Pulmonary Function Tests (PFTs) are operational diagnostic procedures applied to differentiate asthma from chronic obstructive pulmonary disease (COPD) in respiratory disorders. Both result in limitation of airflow but by pathophysiologic heterogeneity of many mechanisms, by different patterns of disease, and with dissimilar responsiveness to therapy.
Why and how two conditions differ and how and why PFTs and other investigations discriminate between asthma and COPD are explained below in order to make the proper diagnosis and treatment. Overview of Pulmonary Function Tests (PFTs)
PFTs are simple measurements of volumes, capacities, airflow rates, and efficiency of lung gas exchange.
Features of PFTs
- Spirometry measures forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC).
- The Bronchodilator Reversibility Test—if airflow obstruction improves after using a bronchodilator.
- Peak Expiratory Flow Rate: Estimates peak expiratory flow.
- Lung Volumes and Capacities: Plethysmography or helium dilution technique.
- Diffusion Capacity of the Lung for Carbon Monoxide (DLCO): Measures how well the lungs transfer oxygen into the blood.
- Fractional Exhaled Nitric Oxide (FeNO): airway inflammation (asthma)
Asthma and COPD pathophysiology
- AsthmaPulmonary Function Tests (PFTs): Asthma and COPD
- Reversible airflow limitation is secondary to chronic airway inflammatory disease.
- Triggers: cold air, exercise, infection, or allergy
- Pathological Features:
- Airway hyperresponsiveness
- Eosinophilic inflammation
- Mucosal edema
- Hypersecretion of mucus
- Bronchial smooth muscle hypertrophy
- COPD
- Irreversible airflow limitation secondary to progressive chronic bronchitis or emphysema.
- Epidemiologically secondary to smoking and environmental irritants.
- Major Pathological Features
- Alveolar injury (emphysema)
- Chronic airway inflammation (chronic bronchitis)
- Excess production and hyperplasia of goblet cells and mucus
- Airway obstruction and loss of elastic recoil
The Importance of PFTs in Differentiating Asthma from COPD
- Spirometric results
- Asthma
- FEV1/FVC: Abnormal but reversible with therapy.
- Reversibility to bronchodilators: FEV1 increases by ≥12% and ≥200 mL following an aerosol of a short-acting inhaled bronchodilator (albuterol) administration.
- Variable airflow limitation: Provocations are labile with symptoms.
- COPD
- Ratio FEV1/FVC: <70% on or after bronchodilator, with evidence of air obstruction irreversibility.
- Irreversibility to bronchodilators: Failure to achieve relief by a bronchodilator (<12% and <200 mL improvement).
- Progressive deterioration of lung function: abnormal PFT progressively worsening over time.
- Peak Expiratory Flow Rate (PEFR)
- Asthma: Diurnal variation of >20% is characteristic of asthma.
- COPD: PEFR is non-diurnally variable, but so.
- Lung Volumes and Capacities
- Asthma: Normal lung capacities in the acute exacerbation.
- In COPD, excess air gets trapped in the lungs, leading to an increase in the amount of air left after exhaling (residual volume) and the total lung capacity.
- Diffusion Capacity of the Lung for Carbon Monoxide (DLCO)
- Asthma: Slightly elevated or normal DLCO with augmented pulmonary blood flow with inflammation.
- In COPD, especially emphysema, the destruction of air sacs (alveoli) reduces the lungs’ ability to transfer oxygen, leading to a lower diffusion capacity (DLCO).
- Fractional exhaled Nitric Oxide (FeNO)
- In asthma, a high FeNO level (above 50 ppb) indicates inflammation in the airways and a good response to steroid treatment.
- COPD: Slightly raised but not in proportion to disease severity.
PFT Differentiation of Asthma vs. COPD
| Parameter | Asthma | COPD |
| FEV1/FVC Ratio | Low during exacerbation but normalizes | Persistently <70% |
| Reversibility with Bronchodilator | Significant improvement (≥12% and ≥200 mL) | Minimal improvement (<12% and <200 mL) |
| PEFR Variability | >20% variation | Stable |
| Lung Volumes | Normal | Increased RV and TLC due to air trapping |
| DLCO | Normal or increased | Decreased (especially in emphysema) |
| FeNO | Elevated (>50 ppb) | Normal or slightly elevated |
Conclusion
Pulmonary Function Tests (PFTs) are used in differential diagnosis of asthma and COPD to diagnose and treat accordingly. Reversibility test to bronchodilator, DLCO, lung volume, FeNO, and spirometry are utilized for accurate diagnosis. Treatment pattern can be given keeping variation into account for different patients to achieve better patient outcome. MBBS students have to be well trained in interpreting PFT for earlier differential pulmonary diagnosis.
