Pulmonary Function Test (PFT): What to Expect and What Results Mean

What is a pulmonary function test and why might I need one?

A pulmonary function test measures how well your lungs work. The term PFT covers a family of tests; spirometry is the most common and can be done in most outpatient settings. More detailed testing (including lung volume measurement and diffusion capacity) is typically done in a specialized pulmonary function laboratory.

You might be referred for a PFT to: - Diagnose the cause of persistent shortness of breath or cough - Confirm a suspected diagnosis of asthma or COPD 1Ref 1Global Initiative for Chronic Obstructive Lung Disease (2024).Global Strategy for the Diagnosis, Management, and Prevention of Chronic Obstructive Pulmonary Disease (2024 Report).Role of spirometry and FEV1/FVC ratio in diagnosing and staging COPD; GOLD severity classification using FEV1 percent predicted; guidance on bronchodilator reversibility testing2Ref 2Global Initiative for Asthma (GINA) Science Committee (2024).Global Strategy for Asthma Management and Prevention.Use of spirometry in asthma diagnosis; reversibility testing with bronchodilators; interpretation of FEV1/FVC and bronchodilator response in distinguishing asthma from other obstructive conditions - Track how a known lung condition is responding to treatment - Screen for lung disease in people with a significant smoking history - Assess lung function before major surgery - Monitor lung effects of chemotherapy, radiation, or certain medications - Evaluate disability or occupational lung disease claims

What happens during spirometry?

The test is straightforward, though it does require effort and cooperation:

1. You sit upright and a clip is placed on your nose so all breathing goes through your mouth. 2. You seal your lips around a disposable mouthpiece attached to the spirometer. 3. The technician asks you to breathe normally for a few breaths to establish your resting pattern. 4. Then: take the deepest breath you possibly can, then blast all the air out as hard and fast as possible until you cannot exhale any more — this takes about six seconds. 5. You typically repeat this three times. The technician looks for at least two consistent efforts. 6. If you are being tested for reversible airway obstruction (such as asthma), you may receive an inhaled bronchodilator (like albuterol) and repeat the test 15 minutes later to see if your lung function improves.

The effort is real — it can feel uncomfortable, like blowing a trumpet very hard. Some people feel lightheaded briefly from the forceful exhalation. This passes quickly. The test is otherwise safe and suitable for most adults.

What do the key numbers mean?

FVC (Forced Vital Capacity): The total volume of air you can force out in one breath after a maximal inhalation. A low FVC suggests either that the lungs cannot fully expand (restrictive problem) or that air is trapped (obstructive problem).

FEV1 (Forced Expiratory Volume in 1 Second): How much air you can blow out in the first second of a full forced breath. This is a measure of airflow and is reduced in obstructive conditions that narrow the airways.

FEV1/FVC ratio: This is the key ratio. In health, most adults can blow out more than 70% of their vital capacity in the first second. A ratio below 70% (or below the age-adjusted lower limit of normal) suggests obstructive disease — air is struggling to get out, as in asthma or COPD. 1Ref 1Global Initiative for Chronic Obstructive Lung Disease (2024).Global Strategy for the Diagnosis, Management, and Prevention of Chronic Obstructive Pulmonary Disease (2024 Report).Role of spirometry and FEV1/FVC ratio in diagnosing and staging COPD; GOLD severity classification using FEV1 percent predicted; guidance on bronchodilator reversibility testing2Ref 2Global Initiative for Asthma (GINA) Science Committee (2024).Global Strategy for Asthma Management and Prevention.Use of spirometry in asthma diagnosis; reversibility testing with bronchodilators; interpretation of FEV1/FVC and bronchodilator response in distinguishing asthma from other obstructive conditions

TLC (Total Lung Capacity) and RV (Residual Volume): Measured in a full PFT lab using a body plethysmograph or gas dilution technique. These detect restriction (low TLC) and air trapping (high RV).

DLCO (Diffusing Capacity of Carbon Monoxide): Measures how well gases cross from the air sacs into the blood. Reduced DLCO can indicate emphysema, pulmonary fibrosis, or anemia.

Results are always compared against predicted normal values based on your age, height, sex, and ethnicity — not against a single universal number. A result of 80% predicted is interpreted differently than 50% predicted.

What patterns of results indicate different conditions?

Obstructive pattern (reduced FEV1/FVC ratio): Seen in asthma, COPD, bronchiectasis. The airway is narrowed and air cannot exit quickly. If FEV1 improves significantly after a bronchodilator, this suggests reversible obstruction — more characteristic of asthma. Minimal improvement is more typical of COPD. 1Ref 1Global Initiative for Chronic Obstructive Lung Disease (2024).Global Strategy for the Diagnosis, Management, and Prevention of Chronic Obstructive Pulmonary Disease (2024 Report).Role of spirometry and FEV1/FVC ratio in diagnosing and staging COPD; GOLD severity classification using FEV1 percent predicted; guidance on bronchodilator reversibility testing2Ref 2Global Initiative for Asthma (GINA) Science Committee (2024).Global Strategy for Asthma Management and Prevention.Use of spirometry in asthma diagnosis; reversibility testing with bronchodilators; interpretation of FEV1/FVC and bronchodilator response in distinguishing asthma from other obstructive conditions

Restrictive pattern (reduced FVC and TLC, normal or elevated FEV1/FVC ratio): Seen in pulmonary fibrosis, chest wall disease, obesity, or neuromuscular weakness. The lung volume itself is reduced.

Mixed pattern: Both obstruction and restriction are present — this can occur in someone who has, for example, both COPD and obesity.

Normal: A normal spirometry in someone with ongoing symptoms does not end the workup. Your clinician may order additional tests or refer to a pulmonologist.

How should I prepare for a PFT?

To get the most accurate result:

• Avoid short-acting inhalers (e.g., albuterol/salbutamol) for 4–6 hours before the test, unless your clinician specifically says otherwise.
• Avoid long-acting bronchodilators for 12–24 hours before, if your clinician instructs. Ask about your specific medications.
• Do not smoke for at least 4 hours before.
• Avoid caffeine and large meals for 2 hours before — a full stomach can limit deep breathing.
• Wear loose, comfortable clothing — tight clothing around the chest or abdomen affects results.
• Bring your usual inhalers to the appointment so staff know what you use.
• Tell the technician if you have had recent eye surgery, a pneumothorax, or if you feel faint during or after the test.

A Gale primary-care clinician can order spirometry where available and interpret your results, or refer you to a pulmonologist for a full PFT lab evaluation .