Pulmonary Function Tests in Children: A Comprehensive Guide to Interpretation and Common Pediatric Diseases

Pulmonary function tests (PFTs) are an important part of evaluating respiratory disease in children. They help clinicians identify airflow obstruction, abnormal lung volumes, air trapping, impaired gas transfer, ventilation inhomogeneity, and exercise limitation.

However, pediatric PFT interpretation is not simply a matter of looking at whether a value is “above or below 80% predicted.” Children are continuously growing, lung volumes change with age and body size, and the ability to perform a technically acceptable test depends strongly on developmental stage.

For postgraduate pediatric examinations and clinical practice, the most useful approach is to understand what each PFT measures, which pattern it produces, and which diseases commonly produce that pattern.

Modern interpretation relies on appropriate reference equations and the lower limit of normal (LLN) rather than applying adult fixed cutoffs to children. The 2022 ERS/ATS interpretive standard also emphasizes interpreting results in the context of test quality and clinical information.

What Are Pulmonary Function Tests?

Pulmonary function tests are a group of investigations used to assess different aspects of respiratory physiology.

Depending on the child’s age, clinical condition, and ability to cooperate, testing may include:

Spirometry
Bronchodilator responsiveness testing
Lung-volume measurement
Diffusing capacity for carbon monoxide (DLCO/TLCO)
Respiratory oscillometry
Multiple-breath washout (MBW)
Bronchial challenge testing
Fractional exhaled nitric oxide (FeNO)
Exercise testing and cardiopulmonary exercise testing (CPET)
Specialized infant pulmonary function testing

No single test answers every clinical question.

For example:

Spirometry → airflow

Lung volumes → restriction and hyperinflation

DLCO → gas transfer

Oscillometry → respiratory system mechanics during tidal breathing

MBW/LCI → ventilation inhomogeneity

FeNO → type-2/eosinophilic airway inflammation

CPET → exercise limitation

Difference of Pulmonary Function Tests in Children

Children are not simply small adults.

Lung function changes substantially with:

Age
Height
Growth
Pubertal development
Sex
Body composition
Reference population

Therefore, pediatric results should be interpreted using an appropriate reference population and laboratory standards.

The correct pediatric approach

For children, pulmonary function results are best interpreted using:

Age- and size-appropriate reference equations

LLN and z-scores

Quality-assured spirometry

Measured lung volumes when restriction is suspected

RV and RV/TLC when air trapping is being assessed

A practical framework is:

Finding	Pediatric interpretation
FEV₁/FVC below LLN	Suggests airflow obstruction
FVC below LLN	Indicates reduced forced vital capacity and requires further interpretation
TLC below LLN	Confirms a restrictive ventilatory defect
RV or RV/TLC increased	Supports air trapping
FEV₁/FVC below LLN + TLC below LLN	Mixed obstructive and restrictive physiology
DLCO below expected range	Suggests impaired gas transfer, after considering technical and hemoglobin-related factors

The key principle is that pediatric PFTs should be interpreted against appropriate pediatric reference standards rather than a single fixed adult cutoff.

1.Spirometry

Spirometry is the most commonly used pulmonary function test.

It measures how much air a child can move and how quickly that air can be exhaled.

The major measurements include:

FVC – forced vital capacity
FEV₁ – forced expiratory volume in the first second
FEV₁/FVC – ratio of forced expiratory volume in one second to forced vital capacity
PEF – peak expiratory flow
Other forced expiratory flows

The 2019 ATS/ERS spirometry standard emphasizes standardized equipment, patient preparation, maneuver quality, acceptability and repeatability.

FEV₁

FEV₁ is the volume exhaled during the first second of a forced expiration.

It is particularly useful for identifying and quantifying airflow limitation.

A reduced FEV₁ may occur in:

Obstructive disease
Restrictive disease
Mixed disease
Poor-quality or submaximal effort

Therefore, FEV₁ should never be interpreted in isolation.

FVC

FVC is the total volume forcibly exhaled after a full inspiration.

A reduced FVC can occur with:

True restriction
Air trapping
Incomplete expiration
Poor effort
Early termination
Severe airflow obstruction

Therefore, a reduced FVC should prompt consideration of the complete physiological pattern.

When restriction is suspected, TLC is the measurement used to establish whether restriction is actually present.

FEV₁/FVC

The FEV₁/FVC ratio is particularly useful for identifying airflow obstruction.

A value below the age-appropriate LLN suggests an obstructive ventilatory defect.

