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Spirometry measures how much air you can blow out and how fast. It answers a lot, but not everything. Complex lung function testing is the set of extra measurements done in a lung function laboratory: how much air stays in your lungs after you breathe all the way out, how well oxygen crosses from your lungs into your blood, how much resistance your airways offer, and how strong your breathing muscles are. Some of it involves sitting in a clear cabin about the size of a phone box and panting gently; some involves a single deep breath of a harmless test gas held for about ten seconds. The full set takes 45 minutes to an hour and involves several maximal efforts, so it is tiring. Together these tests explain why you are breathless, not just how much.
What is meant by "complex" lung function
Spirometry measures the air a person can move — how much and how fast. It is the right first test, and for many people it is the only one needed. But spirometry cannot measure the air that stays behind after a full breath out, it cannot say how well gas crosses into the blood, and it cannot separate a small lung from a stiff chest wall or a weak muscle.
Complex lung function testing is the group of measurements that answer those questions: static lung volumes, gas transfer, airway resistance, respiratory muscle pressures and bronchial provocation. Together with spirometry they make up what most services call a full lung function test, usually performed in a single 45–60 minute appointment.
Static lung volumes
The volumes spirometry cannot reach are those below a full expiration. The key one is residual volume (RV) — the air remaining in the lungs after breathing all the way out — and from it functional residual capacity (FRC) and total lung capacity (TLC) are derived.
| Method | How it works | Notes |
|---|---|---|
| Body plethysmography (the "box") | The person sits in a sealed cabin and pants against a closed shutter; pressure and volume changes give thoracic gas volume | The reference standard. Measures all intrathoracic gas, including trapped gas behind closed airways |
| Gas dilution (helium dilution or nitrogen washout) | An inert gas equilibrates with the lung and the dilution gives the volume | Only measures communicating gas — underestimates volume where there is significant air trapping |
| Imaging-derived | CT-based volume estimation | Research and surgical planning rather than routine clinical measurement |
The difference between the two methods is itself informative: a plethysmographic TLC substantially larger than a dilution TLC indicates trapped, non-communicating gas, typical of emphysema and severe airflow obstruction.
What the volumes tell you. A raised RV and RV/TLC ratio means gas trapping; a raised TLC means hyperinflation, seen in emphysema and severe asthma. A reduced TLC is the finding that actually defines a restrictive process — spirometry can only suggest restriction, never confirm it.
Gas transfer (DLCO / TLCO)
Gas transfer measures how readily carbon monoxide crosses from alveolar gas into the blood, as a proxy for the whole gas-exchanging surface. The person takes a single deep breath of a test mixture, holds it for about ten seconds, and exhales.
| Pattern | Suggests |
|---|---|
| Reduced DLCO with obstruction | Emphysema — alveolar destruction. Distinguishes it from chronic bronchitis and asthma, where transfer is usually preserved |
| Reduced DLCO with restriction | Interstitial lung disease — see IPF and ILD. Often the earliest abnormality, and the most responsive to change |
| Isolated reduction, normal volumes and flows | Pulmonary vascular disease — pulmonary hypertension, chronic thromboembolic disease, anaemia |
| Raised DLCO | Alveolar haemorrhage, polycythaemia, left-to-right shunt, asthma, obesity |
| Reduced DLCO with preserved KCO | Loss of lung units with normal remaining lung — e.g. after resection |
DLCO must be corrected for haemoglobin, and is affected by carboxyhaemoglobin — recent smoking lowers the result, so smoking status on the day matters. KCO (DLCO per unit alveolar volume) helps separate loss of lung units from disease of the remaining units.
Airway resistance and oscillometry
Body plethysmography also yields airway resistance (Raw) and its reciprocal, specific conductance. Oscillometry (forced oscillation technique) is a newer, entirely passive alternative: small pressure oscillations are applied during tidal breathing and the resulting impedance is measured. It requires no effort, works in children and the very breathless, and is sensitive to small-airway change — but reference values and clinical thresholds are still consolidating.
Respiratory muscle strength
Where weakness is suspected — neuromuscular disease, unexplained restriction, unexplained breathlessness — muscle pressures are measured directly:
- Maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) — simple mouth-pressure manoeuvres, effort-dependent and technique-sensitive.
- Sniff nasal inspiratory pressure (SNIP) — a natural manoeuvre, often more reliable than MIP where mouth seal is poor.
- Supine vital capacity — not a pressure measurement but the most useful bedside test of diaphragm function. A fall of more than about 20% from sitting to supine points to diaphragmatic weakness.
- Oesophageal and transdiaphragmatic pressures — invasive, and reserved for specialist assessment.
These are the measurements that drive decisions in motor neurone disease and other neuromuscular conditions — see assisted cough for how they combine with peak cough flow.
