Key Facts

Lung function testing is fundamental to respiratory medicine, providing objective measurements for diagnosis, disease severity grading, monitoring progression, treatment response assessment, and pre-operative risk stratification.

Epidemiology

Millions of spirometry tests are performed annually in the UK across primary and secondary care. Quality spirometry is essential for the accurate diagnosis of COPD (estimated 1.2 million diagnosed, with a similar number undiagnosed). Under-utilisation and poor-quality spirometry in primary care lead to significant misdiagnosis.

Aetiology

Lung function testing is indicated for:

• Diagnosis: COPD, asthma, ILD, neuromuscular disease, upper airway obstruction
• Monitoring: disease progression (COPD, IPF), treatment response (asthma, ILD)
• Pre-operative assessment: lung resection surgery, major surgery risk
• Disability assessment: occupational lung disease, insurance purposes
• Screening: occupational health, cystic fibrosis, alpha-1 antitrypsin deficiency

Pathophysiology

Obstructive defect: narrowing of airways (bronchoconstriction, mucus, wall thickening, loss of elastic recoil in emphysema) → air trapping → disproportionate reduction in FEV₁ relative to FVC → low FEV₁/FVC ratio. The flow-volume loop shows a concave (scooped) expiratory limb.

Restrictive defect: reduced lung expansion (parenchymal fibrosis, chest wall deformity, neuromuscular weakness, pleural disease, obesity) → reduced TLC and FVC with preserved/elevated FEV₁/FVC ratio.

Gas transfer impairment: thickening of the alveolar-capillary membrane (ILD), destruction of alveolar surface area (emphysema), or reduced pulmonary capillary blood volume (PE, pulmonary hypertension) → reduced DLCO.

When to Perform Spirometry

• Dyspnoea of unexplained aetiology
• Chronic cough (>8 weeks)
• Suspected COPD (smoker/ex-smoker >35 with symptoms)
• Suspected or known asthma (diagnosis and monitoring)
• Pre-operative assessment for thoracic surgery
• Monitoring ILD progression or treatment response

Interpreting the Flow-Volume Loop

• Normal: symmetric, peaked expiratory curve
• Obstructive (COPD/asthma): concave (scooped) expiratory limb, reduced peak flow
• Restrictive (ILD): narrow, tall loop with reduced volumes but normal shape
• Fixed upper airway obstruction: flattened inspiratory AND expiratory limbs
• Variable extrathoracic obstruction: flattened inspiratory limb only
• Variable intrathoracic obstruction: flattened expiratory limb only

Red Flags

• Rapidly declining FEV₁ or FVC (accelerated disease progression)
• FVC <1L in neuromuscular disease (consider ventilatory support)
• Unexplained restrictive pattern in young patient (consider neuromuscular or metabolic cause)
• Fixed upper airway obstruction pattern (stridor, tracheal lesion)

Bedside

• Spirometry (FEV₁, FVC, FEV₁/FVC): the foundation of lung function assessment; requires trained operator, calibrated equipment, and patient cooperation
• Peak expiratory flow (PEF): useful for asthma monitoring; not a substitute for spirometry
• FeNO (fractional exhaled nitric oxide): >40 ppb supports eosinophilic inflammation; useful in asthma diagnosis (NICE NG80)

Bloods

• FBC: polycythaemia (chronic hypoxia), anaemia (affects DLCO interpretation)
• Alpha-1 antitrypsin level: if COPD in young non-smoker or lower-lobe emphysema

Imaging

• CXR: hyperinflation (COPD), bilateral infiltrates (ILD), mass lesion
• HRCT: emphysema distribution, ILD pattern, bronchiectasis

Special Tests

• Full lung volumes (body plethysmography/helium dilution): TLC, RV, FRC — essential to confirm restriction (TLC <80% predicted) vs pseudo-restriction
• DLCO (gas transfer): single-breath carbon monoxide method; correct for Hb; reduced in emphysema, ILD, PE
• Bronchodilator reversibility: administer salbutamol 400 mcg via spacer; repeat spirometry after 15-20 min; positive if FEV₁ improves ≥12% AND ≥200 mL
• Bronchial challenge testing (methacholine/histamine): PC₂₀ <8 mg/mL = positive; diagnoses airway hyperresponsiveness when spirometry normal
• Exercise testing (6MWT, CPET): functional capacity, desaturation on exertion, exercise limitation
• Respiratory muscle pressures (MIP/MEP, SNIP): if neuromuscular cause suspected

Non-pharmacological

• Quality assurance: spirometry must meet ATS/ERS acceptability and repeatability criteria (≥3 acceptable blows, 2 best FEV₁ within 150 mL)
• Training: all operators should be ARTP-certified (UK standard)
• Patient preparation: no bronchodilator use for specified washout period if diagnostic; withhold SABA 4h, LABA 12h, LAMA 24h
• Contraindications to spirometry: recent MI (<1 month), pneumothorax, haemoptysis, unstable angina, recent thoracic/abdominal/eye surgery

Pharmacological

• Treatment is directed by the results — not the test itself
• Obstructive + reversibility: suggests asthma → ICS ± LABA
• Obstructive + irreversible: suggests COPD → bronchodilators, ICS if eosinophilic, per NICE NG115
• Restrictive + low DLCO: suggests ILD → consider antifibrotics (IPF) or immunosuppression (non-IPF ILD)

Surgical/Interventional

• Pre-operative lung function: FEV₁ >1.5L for lobectomy, >2.0L for pneumonectomy (NICE guidelines); calculate predicted post-operative FEV₁ (ppo-FEV₁) using perfusion scintigraphy

Referral Criteria

• Respiratory specialist referral for unexplained abnormal spirometry
• Diagnostic uncertainty (especially mixed patterns or discordant results)
• Suspected ILD, neuromuscular disease, or pulmonary hypertension
• Pre-operative assessment for lung resection

Lung function testing is prognostic in many conditions. In COPD, FEV₁ is the strongest predictor of mortality and is incorporated into the BODE index. In IPF, FVC decline >10% over 6-12 months predicts mortality. In neuromuscular disease, FVC <1L indicates need for ventilatory support. Annual FEV₁ decline >40 mL/year in COPD suggests accelerated disease. Normal lung function in a symptomatic patient should prompt consideration of alternative diagnoses or dynamic testing.

GOLD Classification of COPD Severity (by FEV₁)

Key Lung Function Parameters

Common DLCO Patterns