The LLN is generally related to the lower fifth percentile of the reference population, corresponding approximately to a z-score of −1.645 when the reference model is appropriately constructed.

Thus, in children:

Use the age-appropriate LLN to identify airflow obstruction.

The Flow-Volume Loop

The flow-volume loop provides visual information about expiratory and inspiratory airflow.

Obstructive pattern

Typically:

Reduced expiratory flow
Concave or “scooped” expiratory limb
Reduced FEV₁/FVC

Common examples include:

Asthma
Cystic fibrosis
Bronchiectasis
Bronchiolitis obliterans

Restrictive pattern

A restrictive ventilatory defect generally produces:

Reduced lung volumes
Relatively preserved or increased FEV₁/FVC
A smaller but proportionally shaped loop

However, spirometry alone cannot reliably confirm restriction.

Upper-airway obstruction

Characteristic flattening may occur depending on whether obstruction is:

Variable extrathoracic
Variable intrathoracic
Fixed

This is an important additional use of the flow-volume loop.

Pediatric Spirometry: Always Check Test Quality First

Before interpreting numbers, ask:

1. Was the maneuver acceptable?

Consider:

Good start of test
Adequate inspiration
Rapid maximal expiration
No cough during critical portions
No glottic closure
No leak
No early termination

2. Are the maneuvers repeatable?

The best results should be supported by appropriately reproducible maneuvers according to current technical standards.

3. Is the child able to perform the test?

Younger children may require extensive coaching.

Importantly, preschool children are not automatically unable to perform spirometry. Many can produce useful measurements with appropriate coaching, although feasibility and quality vary considerably.

Spirometry by Age

Infants

Routine conventional spirometry is generally not feasible in an awake infant.

Specialized infant pulmonary function laboratories can perform techniques such as:

Tidal breathing analysis
Infant forced expiratory maneuvers
Respiratory mechanics
Infant plethysmography

These techniques require specialized equipment and expertise.

Preschool Children

Some preschool children can perform acceptable spirometry, particularly with experienced coaching.

When forced maneuvers are difficult or unreliable, other techniques may be useful, including:

Oscillometry
Multiple-breath washout
Other specialized preschool lung-function techniques

Therefore, the appropriate statement is:

Spirometry can be performed successfully in many preschool children, while oscillometry and other tidal-breathing techniques provide useful alternatives when forced maneuvers are difficult.

School-Aged Children and Adolescents

Standard spirometry is generally feasible in most cooperative school-aged children and adolescents.

At this age, interpretation should emphasize:

Test quality
Reference equations
FEV₁/FVC
FEV₁
FVC
Bronchodilator response when indicated
Flow-volume loop morphology

A Practical Approach to Spirometry Interpretation

A useful postgraduate approach is:

Step 1: Check test quality

Confirm that the results are technically reliable.

Step 2: Look at FEV₁/FVC

Below LLN → suggests obstruction
Within the expected range → obstruction is not demonstrated by the ratio

Step 3: Assess FVC

Within expected range → obstruction may be isolated
Reduced → consider the complete pattern, including restriction, air trapping and test quality

Step 4: If restriction is suspected, measure TLC

Reduced TLC → confirms a restrictive ventilatory defect.

Step 5: Examine bronchodilator response when clinically indicated

A meaningful response may support variable airflow limitation, but absence of a response on one occasion does not exclude asthma.

Step 6: Integrate the findings

PFTs describe physiological abnormalities; the final diagnosis requires clinical correlation.

Obstructive Ventilatory Defect

The typical pattern is:

FEV₁/FVC below LLN
FEV₁ may be reduced
FVC may be normal or reduced
Expiratory limb may show concavity

Common pediatric causes include:

Asthma
Cystic fibrosis
Bronchiectasis
Bronchiolitis obliterans
Primary ciliary dyskinesia
Chronic airway disease after severe early-life lung injury

Important exam point

A reduced FEV₁/FVC identifies airflow obstruction, not a specific disease.

Clinical history is required to determine the cause.

Restrictive Ventilatory Defect

A restrictive defect means that the total amount of air contained in the lungs is reduced.

The key physiological finding is:

Reduced TLC

Spirometry may suggest restriction when:

FVC is reduced
FEV₁ is reduced proportionally
FEV₁/FVC is preserved or increased

But:

Confirmation comes from demonstrating reduced TLC.

