What does a pulmonary function test (PFT) reading typically show in asthma?

Pulmonary Function Test Findings in Asthma

In asthma, PFT typically shows reversible airflow obstruction with an FEV1/FVC ratio <70-80% and significant bronchodilator responsiveness (≥12% and ≥200 mL increase in FEV1), though baseline spirometry may be completely normal between symptomatic episodes. 1

Key Spirometric Patterns

Obstructive Pattern

• FEV1 reduced with normal or reduced FVC 2
• FEV1/FVC ratio <70-80% in adults (or <90% in children) indicates obstruction 2
• Flow-volume curves show concavity of the expiratory portion, reflecting decreased flow at lower lung volumes 2
• Baseline spirometry may be completely normal between episodes of bronchospasm, making a single normal test insufficient to exclude asthma 2

Bronchodilator Reversibility (Diagnostic Hallmark)

• ≥12% AND ≥200 mL increase in FEV1 after short-acting bronchodilator (e.g., salbutamol 400 μg) is the gold standard criterion 1, 2
• This criterion has high specificity (90-98%) for asthma diagnosis 1
• Sensitivity is only 35-36%, meaning many asthmatics will not show this degree of reversibility on a single test 1
• Testing should be avoided within 4 hours of short-acting or 15 hours of long-acting bronchodilator use to prevent false negatives 1, 2

Alternative Measures of Variable Airflow Limitation

Peak Expiratory Flow (PEF) Variability

• ≥20% diurnal variation (amplitude % best) with minimum change of 60 L/min over 2 weeks is highly suggestive of asthma 2
• ≥10% variability in adults and ≥13% in children when measured serially supports the diagnosis 2
• PEF is inferior to FEV1 for clinic measurements due to lower reproducibility and tendency to underestimate obstruction 2

Airway Hyperresponsiveness Testing

• Methacholine or histamine challenge demonstrates increased airway sensitivity even when baseline spirometry is normal 2
• A negative test in an untreated patient should prompt reconsideration of the asthma diagnosis 2

Clinical Interpretation Caveats

When Spirometry Appears Normal

• Spirometric measures should return to normal in most adequately treated asthmatics, distinguishing it from COPD 2
• Consider serial home PEF monitoring or exercise challenge testing (>10% decrease in FEV1 or >15% decrease in PEF after 6-10 minutes of exercise) 2
• Airway hyperresponsiveness testing may be positive even with normal baseline spirometry 2

Distinguishing Asthma from COPD

• Post-bronchodilator FEV1 <80% predicted with FEV1/FVC <70% has 100% sensitivity but only 38% specificity for COPD, making it inadequate to distinguish between the two conditions 3
• In asthma, diffusing capacity remains normal, whereas it is typically reduced in COPD 2
• Complete reversibility to normal with adequate treatment favors asthma over COPD 2

Severity Assessment

• FEV1 % predicted is used to grade severity but correlates poorly with symptoms and may not accurately predict clinical severity for individual patients 2
• Low pre-bronchodilator FEV1 % predicted is a strong independent predictor of future asthma exacerbations 2

Practical Testing Approach

For suspected asthma with abnormal baseline spirometry (FEV1 <80% or FEV1/FVC <80%):

• Perform pre- and post-bronchodilator spirometry looking for ≥12% and ≥200 mL improvement in FEV1 1

For suspected asthma with normal baseline spirometry:

• Consider bronchodilator testing anyway if clinical history strongly suggests asthma 1
• Alternatively, use serial home PEF monitoring for 2 weeks looking for ≥20% variability 2
• Or perform bronchial challenge testing with methacholine to demonstrate airway hyperresponsiveness 2

Critical pitfall: A single negative bronchodilator test does not exclude asthma, as reversibility varies over time and may require serial testing or trial of inhaled corticosteroids with repeat testing 2