Pulmonary Function Testing (PFT): Clinical Overview

Pulmonary function testing (PFT) is a group of diagnostic tests used to measure how well the lungs and respiratory system are working. These tests help evaluate airflow, lung volumes, gas exchange, respiratory muscle strength, and the body’s response to exercise.

For respiratory therapists and other healthcare providers, PFT results provide objective data that can help identify obstructive, restrictive, and mixed lung disorders. However, the results must always be interpreted with patient history, symptoms, imaging, blood gases, physical assessment findings, and test quality in mind.

What Is Pulmonary Function Testing?

Pulmonary function testing (PFT) is the systematic measurement of lung function. It evaluates how air moves into and out of the lungs, how much air the lungs can hold, how well gases move across the alveolar-capillary membrane, and how effectively the respiratory system supports ventilation and oxygenation.

The lungs have two primary jobs:

• Oxygenating mixed venous blood
• Removing carbon dioxide from the body

These functions depend on many structures working together, including the airways, alveoli, pulmonary blood vessels, respiratory muscles, chest wall, diaphragm, and neurologic control of breathing. Because of this, no single test can fully describe lung function.

Instead, PFTs include several types of tests that examine different parts of the respiratory system. A complete evaluation may include spirometry, lung volume measurements, diffusing capacity, blood gases, airway resistance, respiratory muscle pressures, bronchoprovocation testing, and cardiopulmonary exercise testing.

Pulmonary function testing is not just a collection of numbers. It is a clinical process that combines physiology, patient effort, equipment accuracy, test quality, and clinical judgment.

Why Pulmonary Function Testing Is Important

PFTs are important because they give measurable information about respiratory function. Symptoms such as shortness of breath, wheezing, cough, sputum production, and exercise intolerance can have many possible causes. Pulmonary function testing helps narrow the possibilities by showing whether the problem is related to airflow, lung expansion, gas exchange, respiratory muscle weakness, or exercise limitation.

PFTs can help answer questions such as:

• Does the patient have airflow obstruction?
• Is there evidence of restrictive lung disease?
• Is gas exchange impaired?
• Is the patient responding to bronchodilator therapy?
• Is dyspnea caused by pulmonary disease, cardiac disease, deconditioning, obesity, or another problem?
• Is the patient at increased risk for surgery?
• Is lung disease progressing or improving?
• Is occupational or environmental exposure affecting lung function?
• Does the patient have measurable impairment or disability?

Note: These tests are used for diagnosis, monitoring, treatment planning, surgical risk assessment, occupational surveillance, disability evaluation, and research.

Major Categories of Pulmonary Function Tests

Pulmonary function tests can be grouped into several major categories. Each category measures a different part of respiratory function.

Common categories include:

• Airway function tests
• Lung volume tests
• Diffusing capacity tests
• Blood gas and gas exchange tests
• Ventilation and ventilatory control tests
• Respiratory muscle strength tests
• Bronchoprovocation tests
• Cardiopulmonary exercise tests
• Metabolic measurements

Airway function tests measure how quickly and effectively air can move through the conducting airways. Lung volume tests measure how much air the lungs can hold. Diffusing capacity tests evaluate gas transfer across the alveolar-capillary membrane. Blood gas testing evaluates oxygenation, ventilation, and acid-base status. Exercise testing examines the integrated function of the lungs, heart, circulation, muscles, and metabolism during physical activity.

Many lung diseases affect more than one measurement. For example, emphysema may cause airflow obstruction, air trapping, hyperinflation, and reduced diffusing capacity. Pulmonary fibrosis may cause reduced lung volumes and reduced diffusing capacity. Neuromuscular disease may reduce vital capacity and respiratory muscle pressures while leaving airway resistance relatively normal. For this reason, PFT interpretation depends on patterns rather than isolated values.

Indications for Pulmonary Function Testing

Pulmonary function testing is ordered when clinicians need objective information about lung function. It may be used to diagnose disease, measure severity, monitor treatment, assess surgical risk, or evaluate impairment.

Common indications include:

• Dyspnea
• Wheezing
• Chronic cough
• Sputum production
• Abnormal breath sounds
• Abnormal chest imaging
• Abnormal blood gases
• Low oxygen saturation
• Suspected asthma
• Suspected COPD
• Suspected restrictive lung disease
• Suspected interstitial lung disease
• Neuromuscular weakness
• Occupational or environmental exposure
• Preoperative risk assessment
• Disability evaluation
• Pulmonary rehabilitation evaluation
• Monitoring response to therapy

PFTs are also used in patients with known pulmonary diseases, including asthma, chronic bronchitis, emphysema, bronchiectasis, cystic fibrosis, pulmonary fibrosis, pneumoconiosis, and pulmonary vascular disease.

Note: They may also be used when nonpulmonary diseases affect the lungs, such as congestive heart failure, systemic lupus erythematosus, rheumatoid arthritis, obesity, chest wall disorders, and neuromuscular conditions.

Contraindications for Pulmonary Function Testing

Although PFTs are generally safe, they are not appropriate for every patient. Many tests require forceful breathing, repeated maximal efforts, or breath-holding. These actions can increase intrathoracic pressure, provoke coughing, worsen symptoms, or place stress on the cardiovascular system.

Common contraindications include:

• Recent myocardial infarction
• Unstable angina
• Uncontrolled hypertension
• Hemoptysis
• Pneumothorax
• Acute pulmonary embolism
• Acute chest pain
• Acute abdominal pain
• Acute asthmatic attack
• Acute COPD exacerbation
• Recent eye surgery, such as cataract surgery
• Inability to follow instructions
• Severe confusion or poor cooperation

Note: Testing should be postponed or modified when patient safety is a concern or when valid results cannot be expected. Because PFTs depend heavily on patient effort, a patient who cannot understand or perform the required maneuvers may produce unreliable data.

Spirometry

Spirometry is the most common pulmonary function test and often serves as the starting point for evaluating lung disease. It measures the volume and flow of air during breathing maneuvers, especially forced exhalation after a maximal inspiration.

Important spirometry values include:

• Forced vital capacity
• Forced expiratory volume in one second
• FEV₁/FVC ratio
• Peak expiratory flow
• Forced expiratory flows
• Slow vital capacity
• Maximal voluntary ventilation

Note: Spirometry is especially useful for identifying airflow obstruction, grading severity, assessing bronchodilator response, and monitoring disease over time.