Potential causes include:

Interstitial lung disease
Chest-wall disorders
Severe neuromuscular weakness
Severe obesity
Pleural disease
Certain developmental lung disorders

Mixed Ventilatory Defect

A mixed defect combines:

Obstruction → low FEV₁/FVC
Restriction → low TLC

Therefore, a genuinely mixed pattern requires evidence of both abnormalities.

A low FEV₁/FVC combined with low FVC on spirometry should be further evaluated with lung-volume measurement to determine whether true restriction is present.

This distinction is particularly important in examination questions.

Air Trapping and Hyperinflation

Lung-volume testing can reveal abnormalities that spirometry may not fully demonstrate.

Important measurements include:

RV – residual volume
FRC – functional residual capacity
TLC – total lung capacity

Air trapping

Usually supported by increased:

RV/TLC

Hyperinflation

May be associated with an increased:

TLC

and/or increased FRC, depending on the underlying physiology.

Air trapping is particularly relevant in:

Asthma
Bronchiolitis obliterans
Cystic fibrosis
Bronchiectasis
Small-airway disease

Bronchodilator Responsiveness

Bronchodilator testing assesses whether airflow changes after administration of a short-acting bronchodilator.

The modern ERS/ATS interpretive approach uses change relative to the predicted value rather than relying on the older adult criterion of “>12% and >200 mL.”

The 2022 ERS/ATS approach defines a significant bronchodilator response as an increase of more than 10% of the predicted value in FEV₁ or FVC.

The result must still be interpreted in clinical context.

Important point

A negative bronchodilator response does not by itself exclude asthma.

Similarly, a positive response does not independently establish asthma.

2. Lung Volumes

Spirometry cannot measure all lung volumes.

Important lung-volume measurements include:

Total lung capacity (TLC)

The total amount of air in the lungs after maximal inspiration.

Reduced TLC → confirms restriction.

Residual volume (RV)

The air remaining in the lungs after maximal expiration.

Increased RV → supports air trapping.

Functional residual capacity (FRC)

The volume remaining in the lungs at the end of normal expiration.

It may increase with hyperinflation and decrease in conditions that reduce resting lung volume.

RV/TLC

An increased RV/TLC ratio supports air trapping.

How Lung Volumes Help Solve Difficult Spirometry

Consider a child with:

Low FEV₁
Low FVC
Low FEV₁/FVC

You might initially think:

Mixed obstructive and restrictive disease.

But lung-volume testing could show:

TLC normal
RV increased

This suggests obstruction with air trapping rather than true restriction.

Therefore:

TLC is the key measurement for confirming restriction.

3. Diffusing Capacity: DLCO/TLCO

DLCO assesses the transfer of gas from the alveoli into pulmonary capillary blood.

It depends on several factors, including:

Alveolar-capillary surface area
Membrane characteristics
Pulmonary capillary blood volume
Hemoglobin concentration

A reduced DLCO may be seen in selected:

Interstitial lung diseases
Pulmonary vascular diseases
Emphysematous disorders
Conditions associated with anemia

DLCO should be interpreted with attention to hemoglobin and technical factors.

Pediatric exam pearl

If a child has:

Restriction + reduced DLCO

think particularly about intrinsic/parenchymal lung disease.

If there is:

Restriction + relatively preserved DLCO

consider extraparenchymal causes such as:

Chest-wall restriction
Neuromuscular weakness
Some obesity-related patterns

This is a useful pattern-recognition approach, but individual diseases may vary.

4. Respiratory Oscillometry

Oscillometry measures respiratory system mechanics during normal tidal breathing.

It is especially useful when a child cannot reliably perform forced expiratory maneuvers.

Parameters can include:

Resistance
Reactance
Frequency dependence
Area under the reactance curve

Oscillometry can be particularly useful in:

Preschool children
Asthma
Cystic fibrosis
Other chronic airway diseases

Why is it useful in young children?

The child generally breathes normally through the device rather than performing repeated maximal forced expirations.

Therefore, it can complement—not necessarily replace—spirometry.

5. Multiple-Breath Washout and Lung Clearance Index

Multiple-breath washout (MBW) assesses how evenly ventilation is distributed throughout the lungs.

One important outcome is:

Lung clearance index (LCI)

An elevated LCI indicates increased ventilation inhomogeneity.

MBW can detect abnormalities that may occur before conventional spirometry becomes clearly abnormal.

It has particular clinical and research importance in:

Cystic fibrosis
Small-airway disease
Preschool children

The ATS preschool MBW technical statement specifically addresses the methodological challenges of performing MBW in young children.

