What is the role of spirometry in diagnosing and managing respiratory diseases?

Spirometry: Essential Diagnostic Tool for Respiratory Disease

Spirometry is required to establish a diagnosis of COPD and asthma, and should be obtained in all patients over age 40 with respiratory symptoms such as dyspnea, chronic cough, or wheezing, but should not be used to screen asymptomatic individuals. 1

When to Perform Spirometry

Diagnostic Indications (Symptomatic Patients)

• Perform spirometry in individuals over 40 years presenting with progressive dyspnea (worse with exercise), chronic cough (intermittent or persistent), chronic sputum production, recurrent wheeze, or recurrent lower respiratory tract infections 1, 2
• Obtain spirometry in patients with risk factor exposure including tobacco smoke (>40 pack-years is strong predictor), occupational dusts/vapors/fumes/gases, smoke from home cooking/heating fuels, or family history of COPD 1, 2
• Use spirometry for asthma diagnosis when patients report episodic symptoms of cough (worse at night), wheeze, difficulty breathing, or chest tightness that worsen with exercise, viral infections, allergens, irritants, weather changes, or emotional stress 1

Screening Contraindication

• Do not use spirometry to screen asymptomatic individuals, even those with smoking history or other COPD risk factors, as this leads to overdiagnosis without evidence of treatment benefit and increases unnecessary costs 1
• Screening hundreds of asymptomatic adults would be needed to defer even one exacerbation, with estimated 455 adults aged 60-69 requiring screening under best-case assumptions 1

Diagnostic Criteria

COPD Diagnosis

• Post-bronchodilator FEV1/FVC ratio <0.70 confirms airflow limitation and establishes COPD diagnosis when combined with appropriate clinical context 1, 2
• This fixed ratio criterion is simple, independent of reference values, and used in clinical trials, though it may overdiagnose COPD in elderly patients (>70 years) and underdiagnose in adults <45 years 1, 2

Asthma Diagnosis

• Reversibility is demonstrated by FEV1 increase >200 mL AND ≥12% from baseline after short-acting β2-agonist inhalation 1
• Some evidence suggests ≥10% of predicted FEV1 increase may better differentiate asthma from COPD 1

Disease Severity Classification

COPD Spirometric Staging

• Mild COPD: FEV1 ≥80% predicted 2
• Moderate COPD: FEV1 50-80% predicted 2
• Severe COPD: FEV1 30-50% predicted 2
• Very severe COPD: FEV1 <30% predicted 2

Role in Management Decisions

Treatment Initiation Based on Spirometry

• For symptomatic patients with FEV1 <60% predicted, prescribe inhaled bronchodilators (long-acting anticholinergics or long-acting β-agonists as monotherapy) 1
• For symptomatic patients with FEV1 60-80% predicted, consider inhaled bronchodilators though evidence is limited and conflicting; individual patients may benefit 1
• Spirometry results guide medication changes in 48% of patients, with >85% of changes concordant with guideline recommendations 3

Comprehensive Assessment Beyond Spirometry

• Spirometry alone is insufficient for COPD management; must also assess symptom burden using mMRC (threshold ≥2) or CAT score (threshold ≥10), exacerbation history/risk, and comorbidities 1, 2
• Blood eosinophil count predicts exacerbation risk and inhaled corticosteroid response in patients with exacerbation history 2

Technical Performance Standards

Quality Requirements

• 71% of spirometry tests in primary care settings are technically adequate for interpretation when proper training is provided 3
• Concordance between primary care physicians and pulmonary experts is 76% overall, higher for asthma than COPD 3
• Spirometry requires good-quality technique following American Thoracic Society/European Respiratory Society 2005 standardization guidelines, with hands-on training and meaningful feedback essential 4

Common Pitfalls to Avoid

Diagnostic Errors

• Do not rely on physical examination alone, as physical signs of airflow limitation are rarely present until significantly impaired lung function exists 1
• Do not use peak flow meters for diagnosis; they have wide variability and are designed for monitoring, not diagnosis 1
• Avoid using spirometry results to motivate smoking cessation, as evidence shows no independent benefit on quit rates 1

Misinterpretation Risks

• Fixed ratio may misclassify elderly never-smokers with false-positive COPD diagnoses 1
• Spirometry without clinical context leads to overtreatment; always integrate with symptoms, risk factors, and patient history 1, 2

Additional Diagnostic Utility

Beyond COPD and Asthma

• Spirometry differentiates restrictive from obstructive patterns and identifies upper airway obstruction through flow-volume loop analysis 5, 6
• Use for prognosis, therapeutic response monitoring, and disease progression in established respiratory disease 5
• Consider bronchoprovocation testing (methacholine, histamine, cold air, exercise) when asthma is suspected but spirometry is normal or near-normal; positive test confirms airway hyperresponsiveness 1