Forced Vital Capacity

Forced vital capacity (FVC) is the greatest volume of air a patient can exhale forcefully after taking the deepest breath possible. During the maneuver, the patient inhales fully, seals the lips around the mouthpiece, and exhales as hard and fast as possible until no more air can be expelled.

FVC is highly effort dependent. The patient must start forcefully, continue exhaling long enough, avoid coughing during the first second, maintain a good mouth seal, and avoid glottic closure.

A low FVC may suggest restriction, but it does not confirm restriction by itself. In obstructive disease, the FVC may also be low because airways collapse during forced exhalation, trapping air in the lungs. This is sometimes called pseudo-restriction.

Forced Expiratory Volume in One Second

Forced expiratory volume in one second (FEV₁) is the amount of air exhaled during the first second of the FVC maneuver. It is one of the most important spirometry values because it reflects how quickly air can leave the lungs.

In healthy lungs, a large percentage of the FVC is exhaled during the first second. In obstructive lung disease, narrowed airways slow expiratory flow, causing FEV₁ to decrease.

FEV₁ is used to:

• Identify airflow limitation
• Grade severity of obstruction
• Assess response to bronchodilator therapy
• Monitor disease progression
• Evaluate bronchoprovocation testing
• Assess exercise-induced bronchospasm

Note: A low FEV₁ can occur in obstruction, restriction, poor effort, or mixed disease, so it must be interpreted with the FEV₁/FVC ratio and other test findings.

FEV₁/FVC Ratio

The FEV₁/FVC ratio compares the amount of air exhaled in the first second with the total forced vital capacity. It is one of the most useful values for identifying airflow obstruction.

A reduced FEV₁/FVC ratio suggests obstructive lung disease because the patient cannot exhale air quickly. A commonly used screening value is less than 70%, although interpretation should ideally use the lower limit of normal when available.

Obstructive diseases that may reduce the FEV₁/FVC ratio include:

• Asthma
• Chronic bronchitis
• Emphysema
• Bronchiectasis
• Cystic fibrosis
• Some upper airway disorders

Note: A normal or high FEV₁/FVC ratio with a reduced FVC may suggest restriction, poor effort, or another nonobstructive pattern. Lung volume testing is required to confirm true restriction.

Peak Expiratory Flow

Peak expiratory flow (PEFR) is the highest flow achieved during a forced expiratory effort. It usually occurs near the beginning of exhalation.

Peak flow is commonly used to monitor airway tone in asthma. Patients may use portable peak flow meters at home to track changes in airflow, identify worsening obstruction, and evaluate response to therapy. Peak flow is useful, but it is effort dependent and does not replace complete spirometry when a more detailed assessment is needed.

Spirometry Curves

Spirometry results are commonly displayed in two main graphic formats:

• Volume-time curve
• Flow-volume loop

Note: These curves are important because they help determine whether the maneuver was performed correctly and whether the pattern suggests a specific abnormality.

Volume-Time Curve

The volume-time curve shows how much air is exhaled over time. It is especially useful for determining whether the patient exhaled long enough.

A good curve should show a rapid rise in volume early in exhalation, followed by a gradual flattening as the patient approaches the end of the maneuver. If the patient stops too soon, FVC may be falsely low.

Note: Early termination can falsely raise the FEV₁/FVC ratio and make obstruction look like restriction. This is one reason why curve review is essential before interpretation.

Flow-Volume Loop

The flow-volume loop plots airflow against lung volume. It provides visual information that may not be obvious from the numeric report.

The shape of the flow-volume loop can suggest:

• Obstructive lung disease
• Restrictive lung disease
• Fixed upper airway obstruction
• Variable extrathoracic obstruction
• Variable intrathoracic obstruction
• Vocal cord dysfunction
• Airway malacia
• Poor effort
• Cough artifact

In obstructive disease, the expiratory portion of the loop often appears scooped or concave. In restrictive disease, the loop may be narrow because lung volume is reduced. In upper airway obstruction, flattening of the inspiratory limb, expiratory limb, or both may provide important diagnostic clues.

Note: The shape of the curve matters. A numeric table alone can miss clinically useful information.

Spirometry Quality

Quality control is one of the most important parts of pulmonary function testing. Spirometry is effort dependent, and poor technique can produce falsely abnormal values.

A valid spirometry test depends on:

• Clear patient instruction
• Proper demonstration
• Maximal inhalation before the maneuver
• Explosive start of exhalation
• Continued exhalation until complete
• No major cough artifact
• No leak around the mouthpiece
• No glottic closure
• Repeatable results
• Competent coaching by the technologist

At least three acceptable maneuvers should typically be obtained. The largest FVC and largest FEV₁ are reported, even if they come from different maneuvers. Some flow values, such as FEF₂₅–₇₅%, should come from the single best maneuver.

Common spirometry errors include:

• Early termination
• Slow or hesitant start
• Cough during the first second
• Variable effort
• Mouthpiece leak
• Poor mouth seal
• Incomplete inhalation
• Glottic closure
• Poor posture
• Inadequate coaching

Note: Poor-quality results can lead to misdiagnosis, unnecessary testing, inappropriate treatment, incorrect disability classification, or poor surgical decisions. For this reason, interpretation should begin with test quality.

Bronchodilator Testing

Bronchodilator testing compares spirometry before and after inhaled bronchodilator administration. It evaluates whether airflow obstruction improves after medication.

A significant improvement in FEV₁ or FVC suggests reversible airflow limitation. This is often associated with asthma, but some patients with COPD may also show a bronchodilator response.

Bronchodilator testing may be used to:

• Evaluate reversibility of obstruction
• Assess response to therapy
• Help distinguish asthma from fixed obstruction
• Determine whether bronchodilator therapy is useful
• Monitor treatment effectiveness

A significant bronchodilator response supports reversible airway disease, but absence of a formal response does not always mean the patient will not benefit clinically. Some patients feel symptom relief even when spirometry does not meet strict response criteria.

Interpretation must also consider the testing session. Repeated forced maneuvers can sometimes provoke bronchospasm, especially in asthma. If FEV₁ falls across repeated efforts, the therapist should recognize that the patient may be worsening with repeated forced exhalation.

Lung Volumes vs. Lung Capacities

Lung volumes and lung capacities are closely related, but they are not the same thing. A lung volume is a single measured compartment of air in the lungs. A lung capacity is a combination of two or more lung volumes.