Important distinction

MBW does not directly measure airway obstruction in the same way as FEV₁/FVC.

Instead, it assesses ventilation distribution and inhomogeneity.

6. Fractional Exhaled Nitric Oxide (FeNO)

FeNO is a marker associated with type-2/eosinophilic airway inflammation.

It can support the assessment of asthma, particularly when eosinophilic inflammation is suspected.

However:

FeNO is not a stand-alone diagnostic test for asthma.

FeNO may be influenced by:

Atopy
Allergic rhinitis
Inhaled corticosteroid treatment
Age
Smoking exposure
Other inflammatory conditions

Therefore, FeNO should be interpreted alongside symptoms, spirometry and the overall clinical picture.

7. Bronchial Challenge Testing

Bronchial challenge tests evaluate airway hyperresponsiveness.

Depending on the laboratory and clinical question, challenges may involve:

Exercise
Methacholine
Other standardized stimuli

They can be useful when asthma is suspected but baseline spirometry is nondiagnostic.

However, a challenge test should be selected and interpreted according to the clinical context and laboratory standards.

8. Exercise Testing and CPET

Some children have significant exercise intolerance despite relatively normal resting spirometry.

Cardiopulmonary exercise testing (CPET) evaluates the integrated response of:

Respiratory system
Cardiovascular system
Muscles
Gas exchange
Exercise capacity

It can help investigate:

Unexplained exertional dyspnea
Exercise limitation in chronic lung disease
Congenital heart disease
Obesity
Dysfunctional breathing
Deconditioning

Pulmonary Function Tests in Children ( Disease-Wise Interpretation)

The following patterns are useful for postgraduate examinations, but they should be regarded as typical patterns rather than absolute diagnostic rules.

Asthma

Possible findings:

Normal spirometry between episodes
Reduced FEV₁/FVC during airflow obstruction
Reduced FEV₁
Bronchodilator responsiveness in some patients
Air trapping or hyperinflation in more persistent disease
Increased FeNO in some patients with type-2 inflammation

Key point

Normal spirometry does not automatically exclude asthma, particularly when symptoms are intermittent.

Cystic Fibrosis

Possible findings include:

Obstructive physiology
Reduced FEV₁
Air trapping
Hyperinflation
Progressive decline in lung function
Ventilation inhomogeneity on MBW

LCI/MBW may provide additional information, particularly in younger children.

Exam pearl

In CF:

A normal FEV₁ does not necessarily mean completely normal peripheral airway function.

This is one reason MBW and LCI have been studied extensively in pediatric CF.

Bronchiectasis

Possible findings:

Obstructive physiology
Reduced FEV₁
Air trapping
Sometimes mixed abnormalities

The pattern depends on disease severity, extent and underlying cause.

Possible underlying conditions include:

Cystic fibrosis
Primary ciliary dyskinesia
Immunodeficiency
Previous severe infection
Other chronic suppurative lung diseases

Bronchiolitis Obliterans

A typical pattern is:

Persistent airflow obstruction
Reduced FEV₁/FVC
Reduced FEV₁
Significant air trapping
Possible hyperinflation

The obstruction is generally poorly reversible.

This pattern is particularly important after severe lower respiratory tract injury or hematopoietic stem-cell transplantation.

Primary Ciliary Dyskinesia

PFT abnormalities may include:

Airflow obstruction
Reduced FEV₁
Air trapping
Progressive decline in lung function

However, PFTs are not diagnostic of PCD.

Diagnosis requires integration of clinical phenotype with specialized diagnostic testing.

Post-Prematurity Respiratory Disease

Children and adolescents born very preterm may demonstrate heterogeneous lung-function abnormalities.

Possible findings include:

Airflow obstruction
Reduced lung volumes in some patients
Air trapping
Abnormal respiratory mechanics
Exercise limitation

The phenotype varies according to gestational age, neonatal lung disease, oxygen exposure, mechanical ventilation and subsequent development.

Childhood Interstitial Lung Disease

PFTs may show:

Reduced FVC
Reduced TLC
Restrictive physiology
Reduced DLCO in some disorders

A restrictive pattern with impaired gas transfer can support the presence of significant parenchymal lung disease.

However, PFTs should be interpreted with imaging, clinical findings and other investigations.

