Oct 17, 2025 How to Interpret Pulmonary Function Tests: A Quick Reference

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Pulmonary function tests (PFTs) are important measures that healthcare providers use to understand how well a patient’s lungs function. PFTs measure factors like the volume of air exchanged when a patient inhales and exhales, and how well the lungs absorb oxygen and process it into the bloodstream. These metrics can help clinicians diagnose a wide range of obstructive and restrictive pulmonary conditions more precisely. Understanding how to interpret the results of a pulmonary function test correctly is crucial for providing the appropriate treatment to help patients breathe more effectively. 

What Are Pulmonary Function Tests (PFTs)?

A complete pulmonary function test (PFT) includes spirometry, diffusion capacity (DLCO), and lung volume measurements. Older PFTs did not always include diffusion capacity tests, but technologically advanced PFT devices such as the MiniBox+ provide these additional metrics to assist physicians with more granular data that can help them diagnose patients who have more perplexing or complex disease presentations. Just one example of the need for a more robust PFT would be when an allergist tests a patient to determine to what extent they are dealing with asthma, allergies, both, or any other lung conditions. Adding DLCO (diffusion capacity) to spirometry for allergists, pulmonologists, and other healthcare providers can help clinicians more accurately diagnose patients struggling with these and other pulmonary diagnoses:

  • Allergies
  • Asthma
  • Emphysema
  • Chronic Obstructive Pulmonary Disease (COPD)
  • Cystic fibrosis
  • Anemia-related lung conditions
  • Dyspnea
  • Interstitial lung disease (ILD)
  • Pulmonary hypertension
  • Hypoxemia
  • Idiopathic pulmonary fibrosis (IPF)
  • Sarcoidosis

Understanding lung function tests like these also depends on administering the test correctly. A complete PFT is a non-invasive test, but previous versions that involved patients sitting in an enclosed cabinet resulted in stressed patients. Fortunately, newer devices like the MiniBox+ make administering the test much faster, simpler, and comfortable for patients and clinicians alike. Patients need to be comfortable, and clinicians must provide clear instructions for inhaling and exhaling during the test, so the data collected is as accurate as possible. Some tips for collecting high-quality pulmonary function test results include:

  • Tell the patient what to expect during the test to help them feel comfortable.
  • Before the text, explain the breathing procedures so they know what to do:
    • Take in the biggest possible breath to completely inflate the lungs
    • Exhale explosively into the mouthpiece as quickly and completely as possible for a full deflation.
    • Tell the patient not to interrupt the inhalation or exhalation at any point, but to keep the air flowing continuously throughout the course of the test.
  • If you are using PulmOne’s MiniBox+, make sure the patient is comfortably seated in the correct posture, that the noseclip is securely attached, and that the patient has fully closed the space around the mouthpiece.
  • Continue to coach and encourage patients as they breathe through the test, so accurate values can be collected on the first try as often as possible.

Once an accurate and complete PFT has been collected, clinicians need to understand how to interpret pulmonary function tests.

PFT Results Explained: Key Metrics Physicians Should Know

PulmOne’s MiniBox+ collects a wealth of data that physicians, allergists, and other clinicians can use to provide enhanced pulmonary care for their patients. Each patient will display unique “normal” PFT values, but clinicians can use averages based on patient criteria to determine their base PFT normal values, including:

  • Age
  • Weight
  • Height
  • Sex
  • Ethnicity

Clinicians can also use previous tests to measure against current tests to monitor changes over time. A pulmonary function test chart gathers the following metrics:

  • “Forced expiratory volume in the first second, or “FEV1”
  • The amount of air forcefully exhaled after the initial, big inhalation, “Forced vital capacity”, or “FVC”
  • The total amount of air a patient can breathe out after breathing in fully or “vital capacity.”
  • The full volume a patient breathes in and out when breathing normally, or “tidal volume”
  • The total amount of air a patient breathes out in one minute, or “minute volume”
  • The amount of air left over in the lungs after breathing out normally, or “functional residual capacity”
  • The amount of air left in the lungs after the patient exhales as much air as they can, or “residual volume”
  • The total amount of air the lungs can hold after a complete inhalation, or “total lung capacity”
  • The total amount of air a patient breathes out during the first, second, and third seconds of the FVC is called “forced expiratory volume.”
  • The fastest rate at which a patient can force air out of the lungs is called “peak expiratory flow rate.”
  • The average flow rate of air traveling during the middle half of the FVC is called “forced expiratory flow.”

