pulmonary function tests

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indications

• Diagnosis and classification of asthma, COPD, interstitial lung disease
• Quantify severity and track progression of known lung disease
• Pre-operative risk assessment (lung resection, cardiac surgery)
• Unexplained dyspnoea with normal imaging
• Assess respiratory muscle function in neuromuscular disease
• Disability and impairment evaluation
• Screening in occupational exposure (asbestos, silica)

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components

Test	Measures	Method
Spirometry	Airflow (FEV1, FVC, FEV1/FVC)	Forced expiration into spirometer
Lung volumes	TLC, RV, FRC	Body plethysmography (gold standard) or gas dilution
DLCO	Gas exchange efficiency	Single-breath CO uptake

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glossary of volumes and capacities

Term	Definition
FEV1	Forced expiratory volume in 1 second — the volume exhaled during the first second of the FVC manoeuvre.
FVC	Forced vital capacity — the maximum volume of air that can be forcibly exhaled after a maximal inspiration.
VC	Vital capacity — the maximum volume of air that can be exhaled (slowly) after a maximal inspiration (TLC - RV).
TLC	Total lung capacity — the total volume of air in the lungs at maximal inspiration.
RV	Residual volume — the volume of air remaining in the lungs after maximal expiration (cannot be measured by spirometry alone).
FRC	Functional residual capacity — the volume of air remaining in the lungs at the end of a normal (tidal) expiration (ERV + RV).
ERV	Expiratory reserve volume — the maximum volume of air that can be exhaled from the end of a normal expiration (FRC - RV).
IC	Inspiratory capacity — the maximum volume of air that can be inhaled from the end of a normal expiration (TLC - FRC).
DLCO	Diffusing capacity of the lung for carbon monoxide — measures gas transfer across the alveolar-capillary membrane.
KCO (DL/VA)	Transfer coefficient — DLCO normalised for alveolar volume (VA).

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systematic interpretation

1. Review the flow-volume loop — look for quality, shape, and pattern before numbers.
2. FEV1/FVC — below LLN (z-score < -1.645) = obstruction. Do not use fixed 0.70 cutoff (misclassifies older adults).
3. Bronchodilator (BD) response — positive if change in FEV1 or FVC > 10% of predicted value (ERS/ATS 2022). Replaces older > 12% and > 200 mL absolute change criterion.
4. FVC — if reduced with normal FEV1/FVC, suspect restriction (confirm with TLC) or a non-specific pattern.
5. TLC — below LLN = restriction confirmed. Cannot diagnose restriction on spirometry alone.
6. TLC elevated (z-score > +1.645) = hyperinflation. RV elevated = air trapping.
7. DLCO — contextualise with the pattern (obstructive, restrictive, or normal spirometry).

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ventilatory patterns

Pattern	FEV1/FVC	FVC	TLC	FVL Shape
Obstructive	Reduced (< LLN)	Normal or reduced	Normal or increased	Scooped expiratory limb
Restrictive	Normal or high	Reduced	Reduced (< LLN)	Small but normal shape
Mixed	Reduced	Reduced	Reduced	Scooped + small
Non-specific	Normal	Reduced	Normal	Slight late-expiratory scooping
Dysanapsis	Reduced	Normal	Normal	Normal or slightly scooped

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severity grading — by FEV1 z-score

Current standard (ERS/ATS 2022) uses z-scores to normalise for age, sex, and height:

Severity	Z-score	Approximate %Pred
Mild	-1.65 to -2.5	~60% to LLN
Moderate	-2.51 to -4.0	~40% to 60%
Severe	< -4.1	< 40%

For COPD specifically, GOLD (2024) uses post-BD FEV1 with FEV1/FVC < 0.7:

GOLD	Severity	FEV1 %Pred
1	Mild	≥ 80%
2	Moderate	50–79%
3	Severe	30–49%
4	Very severe	< 30%

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DLCO interpretation

DLCO	DL/VA (KCO)	Interpretation	Think
Low	Low	Parenchymal destruction or vascular loss	Emphysema, interstitial lung disease, pulmonary vascular disease
Low	Normal/High	Extra-parenchymal restriction or incomplete inspiration	Chest wall disease, pleural disease, neuromuscular disease, obesity
Normal/High	Normal/High	Airway disease without parenchymal destruction	Asthma (DLCO can be supranormal)
Low	Normal	Reduced alveolar-capillary surface area	Anaemia, pulmonary hypertension, early ILD

Factors that reduce DLCO: emphysema, ILD/fibrosis, pulmonary vascular disease, anaemia, carboxyhaemoglobin (recent smoking), low inspired volume.