Neuromuscular Disease

Respiratory muscle weakness commonly produces:

Reduced FVC
Restrictive physiology
Reduced TLC
Difficulty generating adequate expiratory force

Additional assessment may include:

Maximal inspiratory pressure
Maximal expiratory pressure
Peak cough flow
Supine versus upright FVC
Sleep-related respiratory assessment when indicated

An increased RV may occur when expiratory muscle weakness impairs effective emptying.

Obesity

Obesity can alter respiratory mechanics.

Common findings include:

Reduced ERV
Reduced FRC
Sometimes reduced TLC
Increased airway resistance in some patients

Importantly:

Reduced FRC or ERV does not by itself prove restrictive lung disease.

True restriction requires a reduced TLC.

A 7-Step Pediatric PFT Interpretation Algorithm

When faced with a PFT question in an examination, use the following sequence:

Step 1 — Confirm test quality

Make sure the study is technically reliable.

Step 2 — Assess FEV₁/FVC

Below LLN → airflow obstruction is suggested.

Step 3 — Assess FVC

Determine whether FVC is within the expected range or reduced, then consider the complete physiological pattern.

Step 4 — Check TLC when restriction is suspected

Low TLC → restriction confirmed.

Step 5 — Check RV and RV/TLC

Increased RV or RV/TLC → supports air trapping.

Step 6 — Consider bronchodilator response

A significant response may support variable airflow limitation.

Step 7 — Integrate with the clinical picture

Ask:

What is the child’s age?
What are the symptoms?
Is disease episodic or persistent?
Is there atopy?
Is there chronic infection?
Was the child born prematurely?
Is there neuromuscular weakness?
Are imaging findings available?

High-Yield Pediatric PFT Examination Points

Point 1

Use age-appropriate reference equations and LLN when interpreting pediatric spirometry.

Point 2

Use TLC to establish restriction.

Point 3

Mixed physiology = obstruction plus confirmed restriction.

That means:

Low FEV₁/FVC + low TLC

Point 4

Use RV and RV/TLC to recognize air trapping.

Point 5

Normal spirometry does not exclude asthma.

Asthma is characterized by variable respiratory symptoms and variable airflow limitation.

Point 6

Interpret FEF25–75 cautiously and never use it alone to diagnose small-airway disease.

Point 7

Preschool children can sometimes perform useful spirometry.

Point 8

Oscillometry is particularly useful when forced maneuvers are difficult.

Point 9

MBW/LCI assesses ventilation inhomogeneity.

Point 10

PFTs describe physiology, not the diagnosis by themselves.

Always integrate PFT findings with clinical history, examination and other investigations.

The Simplest Way to Remember Pediatric PFTs

Think of pulmonary function testing as answering five questions:

1. Is airflow obstructed?

Look at:

FEV₁/FVC

2. Are the lungs too small?

Look at:

TLC

3. Is air trapped?

Look at:

RV and RV/TLC

4. Is gas transfer impaired?

Look at:

DLCO/TLCO

5. Is ventilation uneven?

Think about:

MBW and LCI

This framework makes complicated PFT reports much easier to understand.

Final Clinical Summary

Pediatric PFT interpretation should begin with test quality and appropriate reference equations, followed by assessment of airflow, lung volumes and gas transfer where available.

The core pattern recognition is:

Low FEV₁/FVC → obstruction

Low TLC → restriction

Low FEV₁/FVC + low TLC → mixed physiology

High RV/RV/TLC → air trapping

Low DLCO → impaired gas transfer

High LCI → ventilation inhomogeneity

The most important principle is that these patterns are physiological descriptions, not diagnoses. Asthma, cystic fibrosis, bronchiectasis, bronchiolitis obliterans, primary ciliary dyskinesia, childhood interstitial lung disease, neuromuscular disorders and obesity can produce overlapping patterns.

For postgraduate pediatric examinations, the safest strategy is therefore to interpret the PFT systematically rather than trying to memorize one fixed pattern for every disease.

References

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Beydon N, Davis SD, Lombardi E, et al. An official American Thoracic Society/European Respiratory Society statement: pulmonary function testing in preschool children. Am J Respir Crit Care Med. 2007;175:1304-1345.
Robinson PD, Latzin P, Ramsey KA, et al. Preschool Multiple-Breath Washout Testing. An Official American Thoracic Society Technical Statement. Am J Respir Crit Care Med. 2018;197(5).
Culver BH, Graham BL, Coates AL, et al. Recommendations for a Standardized Pulmonary Function Report. An Official American Thoracic Society Technical Statement. Am J Respir Crit Care Med. 2017;196(11):1463-1472.
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