Understanding this difference is important because pulmonary function testing often uses these values to determine whether a patient has normal lung mechanics, air trapping, hyperinflation, or restriction.

Lung Volumes

The four basic lung volumes are tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume.

• Tidal volume is the amount of air inhaled or exhaled during a normal quiet breath.
• Inspiratory reserve volume is the extra air that can be inhaled after a normal inhalation.
• Expiratory reserve volume is the extra air that can be exhaled after a normal exhalation.
• Residual volume is the air that remains in the lungs after maximal exhalation.

Lung Capacities

Lung capacities are formed by adding lung volumes together.

• Vital capacity includes inspiratory reserve volume, tidal volume, and expiratory reserve volume.
• Inspiratory capacity includes tidal volume and inspiratory reserve volume.
• Functional residual capacity includes expiratory reserve volume and residual volume.
• Total lung capacity includes all four lung volumes and represents the total amount of air in the lungs after maximal inspiration.

These measurements help clinicians identify different pulmonary patterns. In restrictive disease, total lung capacity is reduced because the lungs, chest wall, or respiratory muscles cannot achieve normal expansion. In obstructive disease, residual volume and functional residual capacity may increase because air becomes trapped during exhalation.

This is why a reduced forced vital capacity does not automatically confirm restriction. The patient may actually have an obstruction with gas trapping. Lung volume and capacity measurements help clarify the pattern and prevent misinterpretation.

Measuring Lung Volumes and Capacities

Spirometry cannot directly measure residual volume, functional residual capacity, or total lung capacity. These values require lung volume testing.

Important lung volumes and capacities include:

• Tidal volume
• Inspiratory reserve volume
• Expiratory reserve volume
• Residual volume
• Functional residual capacity
• Inspiratory capacity
• Vital capacity
• Total lung capacity

Note: Lung volume testing is especially important when restriction, air trapping, or hyperinflation is suspected.

Total Lung Capacity

Total lung capacity (TLC) is the volume of air in the lungs after maximal inspiration. It is the key value for confirming restriction.

True restrictive lung disease is defined by reduced TLC. A low FVC alone does not confirm restriction because obstruction can reduce FVC through air trapping.

A reduced TLC may occur in:

• Pulmonary fibrosis
• Interstitial lung disease
• Pleural disease
• Chest wall disorders
• Obesity
• Neuromuscular weakness
• Prior lung resection

Residual Volume

Residual volume (RV) is the amount of air remaining in the lungs after maximal exhalation. It cannot be measured by simple spirometry because it is the air that remains after the patient has exhaled as much as possible.

RV often increases in obstructive lung disease because airways close prematurely during exhalation. This traps gas in the lungs.

An increased RV may suggest:

• Air trapping
• Obstructive lung disease
• Hyperinflation
• Small airway collapse

Functional Residual Capacity

Functional residual capacity (FRC) is the amount of air remaining in the lungs at the end of a normal quiet exhalation. FRC may increase in obstructive disease due to hyperinflation and may decrease in restrictive conditions.

RV/TLC Ratio

The RV/TLC ratio shows the fraction of total lung capacity that cannot be exhaled. An increased RV/TLC ratio suggests gas trapping. This is especially useful in obstructive diseases such as COPD, asthma, bronchiectasis, and cystic fibrosis.

Methods for Measuring Lung Volumes

Lung volumes can be measured using several methods.

Common methods include:

• Helium dilution
• Nitrogen washout
• Body plethysmography
• Imaging-based methods in selected cases

Helium Dilution

Helium dilution measures lung volume by having the patient breathe from a closed system containing a known concentration of helium. Because helium does not significantly cross the alveolar-capillary membrane, its dilution reflects the volume of ventilated lung.

However, helium dilution may underestimate lung volume in patients with significant obstruction because poorly ventilated areas may not communicate well with the test gas.

Nitrogen Washout

Nitrogen washout measures lung volume by having the patient breathe oxygen while nitrogen is washed out of the lungs. The amount of nitrogen removed is used to estimate lung volume.

Like helium dilution, nitrogen washout may underestimate lung volume when gas is trapped in poorly ventilated regions.

Body Plethysmography

Body plethysmography measures thoracic gas volume while the patient sits in an airtight box. It can measure gas in poorly communicating lung regions, making it especially useful in obstructive disease.

Plethysmography is often preferred when significant air trapping or hyperinflation is suspected because it can detect gas that dilution techniques may miss.

Diffusing Capacity

Diffusing capacity evaluates how well gases move from the alveoli into the pulmonary capillary blood. The most common measurement is the diffusing capacity of the lung for carbon monoxide (DLCO).

DLCO is usually measured using a single-breath technique. The patient inhales a test gas mixture containing a small amount of carbon monoxide and a tracer gas, holds the breath briefly, and then exhales. The amount of carbon monoxide taken up by the blood reflects gas transfer across the alveolar-capillary membrane.

DLCO is affected by:

• Alveolar surface area
• Membrane thickness
• Pulmonary capillary blood volume
• Hemoglobin level
• Carboxyhemoglobin level
• Inspired volume
• Altitude
• Test technique

A low DLCO may occur in:

• Emphysema
• Pulmonary fibrosis
• Interstitial lung disease
• Pulmonary vascular disease
• Anemia
• Some drug-induced lung injuries

DLCO may be increased in conditions such as pulmonary hemorrhage, polycythemia, and left-to-right shunt.

DLCO is useful because it helps differentiate causes of abnormal spirometry. For example, emphysema often causes obstruction with reduced DLCO because alveolar walls and capillary beds are destroyed. Asthma may cause obstruction with a normal or high DLCO because the alveolar-capillary membrane is usually preserved. Pulmonary fibrosis often causes restriction with reduced DLCO because the gas exchange membrane is thickened or damaged.

Blood Gases and Gas Exchange

Spirometry and lung volumes measure mechanics, but blood gas testing evaluates the main outputs of lung function: oxygenation and carbon dioxide removal.

Arterial blood gas analysis measures:

• pH
• PaCO₂
• PaO₂
• Bicarbonate
• Acid-base status

Blood gas analysis is useful when evaluating hypoxemia, hypercapnia, respiratory failure, acid-base disturbance, oxygen need, shunt, or ventilatory support.

