total lung capacity (TLC), and spirometry cannot measure TLC because it cannot capture residual volume. A low FVC with a preserved ratio only suggests restriction; a body plethysmograph or gas-dilution study is needed to confirm a low TLC.
What makes a spirometry effort acceptable?
A sharp start, no cough in the first second, no early termination, and exhalation continued to a plateau or at least 6 seconds, with the two best FVC and FEV₁ values reproducible within 150 mL. At least three acceptable efforts are obtained.
Should I hold inhalers before the test?
When reversibility is being assessed, short- and long-acting bronchodilators are withheld for a protocol-specified interval beforehand so the baseline is a true pre-bronchodilator measurement; otherwise follow institutional and physician guidance.
Put it to work
Take the spirometry numbers further: test whether the obstruction reverses with a bronchodilator against the 12% and 200 mL threshold.
Open the Bronchodilator Response calculator →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Pulmonary function testing chapters.
- Graham BL, Steenbruggen I, Miller MR, et al. Standardization of Spirometry 2019 Update. An Official American Thoracic Society and European Respiratory Society Technical Statement. Am J Respir Crit Care Med. 2019;200(8):e70-e88.