Spirometry, bronchodilator reversibility and peak flow

01Purpose and principlesWhat the assessment is for and the core concepts behind it.

Spirometry describes physiology, not a disease label. Obstruction may occur in asthma, COPD, bronchiectasis or other airway disease; low FVC can reflect restriction, gas trapping, weak effort or early termination. The history and further tests determine the diagnosis.

Quality is inseparable from interpretation. A technically poor manoeuvre can mimic obstruction or restriction, and reproducibility does not guarantee a correct technique if every blow has the same error.

Bronchodilator reversibility is one form of variable airflow evidence. It is more specific than sensitive at common thresholds, so a positive result supports asthma but a negative result should redirect the diagnostic sequence rather than close it.

Key points

Check identity, age, sex, height, indication, contraindications, current symptoms and recent bronchodilator use before testing.
FEV1 is the volume expired in the first second; FVC is the total forced expiratory volume; the FEV1/FVC ratio identifies airflow obstruction.
Interpret against appropriate reference values and the lower limit of normal where available; a fixed ratio below 0.70 supports COPD only in the right clinical context and can overdiagnose older adults.
A low FEV1/FVC indicates obstruction. A preserved/high ratio with low FVC suggests possible restriction or suboptimal effort and requires lung-volume confirmation, not a spirometric diagnosis of restriction.
For adult asthma, NICE supports bronchodilator reversibility when FEV1 rises by at least 12% and 200 mL from baseline, or at least 10% of predicted FEV1.
The NG245 evidence protocol uses 400 micrograms salbutamol through a spacer, with repeat spirometry after an appropriate interval; record drug, dose and time.
If spirometry is unavailable or delayed in suspected adult asthma, twice-daily PEF for 2 weeks supports asthma when mean diurnal variability is 20% or more.
A negative reversibility or variability test does not exclude asthma, especially when asymptomatic or already taking inhaled corticosteroid.
Peak flow is effort- and device-dependent; use the same meter, technique and personal best for monitoring, not a single isolated reading.
Never perform routine diagnostic spirometry during an acute attack when it would delay treatment or be unsafe.

02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.

Obstructive pattern

FEV1/FVC below the appropriate lower limit; NICE uses post-bronchodilator ratio below 0.70 to confirm persistent obstruction in suspected COPD. Review curve shape, FEV1 severity and clinical context.

Possible restrictive pattern

Reduced FVC with preserved or raised FEV1/FVC. Confirm with total lung capacity; poor inspiration, short exhalation, obesity, neuromuscular weakness and gas trapping can produce the same spirometric pattern.

Positive adult asthma reversibility

Increase in FEV1 at least 12% and at least 200 mL from pre-bronchodilator, or at least 10% of predicted normal FEV1 under NG245. This supports variable airflow obstruction but must fit symptoms.

Significant PEF variability

Twice-daily measurements over 2 weeks with mean diurnal variability at least 20% support asthma in adults when spirometry is delayed or unavailable. Confirm technique and adherence to the diary.

Unsafe or invalid test settingRed flag

Acute cardiorespiratory instability, active significant haemoptysis, recent relevant thoracic/abdominal/eye surgery, untreated pneumothorax, severe infection-control risk or inability to follow instructions requires deferral or specialist assessment.

Upper-airway or effort clue

Abrupt termination, inconsistent starts, cough in the first second, glottic closure or flattened inspiratory/expiratory limbs should prompt repeat coaching and, if persistent, consideration of central/upper-airway obstruction.

03Method and interpretationA systematic approach to the test and its findings.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Pre-test clinical and medicine checkFirst step
Why
Decide whether testing is safe and whether bronchodilator washout is appropriate.
Interpretation and limitations
Record current inhalers and last doses. Do not ask a symptomatic patient to withhold necessary treatment; if medicines were taken, document this and interpret accordingly.
02
Quality-assured flow-volume and volume-time manoeuvres
Why
Produce valid FEV1, FVC and ratio measurements.
Interpretation and limitations
Look for a rapid start, no early cough/leak/closure, adequate exhalation or plateau, and repeatable best blows. Use the service's current ARTP/ERS quality criteria and record grade.
03
FEV1/FVC, FEV1 and FVC against reference values
Why
Classify ventilatory pattern and quantify impairment.
Interpretation and limitations
Low ratio indicates obstruction. Preserved ratio plus low FVC is only possible restriction until total lung capacity is measured. Review z-scores/LLN where available rather than percentages alone.
04
Bronchodilator reversibility
Why
Demonstrate variable airflow obstruction in suspected asthma.
Interpretation and limitations
After baseline spirometry, give 400 micrograms salbutamol via spacer in the NG245 evidence protocol and repeat after the service-defined interval. Adult positivity: FEV1 increase at least 12% and 200 mL, or at least 10% predicted.
05
Twice-daily peak-expiratory-flow diary
Why
Detect day-to-day variability when spirometry is unavailable/delayed or symptoms are intermittent.
Interpretation and limitations
Use the same meter, best of three efforts morning and evening for 2 weeks. Calculate daily amplitude percentage mean; a mean of 20% or more supports adult asthma.
06
Full lung volumes and gas transfer
Why
Confirm restriction, identify hyperinflation/gas trapping and evaluate diffusion impairment.
Interpretation and limitations
Low total lung capacity confirms restriction. Raised residual volume supports gas trapping; reduced transfer factor can suggest emphysema, interstitial or pulmonary vascular disease but needs haemoglobin and imaging context.

04Clinical next stepsHow the result changes management or prompts escalation.

01PatternFrom numbers to a physiological statementFirst stepAny acceptable spirometry result.