Blood gases are especially important because some patients may have significant gas exchange impairment even when spirometry is only mildly abnormal. For example, pulmonary vascular disease may cause exertional desaturation before spirometry becomes abnormal. Interstitial lung disease may have preserved spirometry early but reduced DLCO and oxygen desaturation during exertion.

Note: Blood gas testing can also help validate noninvasive monitoring, such as pulse oximetry, transcutaneous monitoring, and capnography.

Ventilation Measurements

Ventilation testing evaluates how effectively the patient moves air and how much of that ventilation participates in gas exchange.

Important ventilation measurements include:

• Tidal volume
• Respiratory rate
• Minute ventilation
• Alveolar ventilation
• Dead space ventilation
• I:E ratio

Minute ventilation is the total amount of gas exhaled in one minute. It is calculated by multiplying tidal volume by respiratory rate. A normal resting adult minute ventilation is often about 5 to 10 L/min.

Alveolar ventilation is the portion of ventilation that reaches gas-exchanging regions. It is calculated by subtracting dead space from tidal volume and multiplying by respiratory rate.

A patient can have a normal minute ventilation but poor alveolar ventilation if the respiratory rate is high, tidal volume is low, or dead space is increased. This is why minute ventilation should not be interpreted alone.

Dead space ventilation is wasted ventilation that does not effectively participate in gas exchange. Increased dead space may occur in emphysema, pulmonary embolism, pulmonary vascular disease, and some forms of critical illness.

Maximal Voluntary Ventilation

Maximal voluntary ventilation (MVV) measures the greatest volume of air a patient can breathe in and out over a short period, often 12 seconds. The value is then extrapolated to one minute.

MVV reflects the combined function of:

• Airways
• Lung mechanics
• Thoracic cage
• Respiratory muscles
• Patient effort
• Neurologic control

Note: A low MVV may result from obstruction, restriction, neuromuscular weakness, poor effort, chest wall limitation, or severe dyspnea. MVV may be used to evaluate ventilatory reserve during exercise testing, assess respiratory disability, or estimate the patient’s ability to sustain increased ventilation.

Respiratory Muscle Strength Testing

Respiratory muscle strength testing measures the pressure a patient can generate during maximal inspiratory or expiratory efforts.

Common measurements include:

• Maximal inspiratory pressure
• Maximal expiratory pressure

Maximal inspiratory pressure, or MIP, measures the strength of the inspiratory muscles, especially the diaphragm. It is usually measured as the patient inhales forcefully against an occluded airway.

Maximal expiratory pressure, or MEP, measures expiratory muscle strength. It is usually measured as the patient exhales forcefully against an occluded airway.

These tests are useful in:

• Neuromuscular disease
• Diaphragmatic weakness
• Unexplained low vital capacity
• Ventilatory failure risk
• Weaning assessment
• Cough effectiveness evaluation

Note: A decreasing vital capacity or weak inspiratory pressure may suggest respiratory muscle fatigue or risk of ventilatory failure.

Bronchoprovocation Testing

Bronchoprovocation testing evaluates airway hyperresponsiveness. It is often used when asthma is suspected but baseline spirometry is normal or nondiagnostic.

Common bronchoprovocation methods include:

• Methacholine challenge
• Histamine challenge
• Exercise challenge
• Eucapnic voluntary hyperventilation

The test exposes the airways to a controlled stimulus and measures whether FEV₁ falls significantly. A significant decline in FEV₁ supports airway hyperresponsiveness.

Bronchoprovocation testing may be useful in patients with:

• Intermittent wheezing
• Unexplained cough
• Exercise-induced symptoms
• Suspected asthma with normal baseline spirometry
• Occupational asthma concerns

Note: These tests require careful screening, standardized protocols, emergency preparedness, and bronchodilator administration after the test when needed.

Cardiopulmonary Exercise Testing

Cardiopulmonary exercise testing (CPET) evaluates the integrated response of the lungs, heart, circulation, muscles, and metabolism during exercise. It is useful when resting tests do not explain the patient’s symptoms.

CPET may measure:

• Oxygen consumption
• Carbon dioxide production
• Minute ventilation
• Respiratory exchange ratio
• Heart rate
• Blood pressure
• Oxygen saturation
• Electrocardiogram
• Ventilatory reserve
• Blood gases

CPET helps determine whether exercise limitation is due to a pulmonary problem, cardiac problem, pulmonary vascular disease, deconditioning, obesity, dysfunctional breathing, or another cause.

A patient may have normal resting spirometry but still develop exertional dyspnea or desaturation. In that situation, exercise testing can provide important information that resting tests cannot.

Six-Minute Walk Test

The six-minute walk test (6MWT) is a simple field test used to assess functional exercise capacity. The patient walks as far as possible in six minutes while oxygen saturation, symptoms, distance, and sometimes heart rate are monitored.

The 6MWT is less complex than CPET, but it is clinically useful because it reflects everyday activity better than many resting tests.

It may be used to evaluate:

• Functional capacity
• Oxygen desaturation with exertion
• Response to treatment
• Pulmonary rehabilitation outcomes
• Disease progression
• Prognosis in chronic lung disease

Note: The total distance walked can correlate with clinical outcomes in several cardiopulmonary diseases.

Pediatric Pulmonary Function Testing

Pulmonary function testing in children requires special attention because children are not simply small adults. Age, cooperation, lung size, developmental stage, and coaching all affect the test.

Many children can perform spirometry by about age five, but there is no single age at which every child can reliably complete acceptable maneuvers.

In younger children, FEV₁ may be less useful because they can sometimes exhale most of their vital capacity in less than one second. Other values, such as FEV₀.₅ or FEV₀.₇₅, may provide useful information in preschool-aged children.

Pediatric testing often distinguishes between acceptable and usable results. A child may not complete a perfect FVC maneuver but may still produce a usable early exhalation that provides clinically meaningful information.

Pediatric flow-volume loops may help identify:

• Asthma
• Vocal cord dysfunction
• Tracheomalacia
• Bronchomalacia
• Cystic fibrosis-related obstruction
• Variable effort

Note: Repeatability standards may differ in children with small lung volumes. Smaller absolute differences may be used when FVC is low.

Reference Values

PFT interpretation depends on comparing measured values to predicted reference values. Normal lung function varies based on patient characteristics.

