How to interpret Pulmonary Function Tests (PFTs) for diagnosing Chronic Obstructive Pulmonary Disease (COPD) or asthma?

From the Guidelines

The diagnosis of COPD should be based on a post-bronchodilator (BD) forced expiratory volume in 1 s (FEV1)/forced vital capacity (FVC) ratio <0.7, as recommended by the GOLD Science Committee in 2025 1. When interpreting pulmonary function tests (PFTs), it is essential to examine key spirometry values to diagnose obstruction. The following key points should be considered:

• Obstruction is diagnosed when the FEV1/FVC ratio is less than 0.7.
• For COPD diagnosis, look for this reduced ratio plus an FEV1 less than 80% of predicted value, with minimal or no improvement after bronchodilator administration.
• Asthma, in contrast, shows similar obstruction but demonstrates significant bronchodilator reversibility.
• Additional diagnostic clues include reduced diffusion capacity (DLCO) in emphysema-predominant COPD, while normal DLCO is more common in asthma.
• Total lung capacity and residual volume are typically increased in COPD due to air trapping. Some patients may have discordant results between pre- and post-BD spirometry, including "volume" or "flow" responders 1. Volume responders have reduced FVC due to gas trapping, causing FEV1/FVC ≥0.7 pre-BD, but a volume response occurs post-BD with a greater improvement in FVC relative to FEV1, decreasing the ratio to <0.7. Flow responders show a greater FEV1 improvement relative to FVC, which may increase FEV1/FVC from <0.7 pre-BD to ≥0.7 post-BD; these individuals have an increased likelihood of developing post-BD obstruction during follow-up and require monitoring longitudinally. The GOLD 2025 report recommends using pre-BD spirometry to rule out COPD and post-BD measurements to confirm the diagnosis, which will reduce clinical workload 1. Post-BD results close to the threshold should be repeated to ensure a correct diagnosis is made, and post-BD measurements ensure that volume responders are not overlooked and limit COPD overdiagnosis. When interpreting PFTs, consider the clinical context—COPD patients usually have smoking history, progressive symptoms, and fixed obstruction, while asthma patients often report episodic symptoms, triggers, and family history of atopy 1. Severity classification for COPD follows GOLD criteria based on FEV1 percentage, while asthma severity considers both spirometry and symptom frequency. Some patients have overlapping features (asthma-COPD overlap syndrome), and serial PFTs over time can help distinguish between these conditions and track disease progression or treatment response.

From the Research

Interpreting PFTs

To interpret Pulmonary Function Tests (PFTs) and diagnose COPD or asthma, the following steps can be taken:

• Determine if an obstructive defect is present by checking the forced expiratory volume in one second/forced vital capacity (FEV1/FVC) ratio, which is defined as less than 70% or below the fifth percentile based on data from the Third National Health and Nutrition Examination Survey (NHANES III) in adults, and less than 85% in patients five to 18 years of age 2.
• If an obstructive defect is present, determine if the disease is reversible based on the increase in FEV1 or FVC after bronchodilator treatment (i.e., increase of more than 12% in patients five to 18 years of age, or more than 12% and more than 200 mL in adults) 2.
• Asthma is typically reversible, whereas chronic obstructive pulmonary disease is not 2.

Diagnosing COPD

COPD can be diagnosed based on the following criteria:

• A low FEV1/FVC ratio, which is defined as less than 70% or below the fifth percentile based on data from the Third National Health and Nutrition Examination Survey (NHANES III) in adults, and less than 85% in patients five to 18 years of age 2.
• The efficacy of long-acting beta(2)-agonists combined with inhaled corticosteroids (ICS) in the treatment of COPD has also been confirmed for severe and very severe COPD 3.
• Tiotropium has been shown to improve lung function, exacerbation rate, and asthma control, independent of baseline characteristics including age, degree of airway obstruction, and allergic status 4.

Diagnosing Asthma

Asthma can be diagnosed based on the following criteria:

• A low FEV1/FVC ratio, which is defined as less than 70% or below the fifth percentile based on data from the Third National Health and Nutrition Examination Survey (NHANES III) in adults, and less than 85% in patients five to 18 years of age 2.
• Reversibility of the disease based on the increase in FEV1 or FVC after bronchodilator treatment (i.e., increase of more than 12% in patients five to 18 years of age, or more than 12% and more than 200 mL in adults) 2.
• Tiotropium add-on to ICS plus a LABA has been shown to improve lung function and reduce exacerbation risk in patients with symptomatic asthma 4.

Key Considerations

Some key considerations when interpreting PFTs and diagnosing COPD or asthma include:

• The severity of the abnormality is determined by the FEV1 (percentage of predicted) 2.
• If pulmonary function test results are normal, but the physician still suspects exercise- or allergen-induced asthma, bronchoprovocation (e.g., methacholine challenge, mannitol inhalation challenge, exercise testing) should be considered 2.
• Using the lower limit of normal (LLN) to define airflow obstruction would have excluded patients with a higher risk of nonfatal major adverse CV events and a lower risk of exacerbation 5.
• Tiotropium treatment provides clinical efficacy in patients with GOLD stage II disease with an FEV₁ ≥60% predicted, supporting current GOLD guidelines for COPD treatment 6.