Spirometry

Indications

• Diagnosis of suspected COPD, asthma, bronchiectasis, or restrictive lung disease.
• Symptoms of chronic cough, dyspnoea, wheeze, or sputum production.
• Unexplained findings: normal CXR with hypercapnoea, or unexplained crackles on auscultation.
• Screening at-risk individuals — smokers, occupational exposure history, neuromuscular disease (e.g. motor neuron disease, Guillain-Barré syndrome).
• Monitoring treatment response.

Contraindications

Uncooperative patient; recent pneumothorax (within 6 weeks); unstable angina; recent MI (within 4 weeks); haemoptysis (within 48 hours); recent abdominal/thoracic/eye surgery (within 6 weeks); thoracic, aortic or cerebral aneurysm; suspected/confirmed airborne infectious disease (e.g. TB, influenza, COVID-19).

Patient preparation

Withhold SABA/SAMA for 4–6 hours and LABA/LAMA for 12–24 hours where possible; no smoking or strenuous exercise on the day of the test.

What spirometry measures

• Forced vital capacity (FVC) — total volume of air forcibly exhaled.
• FEV1 — volume exhaled in the first second.
• FEV1/FVC ratio — distinguishes obstructive from restrictive patterns.
• Flow/volume loop — shape helps localise the abnormality.

Interpretation

Obstruction: FEV1 reduced more than FVC (FEV1/FVC ratio below the LLN). FEV1 itself should also be low/below the LLN to define obstruction. If FVC is also low, a mixed obstructive/restrictive pattern may be present. In severe obstruction, the flow/volume loop shows a sharp initial spike with a rapid drop-off — good flow in the large airways but poor flow elsewhere.

Restriction: reduced FVC with a normal or high FEV1/FVC ratio. A restrictive pattern should prompt further testing — formal lung volume measurement (to confirm TLC below the LLN) and DLCO (to distinguish parenchymal from extra-parenchymal causes).

Bronchodilator reversibility: if FEV1 is reduced, test reversibility — give 2–4 puffs of salbutamol via spacer and re-test at 10–15 minutes. A clinically significant response is an improvement of ≥200 mL and ≥12% in either FEV1 or FVC. A failure to demonstrate reversibility on the day does not preclude clinical benefit from bronchodilator therapy in practice.

Causes of restrictive vs obstructive patterns

Restrictive	Obstructive
Pulmonary fibrosis	COPD
Neuromuscular disorders	Asthma
Congestive cardiac failure	Bronchiectasis / cystic fibrosis
Sarcoidosis	Bronchiolitis
Obesity	Alpha-1 antitrypsin deficiency