Reference values may account for:

• Age
• Height
• Sex
• Race or ethnicity
• Sometimes weight

In general, taller individuals have larger lung volumes. Lung function usually rises through childhood and adolescence, peaks in early adulthood, and gradually declines with age.

PFT reports may include:

• Measured value
• Predicted value
• Percent predicted
• Lower limit of normal
• Upper limit of normal
• Z score

Percent predicted is commonly used, but it has limitations. A fixed cutoff such as 80% predicted may misclassify some patients. The lower limit of normal is usually more statistically appropriate because it accounts for expected variation in healthy individuals.

The Global Lung Initiative has published multi-ethnic spirometry reference values across a broad age range, helping provide smoother interpretation from childhood through adulthood. However, reference selection for lung volumes and DLCO can be more complicated because methods and available data vary.

Interpreting PFT Patterns

PFT interpretation should follow a logical process. The first step is always quality review. If the test is not acceptable or repeatable, interpretation should be cautious.

A practical interpretation sequence includes:

• Review test quality
• Evaluate the FEV₁/FVC ratio
• Evaluate FVC and FEV₁
• Review flow-volume loop shape
• Check lung volumes if available
• Check TLC for restriction
• Check RV, FRC, and RV/TLC for air trapping
• Evaluate DLCO
• Assess bronchodilator response
• Grade severity
• Compare with prior tests
• Integrate clinical context

Note: PFTs usually identify physiologic patterns, not exact diagnoses. A pattern may suggest asthma, COPD, emphysema, pulmonary fibrosis, neuromuscular weakness, or pulmonary vascular disease, but the final diagnosis requires the full clinical picture.

Obstructive Lung Disease

Obstructive lung disease is characterized by reduced airflow, especially during exhalation. The key spirometric finding is a reduced FEV₁/FVC ratio.

Common obstructive diseases include:

• Asthma
• Chronic bronchitis
• Emphysema
• Bronchiectasis
• Cystic fibrosis

In obstructive disease, patients have difficulty exhaling fully because of narrowed airways, airway inflammation, mucus, bronchospasm, airway collapse, or loss of elastic recoil.

Common PFT findings in obstruction include:

• Reduced FEV₁/FVC ratio
• Reduced FEV₁
• Reduced peak expiratory flow
• Scooped expiratory flow-volume loop
• Increased RV
• Increased FRC
• Increased RV/TLC
• Normal or increased TLC in hyperinflation

Note: DLCO helps refine interpretation. In emphysema, DLCO is often reduced because alveolar-capillary surface area is destroyed. In asthma and chronic bronchitis, DLCO may be normal unless another process is present.

Restrictive Lung Disease

Restrictive lung disease is characterized by reduced lung expansion and decreased lung volumes. The key finding is reduced TLC.

Spirometry may show a reduced FVC with a normal or high FEV₁/FVC ratio, but TLC is required to confirm restriction.

Restrictive disorders may be caused by:

• Interstitial lung disease
• Pulmonary fibrosis
• Pleural disease
• Chest wall deformity
• Obesity
• Neuromuscular weakness
• Prior lung resection

Common PFT findings in restriction include:

• Reduced TLC
• Reduced FVC
• Normal or high FEV₁/FVC ratio
• Reduced lung volumes
• Narrow flow-volume loop
• Reduced DLCO in parenchymal disease

Note: DLCO is useful for separating parenchymal restriction from extrapulmonary restriction. In pulmonary fibrosis, DLCO is often reduced. In pure chest wall restriction or neuromuscular weakness, DLCO may be relatively preserved when adjusted appropriately.

Mixed Lung Disease

A mixed defect includes both obstruction and restriction. It is usually identified by a reduced FEV₁/FVC ratio and a reduced TLC. Mixed patterns may occur when a patient has more than one condition, such as COPD with pulmonary fibrosis or asthma with obesity-related restriction.

Interpretation can be challenging because severe obstruction with air trapping can lower FVC and mimic restriction. This is why lung volumes are necessary when spirometry suggests possible mixed disease.

PFT Equipment

Pulmonary function testing requires specialized equipment. The type of equipment depends on which test is being performed.

Common PFT equipment includes:

• Spirometer
• Peak flowmeter
• Body plethysmograph
• Pneumotachometer
• Turbinometer
• Pulmonary gas analyzer
• Oxygen analyzer
• Blood gas analyzer
• Breathing valves
• Gas-conditioning devices
• Computers and reporting software
• Calibration syringes
• Quality-control tools

PFT equipment may measure volume, flow, pressure, gas concentration, or time. Each device has potential sources of error, so routine calibration, maintenance, and quality control are essential.

Flow sensors can be affected by secretions, condensation, humidity, temperature, resistance, or calibration problems. Gas analyzers require proper calibration and response characteristics. Body plethysmography requires proper panting technique and equipment performance.

Laboratory Quality Control

Pulmonary function laboratories must have strong quality control and quality improvement systems. Poor PFT data can be worse than no data because inaccurate results may point clinicians in the wrong direction.

Quality control includes:

• Equipment calibration
• Leak testing
• Volume accuracy checks
• Flow accuracy checks
• Time validation
• Gas analyzer calibration
• Infection control
• Staff competency
• Procedure manuals
• Documentation of errors and corrections
• Review of test quality
• Regular maintenance records

Note: Daily spirometer checks and documented equipment maintenance are especially important. If equipment is inaccurate or leaking, test results cannot be trusted. Quality control also applies to the patient maneuver. Even a perfectly calibrated device can produce poor data if the patient does not perform the test correctly.

Infection Control During PFT

Pulmonary function testing creates potential exposure to respiratory secretions. Patients breathe forcefully through mouthpieces, valves, filters, tubing, and other equipment. Coughing may occur during testing.

Infection control measures include:

• Hand hygiene before and after patient contact
• Use of standard precautions
• Proper cleaning and disinfection
• Disposable mouthpieces when appropriate
• Filters when indicated
• Gloves when handling contaminated equipment
• Respiratory protection when airborne infection is suspected
• Proper room ventilation
• Cleaning surfaces between patients

Note: Equipment that directly contacts the patient should be discarded, sterilized, or disinfected according to laboratory policy and manufacturer instructions.

Clinical Integration

Pulmonary function testing is most useful when interpreted with the clinical picture. PFTs can identify physiologic abnormalities, but they do not replace patient assessment.

