Pulmonary Function Testing (PFTs)
Pulmonary function tests measure how well the lungs move air in and out, how much air they can hold, and how efficiently they transfer oxygen into the bloodstream. They are among the most common tests ordered to evaluate breathing symptoms.
What are pulmonary function tests?
Pulmonary function tests (PFTs) are a group of non-invasive breathing tests performed in a clinic or hospital lab. They help clinicians understand the type and severity of lung problems.
A complete PFT session typically includes three main components: spirometry (airflow), lung volumes (capacity), and diffusion capacity (gas exchange). Some patients may only need one component depending on the clinical question.
PFTs are painless and involve breathing into a mouthpiece connected to a machine. A nose clip is worn so all air flows through the mouth. The technician will coach you through each maneuver.
When are PFTs ordered?
- Unexplained shortness of breath, chronic cough, or wheezing
- Diagnosing or monitoring asthma, COPD, or interstitial lung disease
- Evaluating the severity of a known lung condition
- Pre-operative assessment before lung or other major surgery
- Monitoring the effects of medications or occupational exposures
- Screening patients with risk factors such as smoking history
- Evaluating disability or impairment for work-related claims
The three main parts of a PFT
1. Spirometry — Airflow
Spirometry is the most common pulmonary function test. You take the deepest breath possible and blow out as hard and fast as you can into the mouthpiece until your lungs are completely empty. This is repeated several times to ensure reliable results.
Key measurements include:
- FVC (Forced Vital Capacity) — the total amount of air you can forcefully blow out after a full breath. A low FVC can suggest restriction (the lungs cannot fully expand) or air trapping.
- FEV1 (Forced Expiratory Volume in 1 second) — how much air you blow out in the first second. This is the single most important number for detecting airflow obstruction.
- FEV1/FVC ratio — the proportion of your total air that comes out in the first second. A low ratio (generally below 0.70) is the hallmark of obstructive diseases like asthma and COPD.
2. Lung Volumes — Capacity
Lung volume testing measures how much air the lungs can hold in total, including air that cannot be exhaled. This is typically done in a body plethysmograph — a clear, phone-booth-sized chamber — or sometimes with a gas dilution technique.
Key measurements include:
- TLC (Total Lung Capacity) — the total amount of air the lungs can hold. Low TLC confirms restriction. High TLC can indicate air trapping or hyperinflation seen in emphysema.
- RV (Residual Volume) — the air remaining in the lungs after you exhale completely. An elevated RV suggests air trapping.
- FRC (Functional Residual Capacity) — the amount of air left in the lungs after a normal, relaxed breath out.
Sitting in the plethysmograph is painless. The door closes briefly while you breathe through the mouthpiece, and internal pressure changes allow precise volume measurement.
3. Diffusion Capacity (DLCO) — Gas Exchange
The DLCO test measures how well oxygen passes from the air sacs of the lungs into the bloodstream. You breathe in a small, harmless amount of carbon monoxide (a tracer gas), hold your breath for about 10 seconds, and then exhale. The machine measures how much of the tracer was absorbed.
A low DLCO can indicate:
- Interstitial lung disease (pulmonary fibrosis) — scarring thickens the barrier between air and blood
- Emphysema — destruction of air sacs reduces the surface area for gas exchange
- Pulmonary hypertension — changes in lung blood vessels
- Anemia — fewer red blood cells to carry the tracer gas
An elevated DLCO can sometimes be seen with conditions such as asthma, obesity, or pulmonary hemorrhage.
Other tests sometimes included
Bronchodilator Reversibility
If spirometry shows obstruction, you may be given an inhaled bronchodilator (typically albuterol) and then repeat spirometry 15–20 minutes later. Significant improvement suggests the obstruction is at least partially reversible, which is characteristic of asthma.
COPD may also show some reversibility, but typically less than asthma. The response (or lack of response) helps guide diagnosis and treatment decisions.
Bronchoprovocation (Methacholine Challenge)
If asthma is suspected but spirometry is normal, a bronchoprovocation test may be performed. You inhale increasing doses of methacholine, a substance that temporarily narrows the airways in people with hyperreactive airways. Spirometry is repeated after each dose.
A significant drop in FEV1 (typically ≥ 20%) at a low dose confirms airway hyperreactivity, supporting an asthma diagnosis. A bronchodilator is given afterward to reverse the effect.
Respiratory Muscle Pressures (MIP/MEP)
These tests measure the strength of the muscles used for breathing. You breathe against a closed valve — breathing in hard (MIP, or maximal inspiratory pressure) and breathing out hard (MEP, or maximal expiratory pressure).
Low values can indicate respiratory muscle weakness from neuromuscular conditions, diaphragm dysfunction, or deconditioning.
Six-Minute Walk Test (6MWT)
While not technically a PFT, the six-minute walk test is frequently performed alongside PFTs. You walk at your own pace along a flat corridor for six minutes while your oxygen level and heart rate are monitored.
The total distance walked and any drop in oxygen saturation help clinicians assess functional capacity and supplemental oxygen needs.
Patterns in PFT results
Obstructive Pattern
An obstructive pattern means airflow is blocked or slowed, usually from narrowed or inflamed airways. The FEV1/FVC ratio is low, and air may get trapped in the lungs (elevated RV).
Common causes: asthma, COPD, bronchiectasis.
Restrictive Pattern
A restrictive pattern means the lungs cannot fully expand. The TLC is low, the FVC may be low, but the FEV1/FVC ratio is usually normal or even high (because whatever air is present comes out quickly).
Common causes: pulmonary fibrosis, chest wall deformity, obesity, neuromuscular weakness.
Mixed Pattern
Some patients show features of both obstruction and restriction. This can occur in conditions where two problems coexist, such as COPD with obesity, or combined pulmonary fibrosis and emphysema.
How results are reported
PFT results are compared to predicted values based on age, height, sex, and race/ethnicity. Results are reported as a percentage of the predicted value. Modern guidelines (GLI, or Global Lung Initiative) use a statistical approach called the lower limit of normal (LLN) rather than fixed cutoffs to determine whether a result is abnormal.
Severity of abnormalities is typically graded as mild, moderate, moderately severe, severe, or very severe based on how far the FEV1 falls below predicted.
What to know before a PFT
Before the test
- You may be asked to hold certain inhaler medications before the test — follow your clinic's specific instructions
- Avoid smoking for at least 4–6 hours before testing
- Avoid heavy meals within 2 hours of the test
- Avoid vigorous exercise for at least 30 minutes before
- Wear comfortable, non-restrictive clothing
- Bring a list of your current medications
During the test
- A nose clip will be placed on your nose
- You will breathe through a mouthpiece — it is important to form a tight seal with your lips
- The technician will coach you through each breathing maneuver
- Some maneuvers require maximal effort — blow as hard and as long as you can when asked
- Each maneuver is repeated multiple times to ensure consistent, reliable results
- A complete PFT session usually takes 30–60 minutes
- You may feel lightheaded briefly after forceful blowing — this is normal and passes quickly