Factors that increase DLCO: asthma (hyperinflated capillary bed), pulmonary haemorrhage (Goodpasture), polycythaemia, left-to-right shunt, obesity (increased pulmonary blood volume).

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flow-volume loop patterns

• Normal — rapid rise to peak expiratory flow (PEF), smooth descent; inspiratory limb is a symmetric curve.
• Obstructive (scooped) — concave expiratory limb with reduced PEF. Severity correlates with degree of scooping.
• Fixed upper airway obstruction — flattening of both inspiratory and expiratory limbs (plateau). Think: tracheal stenosis, goitre.
• Variable extrathoracic obstruction — flattened inspiratory limb only. Think: vocal cord paralysis, tracheomalacia.
• Variable intrathoracic obstruction — flattened expiratory limb only. Think: intrathoracic tracheal tumour.
• Restrictive — normal shape but small volume (reduced FVC with preserved peak flow relative to volume).

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bronchodilator response

Current standard (ERS/ATS 2022):

$\text{BD response} = \frac{(\text{post-BD value} - \text{pre-BD value})}{\text{predicted value}} \times 100$

• Positive: > 10% of predicted for FEV1 or FVC.
• The new method normalises for sex and height differences, reducing misclassification compared to the old > 12% and > 200 mL absolute change criterion.
• A positive BD response supports asthma but does not exclude COPD.

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obstructive patterns — asthma vs COPD

Feature	Asthma	COPD
BD response	Often significant	Often absent or partial
Hyperinflation	Present during exacerbation	Persistent
Air trapping (RV)	Variable	Persistent
DLCO	Normal or increased	Reduced (emphysema)
Flow-volume loop	Scooping, improves post-BD	Scooping, minimal change

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small airways disease

Small airways (<2 mm diameter) contribute minimally to total airway resistance but are the primary site of airflow limitation in COPD and early asthma. On PFTs, small airways dysfunction may manifest as isolated air trapping — FRC elevated (z-score > +1.645) — due to premature airway closure and air trapping (ERS/ATS 2022). Mid-expiratory flow measurements (FEF25-75%) are highly variable, poorly reproducible, and not recommended for diagnosing small airways dysfunction (ERS/ATS 2022). Alternative tests such as oscillometry, multiple-breath nitrogen washout, and imaging may provide more sensitive detection when FEV1/FVC remains normal (ERS/ATS 2022; Toumpanakis, Chest. 2026).

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gas trapping and hyperinflation

Gas trapping reflects incomplete lung emptying due to airway obstruction and is distinct from hyperinflation (increased absolute lung volumes) (ERS/ATS 2022; Smith, Chest. 2014). RV or RV/TLC above the upper limit of normal (z-score > +1.645) indicates air trapping (ERS/ATS 2022). With disease progression, hyperinflation develops with increases in FRC, FRC/TLC, and often TLC (ERS/ATS 2022). An increased FRC/TLC (reduced IC/TLC) is a hallmark of COPD and correlates closely with reduced exercise tolerance and dyspnoea (ERS/ATS 2022).

Gas trapping is associated with smaller airway lumen diameters, greater dyspnoea, and chronic bronchitis, whereas hyperinflation correlates with emphysema on CT (Smith, Chest. 2014). Note that RV/TLC may also be elevated in neuromuscular weakness, suboptimal effort, or restrictive processes when TLC is reduced proportionally more than RV (ERS/ATS 2022).

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neuromuscular disease assessment

Spirometry in suspected diaphragmatic weakness includes sitting/supine comparison:

• Normal drop in FVC from sitting to supine: 5–10%.
• Drop > 20% is significant for diaphragmatic weakness (e.g. ALS).
• Mechanism: In the supine position, abdominal contents shift cranially; a weak diaphragm cannot resist this pressure or effectively descend, leading to a marked reduction in vital capacity.

Additional measurements:

• Peak cough flow (PCF): < 160 L/min = ineffective cough; 160–270 L/min = at risk for respiratory tract infections.
• Maximal inspiratory/expiratory pressures (MIP/MEP): direct assessment of respiratory muscle strength.
• Sniff nasal inspiratory pressure (SNIP): alternative measure of diaphragm strength.

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quality considerations

• Acceptability criteria: good start of test (no hesitation), maximal effort, no cough in first second, adequate exhalation time (FET ≥ 6 seconds).
• Repeatability: largest two FVC and FEV1 values within 150 mL of each other.
• DLCO: inspiratory volume must be ≥ 90% of VC; if not, results may underestimate true diffusing capacity.
• Review all FVLs: a single loop may be misleading. Persistent abnormalities across all loops are more meaningful than isolated findings.