Important clinical factors include:

• Symptoms
• Smoking history
• Occupational exposure
• Medication use
• Physical examination
• Chest imaging
• Blood gases
• Oxygen saturation
• Exercise tolerance
• Prior PFT results
• Test quality
• Patient effort

For example, a patient with dyspnea, smoking history, reduced FEV₁/FVC, increased RV/TLC, and reduced DLCO may have emphysema-predominant COPD. A patient with reduced TLC, reduced FVC, and reduced DLCO may have interstitial lung disease. A patient with normal spirometry but exertional desaturation may need DLCO, exercise testing, blood gases, or pulmonary vascular evaluation.

A patient with variable inspiratory loop flattening may need evaluation for vocal cord dysfunction or upper airway obstruction. A patient with low FVC but poor exhalation quality may need repeat testing before any diagnosis is made.

Note: The numbers matter, but so do the curves, symptoms, effort, equipment, and clinical context.

Common PFT Calculations

Pulmonary function testing often requires basic calculations. These are especially important for students and respiratory therapists preparing for exams.

Common formulas include:

• VC = IRV + VT + ERV
• VC = TLC − RV
• TLC = IRV + VT + ERV + RV
• TLC = FRC + IC
• FRC = RV + ERV
• RV = TLC − VC
• RV = FRC − ERV
• FRC = TLC − IC
• Minute ventilation = VT × respiratory rate
• Alveolar ventilation = (VT − dead space) × respiratory rate
• Percent predicted = measured value ÷ predicted value × 100
• Percent change = new value − old value ÷ old value × 100

Note: Drawing a lung volume graph can help prevent calculation errors because it shows how the volumes and capacities relate to each other.

Role of the Respiratory Therapist

Respiratory therapists play an important role in pulmonary function testing. They may perform the test, coach the patient, evaluate effort, recognize artifacts, maintain equipment, follow infection control procedures, and help interpret results.

The therapist must understand both the technical and clinical sides of testing. This includes knowing when a result is valid, when a maneuver should be repeated, when testing should stop, and when results should be interpreted cautiously.

The therapist’s responsibilities may include:

• Explaining the test to the patient
• Demonstrating the maneuver
• Providing strong coaching
• Monitoring patient safety
• Identifying poor effort
• Reviewing curves
• Documenting test quality
• Performing calibration checks
• Following infection control procedures
• Recognizing abnormal patterns
• Communicating concerns to the healthcare team

Note: A trained respiratory therapist is essential because computer-generated interpretations cannot replace clinical judgment.

Pulmonary Function Testing Practice Questions

1. What is pulmonary function testing?
Pulmonary function testing is a group of diagnostic tests used to measure how well the lungs and respiratory system move air, hold air, exchange gases, and respond to physiologic demands.

2. What is the main purpose of pulmonary function testing?
The main purpose is to identify the presence, type, severity, and progression of pulmonary impairment and help guide treatment decisions.

3. What are the three major components of pulmonary function testing?
The three major components are spirometry, lung volume measurements, and diffusing capacity testing.

4. Why should PFT results be interpreted with the clinical picture?
PFT results should be interpreted with the patient’s history, symptoms, imaging, blood gases, physical assessment findings, and test quality because numbers alone do not diagnose a specific disease.

5. What does spirometry measure?
Spirometry measures airflow and volume during breathing maneuvers, especially forced exhalation after a maximal inspiration.

6. What is forced vital capacity (FVC)?
Forced vital capacity is the greatest volume of air a patient can exhale forcefully after taking the deepest breath possible.

7. What does FEV1 measure?
FEV1 measures the volume of air exhaled during the first second of the forced vital capacity maneuver.

8. Why is the FEV1/FVC ratio important?
The FEV1/FVC ratio is important because a reduced ratio is one of the key findings used to identify airflow obstruction.

9. What does an FEV1/FVC ratio below 70% commonly indicate?
An FEV1/FVC ratio below 70% commonly indicates airflow obstruction, especially when screening for COPD.

10. What type of lung disease is associated with a reduced FEV1/FVC ratio?
Obstructive lung disease is associated with a reduced FEV1/FVC ratio.

11. What are examples of obstructive lung diseases?
Examples of obstructive lung diseases include asthma, chronic bronchitis, emphysema, bronchiectasis, and cystic fibrosis.

12. What type of lung disease is confirmed by a reduced total lung capacity?
Restrictive lung disease is confirmed by a reduced total lung capacity.

13. Why does a low FVC alone not confirm restriction?
A low FVC alone does not confirm restriction because obstruction with air trapping can also reduce FVC.

14. What lung volume is most important for confirming restrictive disease?
Total lung capacity is the most important lung volume for confirming restrictive disease.

15. What does an increased residual volume suggest?
An increased residual volume suggests air trapping, which is commonly seen in obstructive lung disease.

16. What does an increased RV/TLC ratio indicate?
An increased RV/TLC ratio indicates that a larger portion of the total lung capacity is trapped and cannot be exhaled.

17. What is functional residual capacity?
Functional residual capacity is the volume of gas remaining in the lungs at the end of a normal quiet exhalation.

18. What is residual volume?
Residual volume is the amount of air remaining in the lungs after a maximal exhalation.

19. What is total lung capacity?
Total lung capacity is the total amount of air in the lungs after a maximal inspiration.

20. What is body plethysmography used to measure?
Body plethysmography is used to measure thoracic gas volume and lung volumes, including residual volume and total lung capacity.

21. Why is body plethysmography useful in obstructive lung disease?
Body plethysmography is useful in obstructive lung disease because it can measure gas trapped in poorly ventilated lung regions.

22. Why can helium dilution underestimate lung volume in severe obstruction?
Helium dilution can underestimate lung volume because trapped or poorly ventilated gas may not communicate well with the test gas.

23. What does DLCO evaluate?
DLCO evaluates how well gas transfers from the alveoli across the alveolar-capillary membrane into pulmonary capillary blood.

24. Why is carbon monoxide used during DLCO testing?
Carbon monoxide is used because its uptake helps estimate gas transfer across the alveolar-capillary membrane.

25. What conditions can cause a low DLCO?
A low DLCO can occur with emphysema, pulmonary fibrosis, interstitial lung disease, pulmonary vascular disease, anemia, and some drug-induced lung injuries.