Bronchial provocation testing
Where asthma is suspected but spirometry and bronchodilator response are normal, provocation testing looks for airway hyper-responsiveness.
| Type | Agent | What it shows |
|---|---|---|
| Direct | Methacholine, histamine | Acts on airway smooth muscle. Highly sensitive — a negative test is good evidence against current asthma |
| Indirect | Mannitol, hypertonic saline, eucapnic voluntary hyperpnoea, exercise | Acts via mediator release. More specific for active airway inflammation and for exercise-induced bronchoconstriction |
Indirect challenge is generally the more useful test in athletes and in suspected exercise-induced bronchoconstriction. Testing requires medication withholding, and carries a small risk of significant bronchoconstriction — it is done in supervised settings with reversal available.
Putting a full report together
A structured read of a full lung function report follows the same order every time:
- Is the test valid? Acceptability and repeatability criteria, effort, number of manoeuvres.
- Is there obstruction? FEV1/FVC below the lower limit of normal.
- Is there restriction? Reduced TLC — not a reduced FVC, which is unreliable on its own.
- Is there gas trapping or hyperinflation? RV, RV/TLC, and any plethysmography–dilution discrepancy.
- Is gas transfer impaired? DLCO and KCO, corrected for haemoglobin.
- Is there a bronchodilator response? And does it match the clinical picture?
- Is there muscle weakness? MIP/MEP/SNIP, and supine versus seated vital capacity.
- How does it compare with last time? Trend usually matters more than a single absolute value.
Results are reported against reference equations — current practice uses the GLI (Global Lung Function Initiative) equations with z-scores rather than percent predicted, because percent predicted misclassifies at the extremes of age and height.
Preparing for the test
Withhold bronchodilators as instructed by the service — typically short-acting for 4–6 hours and long-acting for 12–36 hours depending on the agent and the question being asked. Avoid smoking on the day (it alters gas transfer), avoid a heavy meal and vigorous exercise beforehand, and wear clothing that does not restrict the chest. Bring a list of current medicines. The full battery takes 45–60 minutes and involves repeated maximal efforts, which is genuinely tiring — and worth warning people about, because someone who is not expecting it may under-perform on the later manoeuvres.
Role of the physiotherapist
Physiotherapists are among the heaviest users of lung function reports and, in many Australian services, perform spirometry themselves. Four practical contributions.
Reading beyond the FEV1. Most treatment reasoning stops at the flow rates. The volumes and gas transfer explain the symptom: hyperinflation, not airflow, is what limits exercise in emphysema, and a low DLCO with normal spirometry points toward pulmonary vascular disease that no amount of airway clearance will help.
Recognising when the pattern does not fit. A reduced FVC with a normal TLC is not restriction — it is submaximal effort, obesity or gas trapping. Chasing "restrictive lung disease" that the volumes do not support is a common and avoidable error.
Using muscle measurements properly. MIP, SNIP and supine vital capacity are the numbers that determine when to introduce cough augmentation and non-invasive ventilation. Tracking them over time in neuromuscular disease is squarely physiotherapy business.
Setting exercise prescription from physiology. Severe gas trapping calls for attention to breathing pattern, pacing and expiratory time. Impaired gas transfer raises the likelihood of exertional desaturation, and should prompt an oxygen assessment rather than an assumption. Where the report cannot answer the question, CPET usually can.
Evidence summary
Framing. Full lung function testing is where physiological reasoning either happens or does not. The commonest failure in cardiorespiratory practice is not misreading a report but never opening it past the first three numbers — treating FEV1, FVC and their ratio as the whole picture, when volumes and gas transfer are what distinguish diseases that share a spirometric pattern and demand different treatment.
Standards and interpretationThe ERS/ATS technical standards define acceptability, repeatability and reporting for spirometry, static volumes and gas transfer, and the 2022 interpretive strategies statement made two changes that matter clinically: reporting in z-scores against GLI reference equations rather than percent predicted, and abandoning fixed severity cut-points based on percent predicted FEV1.1,2 Percent predicted systematically misclassifies at the extremes of age, height and sex; z-scores do not. The same statements formalise that restriction cannot be diagnosed from spirometry — a reduced FVC with a preserved ratio has a differential that includes obesity, gas trapping, submaximal effort and neuromuscular weakness, and only a reduced TLC settles it.2,3 The 2022 removal of race-specific reference equations in favour of race-neutral GLI equations is a substantive change — but it is a change in international recommendation, not yet in Australian primary-care practice, and the two should not be confused. The ATS 2023 statement recommends the race-neutral GLI Global average equation in place of race and ethnicity-specific equations.4 TSANZ SPC01 still specifies GLI 2012, with the “GLI Other” reference category for Aboriginal and Torres Strait Islander patients unless the GLI Global dataset is in use. GLI Other is not the race-neutral equation — GLI Global is a weighted average across the GLI categories, GLI Other is one of those categories. Read every report against the equation set it names, and see what changed in lung function standards for the Australian position.
