How do you interpret pulmonary function tests (PFTs) for chronic obstructive pulmonary disease (COPD) or asthma diagnosis?

From the Guidelines

Pulmonary function tests (PFTs) for COPD or asthma diagnosis should be interpreted by examining specific patterns of airflow limitation, with a focus on post-bronchodilator FEV1/FVC ratio and reversibility of airflow limitation, as recommended by the most recent GOLD Science Committee report 1. To diagnose COPD, look for a post-bronchodilator FEV1/FVC ratio less than 0.7, indicating persistent airflow limitation that doesn't fully reverse with bronchodilators. The severity is then classified based on FEV1 percentage of predicted value:

• Mild (≥80%)
• Moderate (50-79%)
• Severe (30-49%)
• Very severe (<30%) For asthma, the key finding is reversible airflow limitation, demonstrated by a 12% and 200mL improvement in FEV1 after bronchodilator administration. Some key points to consider when interpreting PFTs for COPD or asthma diagnosis include:
• Asthma may also show normal spirometry between exacerbations, increased airway hyperresponsiveness on methacholine challenge testing, and greater variability in peak expiratory flow measurements
• Additional tests like lung volumes may reveal hyperinflation in COPD, while diffusion capacity (DLCO) is typically reduced in emphysema but preserved in asthma
• These distinct patterns help differentiate between the fixed airflow limitation characteristic of COPD and the variable, reversible obstruction seen in asthma, guiding appropriate treatment selection It's worth noting that the differentiation between COPD and asthma can be challenging, especially in older subjects, and should be based on a combination of clinical features, imaging, and PFT results, as suggested by the European Respiratory Society task force 1 and the BTS guidelines for the management of COPD 1. However, the most recent and highest quality study, the GOLD Science Committee report 1, provides the most up-to-date recommendations for the interpretation of PFTs in COPD diagnosis.

From the Research

Interpreting Pulmonary Function Tests (PFTs) for COPD or Asthma Diagnosis

To interpret PFTs for COPD or asthma diagnosis, several factors and test results must be considered:

• The presence of a post-bronchodilator FEV1/FVC < 0.70 confirms the presence of persistent airflow limitation and the diagnosis of COPD 2.
• The classification of severity of airflow limitation in COPD is based on %FEV1, with gas trapping and static hyperinflation occurring as airflow limitation worsens 2.
• Measurement of diffusing capacity (DLco) provides information on the functional impact of emphysema in COPD 2.
• Spirometry testing enhances primary care clinicians' ability to differentiate between asthma and COPD 3.
• More advanced diagnostic tests used in hospital care settings do not seem to provide a better overall diagnostic differentiation between asthma and COPD in primary care patients 3.

Key PFTs for COPD Diagnosis

The following PFTs are essential for COPD diagnosis:

• Spirometry: measures FEV1 and FVC to assess airflow limitation 4, 2.
• Diffusing capacity: measures DLco to assess gas exchange 2, 5.
• Lung volume measurement: measures functional residual capacity, residual volume, and total lung capacity to assess gas trapping and hyperinflation 2.
• Exercise testing: assesses exercise capacity and symptoms 4, 5.

Severity Classification of COPD

The severity of COPD can be classified using the Global Initiative for Chronic Obstructive Lung Disease (GOLD) stages or the new STaging of Airflow obstruction by Ratio (STAR) classification:

• GOLD stages: based on FEV1% predicted (ppFEV1) 6.
• STAR classification: based on FEV1/FVC ratio, providing a more uniform gradation of disease severity and better discrimination for mortality 6.

Additional PFTs for COPD Management

Incorporating additional PFTs into everyday clinical evaluation of COPD patients may enhance management:

• Resting volume, capacity, and airway resistance measurements 5.
• Diffusion capacity measurements 5.
• Forced oscillation technique 5.
• Field and cardiopulmonary exercise testing 5.
• Muscle strength evaluation 5.