26. What is the main purpose of bronchodilator testing?
Bronchodilator testing is used to determine whether airflow obstruction improves after inhaled bronchodilator medication.

27. What does a significant bronchodilator response suggest?
A significant bronchodilator response suggests reversible airflow limitation, which is commonly associated with asthma but may also occur in some patients with COPD.

28. Does a negative bronchodilator response always mean bronchodilator therapy will not help?
No. Some patients may experience symptom relief even if their spirometry results do not meet formal response criteria.

29. Why can repeated forced spirometry maneuvers sometimes be a problem?
Repeated forced maneuvers can cause fatigue or provoke bronchospasm, especially in patients with asthma or reactive airways.

30. What is maximal voluntary ventilation (MVV)?
Maximal voluntary ventilation is the greatest volume of air a patient can inhale and exhale over a short period, usually measured over 12 seconds and extrapolated to one minute.

31. What does a decreased MVV indicate?
A decreased MVV may indicate obstructive disease, restrictive disease, neuromuscular weakness, poor effort, or limited ventilatory reserve.

32. What does maximal inspiratory pressure (MIP) evaluate?
Maximal inspiratory pressure evaluates the strength of the inspiratory muscles, especially the diaphragm.

33. What does maximal expiratory pressure (MEP) evaluate?
Maximal expiratory pressure evaluates the strength of the expiratory muscles used for forceful exhalation and coughing.

34. Why are MIP and MEP useful in neuromuscular disease?
MIP and MEP are useful because neuromuscular disease can weaken the respiratory muscles and reduce the patient’s ability to ventilate or cough effectively.

35. What is bronchoprovocation testing?
Bronchoprovocation testing is a diagnostic test used to evaluate airway hyperresponsiveness when asthma is suspected but baseline spirometry is normal or unclear.

36. What is a methacholine challenge test used to assess?
A methacholine challenge test is used to assess airway hyperresponsiveness by measuring whether FEV1 decreases after inhaling increasing concentrations of methacholine.

37. What finding supports airway hyperresponsiveness during bronchoprovocation testing?
A significant decline in FEV1 after exposure to the challenge agent supports airway hyperresponsiveness.

38. When might bronchoprovocation testing be indicated?
Bronchoprovocation testing may be indicated for intermittent wheezing, unexplained cough, exercise-induced symptoms, suspected asthma, or occupational asthma concerns.

39. Why must bronchoprovocation testing be performed carefully?
It must be performed carefully because it can intentionally trigger bronchoconstriction and requires screening, monitoring, emergency readiness, and post-test bronchodilator treatment when needed.

40. What is cardiopulmonary exercise testing (CPET)?
Cardiopulmonary exercise testing evaluates the combined response of the lungs, heart, circulation, muscles, and metabolism during exercise.

41. Why is CPET useful when resting PFTs are normal?
CPET is useful because some patients have exertional dyspnea or desaturation that does not appear during resting pulmonary function testing.

42. What measurements may be collected during CPET?
CPET may measure oxygen consumption, carbon dioxide production, ventilation, heart rate, rhythm, blood pressure, oxygen saturation, and ventilatory reserve.

43. What conditions can CPET help distinguish?
CPET can help distinguish pulmonary limitation, cardiac limitation, pulmonary vascular disease, deconditioning, obesity, and dysfunctional breathing.

44. What is the six-minute walk test?
The six-minute walk test is a field test that measures how far a patient can walk in six minutes while monitoring symptoms and oxygenation.

45. What does the six-minute walk test help evaluate?
The six-minute walk test helps evaluate functional capacity, exertional oxygen desaturation, treatment response, pulmonary rehabilitation outcomes, and disease progression.

46. Why is the six-minute walk test clinically useful?
It is clinically useful because it is simple, practical, inexpensive, and reflects functional activity better than many resting tests.

47. What is minute ventilation?
Minute ventilation is the total volume of gas exhaled in one minute.

48. What is the normal resting minute ventilation for an alert adult?
The normal resting minute ventilation for an alert, afebrile adult is approximately 5 to 10 L/min.

49. Why should minute ventilation not be interpreted alone?
Minute ventilation should not be interpreted alone because it depends on both tidal volume and respiratory rate, and a normal value can occur with an abnormal breathing pattern.

50. What is alveolar ventilation?
Alveolar ventilation is the portion of ventilation that reaches the alveoli and participates in gas exchange.

51. What is dead space ventilation?
Dead space ventilation is the portion of ventilation that does not participate effectively in gas exchange.

52. How is alveolar ventilation calculated?
Alveolar ventilation is calculated by subtracting dead space from tidal volume, then multiplying by respiratory rate.

53. What is the common bedside estimate for anatomic dead space?
A common bedside estimate for anatomic dead space is 1 mL/lb or 2.2 mL/kg of ideal body weight.

54. Why can a patient have normal minute ventilation but poor alveolar ventilation?
A patient can have normal minute ventilation but poor alveolar ventilation if tidal volume is too low, respiratory rate is too high, or dead space is increased.

55. What is a normal I:E ratio in a spontaneously breathing adult?
A normal I:E ratio in a spontaneously breathing adult is usually about 1:2 to 1:4.

56. What does a prolonged expiratory time suggest?
A prolonged expiratory time commonly suggests airflow obstruction, such as asthma or COPD.

57. What does a prolonged inspiratory time suggest?
A prolonged inspiratory time may suggest upper airway obstruction.

58. What is the purpose of reviewing the volume-time curve?
The volume-time curve helps determine whether the patient exhaled long enough to reach an adequate plateau.

59. What can early termination of exhalation cause during spirometry?
Early termination can falsely lower FVC and may falsely elevate the FEV1/FVC ratio.

60. How can early termination mimic restriction?
Early termination can make FVC appear reduced even when the patient does not have true restrictive lung disease.

61. What is the purpose of reviewing the flow-volume loop?
The flow-volume loop helps assess test quality and identify patterns such as obstruction, restriction, upper airway obstruction, poor effort, or cough artifact.

62. What does a scooped expiratory flow-volume loop suggest?
A scooped expiratory flow-volume loop suggests obstructive lung disease.

63. What can flattening of the inspiratory limb suggest?
Flattening of the inspiratory limb can suggest variable extrathoracic upper airway obstruction.

64. What can flattening of both inspiratory and expiratory limbs suggest?
Flattening of both limbs can suggest a fixed upper airway obstruction.