Gas transfer as the discriminating measurementDLCO frequently carries more diagnostic information than the flows. In obstructive disease it separates emphysema, where alveolar destruction reduces transfer, from asthma and chronic bronchitis, where it is preserved — a distinction that changes both prognosis and the plausibility of airway clearance as a treatment. In restrictive disease it separates parenchymal from extrapulmonary causes. An isolated reduction with normal volumes and flows is the classic signature of pulmonary vascular disease, and is the finding most often overlooked in a patient labelled as deconditioned.5 Standardisation of the single-breath measurement, including haemoglobin and carboxyhaemoglobin correction, is set out in the ERS/ATS technical standard.5
Respiratory muscle assessmentThe ERS statement on respiratory muscle testing at rest and during exercise is the current reference for MIP, MEP, SNIP and invasive pressures, superseding the 2002 ATS/ERS statement, and it emphasises that non-invasive mouth pressures are effort-dependent with wide normal ranges, so a low value warrants confirmation rather than immediate action.6 Supine versus seated vital capacity is the most accessible index of diaphragm function, and a postural fall exceeding roughly 20% is the practical trigger for further assessment. In neuromuscular disease these measurements, alongside peak cough flow, drive the timing of cough augmentation and non-invasive ventilation, and are typically tracked by the physiotherapy service.
Physiotherapy implications- Read the volumes before forming a plan. Hyperinflation and gas trapping explain exertional limitation that FEV1 does not, and they are what breathing-pattern and pacing work actually target.
- Never accept "restrictive pattern" from spirometry alone — check TLC, or say the pattern is unconfirmed.
- A plethysmography–dilution TLC discrepancy is a free measure of trapped gas; look for it when both are reported.
- Treat an isolated low DLCO as a prompt to consider pulmonary vascular disease and to assess for exertional desaturation before prescribing.
- Track MIP/SNIP and supine vital capacity in neuromuscular disease at every review — they are the numbers that determine when equipment is needed.
- Check the withholding instructions before a test you have influenced. A patient who took their LABA that morning may generate a report that misleads for months.
Work from the pattern to the mechanism, not from the label to the treatment. Two patients with an identical FEV1 of 45% predicted — one with a raised RV/TLC and a DLCO of 40%, the other with normal volumes and preserved transfer — have different diseases, different limiting factors and different prescriptions. And when the report is normal but the patient is symptomatic, that is a finding: it moves the question toward breathing pattern disorder, laryngeal obstruction, deconditioning or pulmonary vascular disease, all of which need a different test rather than a louder repetition of the same one.
Evidence gapsThe clinical consequences of the shift to race-neutral GLI equations — on diagnosis rates, transplant listing and disability assessment — are still being worked through. Oscillometry lacks agreed clinical thresholds and outcome-linked evidence despite obvious practical appeal. Minimal clinically important differences for DLCO and for static volumes are poorly defined, so interpreting change in an individual remains partly subjective. Reference values for respiratory muscle pressures are old, small and inconsistent between studies. And there is essentially no trial evidence that any physiotherapy intervention is better targeted by full lung function than by spirometry plus clinical assessment — the case rests on mechanism, not on outcome data.
References & evidence base
- Graham BL, Steenbruggen I, Miller MR, et al. Standardization of spirometry 2019 update: an official ATS and ERS technical statement. Am J Respir Crit Care Med 2019;200:e70–88.
- Stanojevic S, Kaminsky DA, Miller MR, et al. ERS/ATS technical standard on interpretive strategies for routine lung function tests. Eur Respir J 2022;60:2101499.
- Wanger J, Clausen JL, Coates A, et al. Standardisation of the measurement of lung volumes. Eur Respir J 2005;26:511–22.
- Bhakta NR, Bime C, Kaminsky DA, et al; American Thoracic Society Committees on Pulmonary Function Testing and on Health Equity and Diversity. Race and ethnicity in pulmonary function test interpretation: an official American Thoracic Society statement. Am J Respir Crit Care Med 2023;207(8):978–995.
- Graham BL, Brusasco V, Burgos F, et al. 2017 ERS/ATS standards for single-breath carbon monoxide uptake in the lung. Eur Respir J 2017;49:1600016.
- Laveneziana P, Albuquerque A, Aliverti A, et al. ERS statement on respiratory muscle testing at rest and during exercise. Eur Respir J 2019;53(6):1801214. Supersedes the 2002 ATS/ERS statement.
References are numbered in citation order (Vancouver/BMJ style) and were current at the time of writing. Guidelines are living documents — verify against the latest version before clinical use.
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