Pulmonary Function Test Basics
A complete reference guide to interpreting pulmonary function tests — spirometry values, lung volumes, diffusion capacity, and recognizing obstructive vs restrictive disease patterns.
Written for respiratory therapists, students, and clinicians.
What Are Pulmonary Function Tests?
Pulmonary function tests (PFTs) are a group of non-invasive diagnostic tests that measure how well the lungs work. They assess the volume of air the lungs can hold, the rate at which air can be moved in and out, and the efficiency of gas exchange across the alveolar-capillary membrane.
PFTs are essential in diagnosing and monitoring obstructive lung diseases (COPD, asthma), restrictive lung diseases (pulmonary fibrosis, chest wall disorders), and mixed patterns. They also guide treatment decisions, assess surgical risk, and evaluate disease progression over time.
Respiratory therapists are the primary practitioners responsible for performing and ensuring quality spirometry and complete PFT studies in most clinical settings. Understanding the technical standards, normal values, and interpretation framework is fundamental to the RT scope of practice.
Spirometry: Core Measurements
Spirometry is the most common and clinically important component of PFT testing. It measures the volume of air inhaled and exhaled and the speed of air movement. Key spirometry values include:
FVC — Forced Vital Capacity
Normal: ≥80% predicted
Total air forcefully exhaled after max inspiration
FEV₁ — Forced Expiratory Volume in 1 sec
Normal: ≥80% predicted
Air exhaled in the first second
FEV₁/FVC — FEV₁ to FVC Ratio
Normal: ≥0.70 (70%)
Key ratio for obstruction detection
FEF 25–75% — Mid-Expiratory Flow
Normal: ≥65% predicted
Sensitive for small airway disease
PEF — Peak Expiratory Flow
Normal: Variable by age/sex
Peak flow rate during forced exhalation
MVV — Maximum Voluntary Ventilation
Normal: ≥80% predicted
Max air moved per minute
Lung Volumes and Capacities
Complete lung volume measurements require body plethysmography or gas dilution techniques (helium dilution or nitrogen washout) because spirometry alone cannot measure gas trapped in the lungs.
TLC — Total Lung Capacity
Total volume at full inspiration; increased in obstruction, decreased in restriction
RV — Residual Volume
Air remaining after maximal exhalation; increased in air trapping
FRC — Functional Residual Capacity
Volume at end of normal expiration; reflects natural lung recoil balance
VC — Vital Capacity
Maximum volume from full inspiration to full exhalation
IC — Inspiratory Capacity
Volume from FRC to full inspiration
ERV — Expiratory Reserve Volume
Volume that can be forcefully exhaled from FRC
Obstructive vs Restrictive Patterns
The most important clinical decision in PFT interpretation is identifying whether an abnormality is obstructive, restrictive, or mixed. This determination guides diagnosis and treatment.
Obstructive Pattern
The hallmark is reduced airflow — specifically a decreased FEV₁/FVC ratio below 0.70.
- • FEV₁/FVC < 0.70 (post-bronchodilator defines fixed obstruction)
- • FEV₁ reduced (may be severely so)
- • FVC may be normal or reduced due to air trapping
- • TLC normal or increased; RV increased
- • Common diseases: COPD, asthma, bronchiectasis, cystic fibrosis
Restrictive Pattern
The hallmark is reduced lung volumes with preserved or increased flow rates relative to volume.
- • TLC < 80% predicted (definitive for restriction)
- • FVC reduced
- • FEV₁ reduced proportionally to FVC — FEV₁/FVC normal or elevated
- • Common diseases: pulmonary fibrosis, obesity, scoliosis, neuromuscular disease, pleural effusion
Mixed Pattern
Both obstructive and restrictive features are present — FEV₁/FVC below 0.70 AND TLC below 80% predicted. Seen in patients with overlapping diseases such as COPD with pulmonary fibrosis.
DLCO: Diffusion Capacity
The diffusing capacity of the lungs for carbon monoxide (DLCO) measures how efficiently the lungs transfer gas from the alveoli into the bloodstream. It reflects the functional surface area of the alveolar-capillary membrane, hemoglobin availability, and pulmonary blood flow.
DLCO is measured by having the patient inhale a small amount of carbon monoxide, hold the breath for 10 seconds, then exhale. The amount of CO absorbed reflects gas transfer efficiency.
A reduced DLCO (below 70–75% predicted) suggests:
- • Pulmonary fibrosis or interstitial lung disease
- • Emphysema (alveolar destruction)
- • Pulmonary hypertension
- • Pulmonary embolism (reduced vascular bed)
- • Anemia (insufficient hemoglobin)
An elevated DLCO may be seen in polycythemia, alveolar hemorrhage, or left-to-right cardiac shunts.
COPD Severity Classification (GOLD)
The GOLD (Global Initiative for Chronic Obstructive Lung Disease) criteria classify COPD severity based on post-bronchodilator FEV₁ % predicted:
GOLD 1 — Mild
FEV₁: ≥ 80% predicted
Usually minimal symptoms; airflow limitation present
GOLD 2 — Moderate
FEV₁: 50–79% predicted
Increased breathlessness, usually seek medical attention
GOLD 3 — Severe
FEV₁: 30–49% predicted
Significant impact on quality of life, exacerbations common
GOLD 4 — Very Severe
FEV₁: < 30% predicted
Chronic respiratory failure; life-threatening exacerbations
Frequently Asked Questions
What is the bronchodilator response test?
After baseline spirometry, the patient receives a short-acting bronchodilator (typically albuterol 2.5 mg nebulized or 400 mcg MDI). Spirometry is repeated 15–20 minutes later. A positive bronchodilator response is defined as an increase in FEV₁ or FVC of ≥12% AND ≥200 mL from baseline, suggesting reversible airflow obstruction consistent with asthma.
What makes a spirometry effort acceptable?
ATS/ERS criteria require: a forceful, maximal effort from the start; at least 6 seconds of exhalation (3 seconds in children); no coughs in the first second; no Valsalva; no early termination. At least 3 acceptable maneuvers are needed, with the two best FVC values within 150 mL of each other for reproducibility.
Why is FEV₁/FVC more important than FEV₁ alone?
FEV₁/FVC is the primary indicator of obstructive physiology. FEV₁ alone can be reduced in both obstruction AND restriction. The ratio contextualizes FEV₁ against total exhaled volume — if both FEV₁ and FVC are proportionally reduced (normal ratio), the pattern is restrictive rather than obstructive.
What is the RV/TLC ratio and why does it matter?
The RV/TLC ratio (normal: 20–35%) reflects the proportion of total lung capacity that cannot be exhaled. Values above 35–40% indicate significant air trapping, commonly seen in COPD and emphysema. A markedly elevated RV/TLC supports hyperinflation and air trapping even when FEV₁/FVC may only be mildly reduced.
Summary
- FEV₁/FVC ratio below 0.70 defines obstruction; reduced TLC defines restriction
- Spirometry measures airflow; lung volumes require body plethysmography or gas dilution
- DLCO reflects alveolar-capillary gas exchange efficiency — reduced in fibrosis and emphysema
- GOLD staging classifies COPD severity using post-bronchodilator FEV₁ % predicted
- Quality spirometry requires ATS/ERS-acceptable efforts and reproducible results
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