65. Why is spirometry considered effort dependent?
Spirometry is effort dependent because the patient must inhale fully, start exhalation forcefully, continue blowing, and avoid artifacts for valid results.

66. How many acceptable spirometry maneuvers should usually be obtained?
At least three acceptable spirometry maneuvers should usually be obtained for evaluation.

67. Which FVC and FEV1 values should be reported?
The largest FVC and largest FEV1 should be reported, even if they come from different acceptable maneuvers.

68. What is a common cause of falsely low FEV1?
A slow, hesitant, or poor start to exhalation can cause a falsely low FEV1.

69. Why can coughing during the first second invalidate spirometry?
Coughing during the first second can distort FEV1 and make the maneuver unreliable.

70. What is the role of the respiratory therapist during spirometry?
The respiratory therapist explains the procedure, demonstrates the maneuver, coaches the patient, monitors safety, and evaluates acceptability and repeatability.

71. Why should computer interpretation not replace clinical judgment?
Computer interpretation should not replace clinical judgment because software may not recognize poor effort, artifacts, clinical context, or unusual curve patterns.

72. What is the lower limit of normal?
The lower limit of normal is the lowest value expected in a healthy person based on reference data and patient characteristics.

73. Why is the lower limit of normal preferred over a fixed percent predicted cutoff?
The lower limit of normal is preferred because fixed cutoffs can misclassify patients, especially older adults.

74. What patient factors are commonly used to calculate predicted PFT values?
Predicted PFT values are commonly based on age, height, sex, and race or ethnicity.

75. Why are reference values important in pulmonary function testing?
Reference values are important because they allow measured results to be compared with expected normal values for a similar patient.

76. What is percent predicted in pulmonary function testing?
Percent predicted compares the patient’s measured value with the expected normal value for someone with similar characteristics.

77. How is percent predicted calculated?
Percent predicted is calculated by dividing the measured value by the predicted value, then multiplying by 100.

78. Why can fixed percent predicted cutoffs be misleading?
Fixed percent predicted cutoffs can be misleading because normal variation changes with age, height, sex, and other patient factors.

79. What does a reduced TLC with a normal or high FEV1/FVC ratio suggest?
A reduced TLC with a normal or high FEV1/FVC ratio suggests a restrictive ventilatory defect.

80. What does a reduced FEV1/FVC ratio with a reduced TLC indicate?
A reduced FEV1/FVC ratio with a reduced TLC indicates a mixed obstructive and restrictive pattern.

81. Why can severe obstruction mimic a mixed pattern on spirometry?
Severe obstruction can mimic a mixed pattern because air trapping may reduce FVC, making restriction appear possible.

82. What PFT finding is commonly associated with hyperinflation?
Increased FRC, RV, TLC, or RV/TLC may be associated with hyperinflation.

83. How does hyperinflation affect breathing mechanics?
Hyperinflation can flatten the diaphragm, increase the work of breathing, and contribute to dyspnea and exercise limitation.

84. Why is DLCO often reduced in emphysema?
DLCO is often reduced in emphysema because alveolar walls and pulmonary capillary surface area are destroyed.

85. Why is DLCO often reduced in pulmonary fibrosis?
DLCO is often reduced in pulmonary fibrosis because thickening or scarring of the alveolar-capillary membrane impairs gas transfer.

86. Why can anemia lower DLCO?
Anemia can lower DLCO because there is less hemoglobin available to bind carbon monoxide during the test.

87. Why may DLCO need correction for hemoglobin?
DLCO may need correction for hemoglobin because abnormal hemoglobin levels can make gas transfer appear falsely low or high.

88. What condition may cause a higher-than-normal DLCO?
Polycythemia may cause a higher-than-normal DLCO because increased hemoglobin can bind more carbon monoxide.

89. What is the purpose of arterial blood gas analysis in PFT evaluation?
Arterial blood gas analysis evaluates oxygenation, ventilation, and acid-base status.

90. Why can blood gases be useful even when spirometry is only mildly abnormal?
Blood gases can reveal hypoxemia, hypercapnia, or acid-base problems that may not be fully explained by spirometry alone.

91. What is the role of pulse oximetry during pulmonary function or exercise testing?
Pulse oximetry monitors oxygen saturation and helps detect resting or exertional desaturation.

92. What does exertional desaturation suggest?
Exertional desaturation suggests that gas exchange may worsen during activity, which can occur in interstitial lung disease, COPD, or pulmonary vascular disease.

93. Why are PFTs used before pulmonary rehabilitation?
PFTs are used before pulmonary rehabilitation to establish baseline lung function, assess disease severity, and help guide the rehabilitation plan.

94. Why are PFTs used before surgery?
PFTs are used before surgery to estimate pulmonary risk and identify patients who may be more likely to develop postoperative complications.

95. Why are PFTs used in occupational health?
PFTs are used in occupational health to monitor lung function in workers exposed to dusts, fumes, chemicals, or other inhaled hazards.

96. What is the role of PFTs in disability evaluation?
PFTs provide objective measurements that help quantify pulmonary impairment for disability or work-capacity assessment.

97. Why is infection control important during pulmonary function testing?
Infection control is important because patients breathe forcefully into equipment that may contact saliva, mucus, blood, or respiratory secretions.

98. What infection control practices should be used during PFT?
Hand hygiene, standard precautions, proper cleaning, disposable mouthpieces, filters when indicated, and disinfection of patient-contact equipment should be used.

99. Why must PFT equipment be calibrated regularly?
PFT equipment must be calibrated regularly to ensure accurate measurements and prevent incorrect clinical interpretation.

100. What is the most important principle when interpreting pulmonary function testing?
The most important principle is to interpret valid, high-quality results as patterns and connect them with the patient’s clinical condition.

Final Thoughts

Pulmonary function testing (PFT) is a broad set of tools used to evaluate respiratory mechanics, lung volumes, gas exchange, respiratory muscle strength, and exercise response. Spirometry is often the starting point, but complete interpretation may also require lung volumes, DLCO, blood gases, bronchodilator testing, bronchoprovocation testing, or exercise testing.

The most important patterns are obstruction, restriction, and mixed disease. However, accurate interpretation depends on test quality, reference values, patient effort, equipment accuracy, and clinical context.

For respiratory therapists, understanding PFTs is essential for assessment, treatment planning, monitoring, and safe patient care.