Each one of these metrics can help clinicians form a well-rounded picture of a patient’s pulmonary condition, insofar as the test is administered correctly and the patient has followed the instructions.

Pulmonary Function Test Interpretation Steps

Interpreting a PFT correctly requires four key steps as outlined in the American Thoracic Society (ATS) and European Respiratory Society (ERS) guidelines¹:

1. Validating PFT test quality

Double-check that the PFT was performed according to the ATS and ERS guidelines in a manner that can be reproduced, with an approved pulmonary function test machine such as the MiniBox+.

2. Compare the FEV1 and FVC values with normal lung function using the z-score

Average lung function scores from different population cohorts, such as age, sex, height, weight, etc., are compared with the patient’s results to arrive at a “z-score”² that indicates, relative to those criteria, how well the patient’s lungs are functioning. The more a patient’s z-score deviates from population averages, the more likely the patient is to have inadequate lung function.

3. Calculate the FEV1/FVC Ratio

The clinician assesses the FEV1/FVC ratio and arrives at a decimal value. This value is, again, measured against averages for the patient’s demographic data (height, sex, weight, age, ethnicity). This value can indicate whether the patient has an airway obstruction.

4. Are Results Reversible with a Bronchodilator?

If the patient uses an inhaler (bronchodilators) to manage their condition, can the abnormal PFT scores be reversed or moderated to a score that is closer to the normal range for their demographic averages?

PFT Normal Values vs. Abnormal Results

PFT values can help clinicians determine whether a patient is struggling with an obstructive lung condition or a restrictive lung disease, or a more complex presentation that involves a combination of both obstructive and restrictive lung conditions. Deviations from PFT normal values can provide diagnostic insight.

  • Firstly, normal PFT values that indicate normal lung function will show FEV1 and FVC values above 80% of the predicted value. The FEV1/FVC ratio will be above 0.7.

Obstructive Lung Diseases:

When airflow through the lungs is slowed because of resistance in the airway, an obstructive lung condition exists. Asthma, COPD, and cystic fibrosis are considered obstructive lung diseases.

FEV1 values will fall below 80% of the average predicted value. FVC may be normal or possibly reduced, but usually not as obvious as the lower FEV1 value. The FEV1/FVC ratio will fall below 0.7.

The obstruction can come from excess mucus, inflammation, or muscle contraction. These diseases will cause a lower FEV1/FVC ratio. The lower this value is, the worse the obstruction level.

Restrictive Lung Diseases

Restrictive lung conditions reduce overall lung volume. Obesity, pulmonary fibrosis, heart failure, and pulmonary edema are examples of diseases that can lower the amount of air the lungs can process. Flow values for FEV1/FVC may even be elevated, but FVC and VC values will be lower than normal, as follows:

FEV1 may appear normal or slightly lower. FVC will fall below 80% of the average predicted value. The FEV1/FVC ratio will appear normal, above 0.7.

Interpreting Reversibility and Bronchodilator Response

In the past few years, the American Thoracic Society and European Respiratory Society have updated their guidance about measuring how bronchodilators reverse decreases in lung function.‌³‌ Clinicians can better assess a patient’s condition by performing two PFTs: one where the patient does not use their inhaler, and another in which they do use their inhaler. A positive change of 10% or more in predicted values quantifies a positive response to using a bronchodilator.

Knowing How to Interpret Pulmonary Function Tests Empowers Better Respiratory Decisions

Clinicians who care for patients with pulmonary conditions can provide better patient care with better PFT tools. The PulmOne MiniBox+ is an easy-to-use, portable diagnostic tool that registers fast, precise results while helping patients feel more comfortable so they can perform a more accurate test. Contact us today to learn more about how the MiniBox+ is revolutionizing spirometry for allergists, pulmonologists, and physicians.

 

References

¹PFT Interpretation -Rapid Guide. (n.d.). https://www.thoracic.org/professionals/career-development/ats-virtual-network/resources/2021-rbc-pft-interpretation-card.pdf

²Chhabra, S. K. (2024). Understanding the use of z-scores and LLN in pulmonary function test reports. Lung India, 42(1), 1–3. https://doi.org/10.4103/lungindia.lungindia_477_24

‌³‌American Thoracic Society | Clinical Practice Guidelines, Statements…. (n.d.). American Thoracic Society. https://site.thoracic.org/clinicians-researchers/clinical-practice-guidelines-statements-reports