11. Confirm acceptability, repeatability, reference equation and whether result is pre- or post-bronchodilator.
22. Read FEV1/FVC first: below LLN indicates obstruction; then use FEV1 to describe impairment without equating it to symptom burden.
33. If the ratio is preserved but FVC is low, state 'possible restriction or incomplete exhalation' and arrange lung volumes where clinically indicated.
44. Inspect the curves for poor start, cough, truncation, scooping or fixed/variable upper-airway patterns; integrate symptoms and imaging before naming disease.

02Asthma evidenceObjective confirmation in an adultEpisodic wheeze, cough, breathlessness or chest tightness with suspected asthma.

1Follow NG245's sequence: clinical assessment, blood eosinophils or FeNO; if not confirmed, perform spirometry with bronchodilator reversibility.
2A rise in FEV1 of at least 12% and 200 mL, or at least 10% predicted, supports asthma. If negative, do not simply label the person non-asthmatic.
3If spirometry is unavailable/delayed, collect twice-daily PEF for 2 weeks; variability at least 20% supports diagnosis. If uncertainty persists, refer for bronchial challenge or specialist review.
4AlternativeRecord objective evidence in the diagnosis and establish a baseline for future comparison; review alternative diagnoses when the pattern and symptoms diverge.

03COPD confirmationPersistent obstruction in contextAge over 35, smoking/exposure history and exertional breathlessness, chronic cough or sputum.

1Perform quality-assured post-bronchodilator spirometry when clinically stable; FEV1/FVC below 0.70 supports persistent airflow obstruction under NICE NG115.
2Check whether the fixed ratio is plausible for age and symptoms; consider LLN, asthma, bronchiectasis, heart failure or other causes when discordant.
3Grade airflow obstruction with post-bronchodilator FEV1 percentage predicted, but separately record MRC dyspnoea, exacerbations, oxygenation, BMI and comorbidity because FEV1 alone does not define disease impact.

04Poor-quality resultDo not diagnose from an unreliable blowNon-repeatable values, cough, slow start, early termination, leak or inconsistent curves.

1Re-explain and demonstrate; ensure upright posture, correct mouth seal and nose clip if used, then allow adequate rest between attempts.
2Stop if the person develops chest pain, syncope, severe distress or another safety concern. Do not keep repeating beyond the service protocol when quality is not improving.
3DefinitiveAlternativeDocument why testing was incomplete and arrange a trained repeat or alternative physiology assessment; never convert a technically limited trace into a definitive COPD or restrictive diagnosis.

05Procedure and medicine safetyRelevant preparation, treatment and contraindications.

Tests acute change in airflow after a short-acting beta2 agonist.

Salbutamol for bronchodilator reversibility testing

400 micrograms inhaled through a spacer (typically 4 actuations of a 100 microgram metered-dose inhaler), then repeat spirometry after the protocol interval, commonly about 15 minutes.

Document pre-test bronchodilator use. May cause tremor, tachycardia or hypokalaemia; defer or seek senior advice in unstable patients. A negative response does not exclude asthma and a positive response is not uniquely diagnostic.

Alternative or additional bronchodilator assessment, particularly where beta2-agonist response is unsuitable or COPD physiology is being characterised.

Ipratropium as an alternative reversibility agent in specialist protocols

A common protocol uses 160 micrograms inhaled (for example 8 actuations of a 20 microgram inhaler) with repeat testing after about 30 minutes; follow the physiology laboratory protocol and product licence.

Not the NG245 default asthma diagnostic protocol. Avoid spray into eyes; use caution with narrow-angle glaucoma and bladder-outflow problems. Record exact device, dose and interval.

06Risks, monitoring and follow-upComplications, safety checks and further assessment.

Save numerical results, reference equation, quality grade and flow-volume/volume-time curves rather than only a diagnostic label.
Record bronchodilator name, dose, delivery device, time and any pre-test withholding or unavoidable recent use.
Repeat spirometry when the person is clinically stable if an acute infection/exacerbation or poor technique invalidated the first test.
In asthma, do not use routine spirometry alone to judge control; combine symptoms, exacerbations, reliever use, technique and action-plan review.
In COPD, review MRC dyspnoea and exacerbation frequency at least annually; repeat spirometry when it will change diagnosis or management rather than by reflex.

07Special situationsVariants, exceptions and circumstances that change the usual approach.

The ratio comes before FEV1

A low FEV1 does not automatically mean obstruction. Read FEV1/FVC first, then decide whether low FEV1 reflects obstruction, restriction, mixed physiology or technique.

Spirometry cannot prove restriction

Restriction is defined by low total lung capacity. Spirometry can only raise suspicion when FVC is low and the ratio preserved.

Reversibility is threshold-dependent

A person can have clinically important variability below a binary cut-off, and some COPD patients meet reversibility thresholds. Treat the result as weighted evidence, not a disease barcode.

PEF is best within-person

Peak flow is useful for variability and action plans because it tracks one person's airway calibre over time; it is less complete than spirometry and highly dependent on technique and meter.

A normal test can be correctly normal

Intermittent asthma may produce normal spirometry between episodes. The next step is timed objective testing, PEF variability or bronchial challenge—not inventing obstruction from symptoms alone.

08Common pitfallsFrequent interpretation and management errors.

01
Diagnosing COPD from pre-bronchodilator obstruction or from a ratio alone without typical symptoms/exposure.
02
Calling low FVC 'restriction' without measuring total lung capacity.
03
Using only percentage predicted and ignoring LLN/z-score, age and curve quality.
04
Excluding asthma after one negative reversibility test while the patient is well or already using ICS.
05
Comparing PEF values from different meters or accepting a diary without technique review.
06
Withholding clinically necessary bronchodilator solely to satisfy a test protocol.