Pulmonary Function Testing

What Pulmonary Function Testing Evaluates

Pulmonary function testing (PFT) is not a single test but a family of breathing tests that together assess how well the lungs move air and exchange gas. Rather than looking at the lungs directly the way an imaging study does, PFT measures how the lungs actually perform.

Lung Capacity

How much air the lungs can hold.

Airflow Speed

How quickly air moves in and out of the lungs.

Gas Exchange

How efficiently oxygen crosses from the air you breathe into your bloodstream.

Physicians order PFT for two broad reasons:

A full PFT session may combine several components, each answering a different question about how the lungs are functioning. Not every patient needs every component: your physician selects which parts of the battery are appropriate based on your symptoms and history. The sections below describe what each component evaluates at a conceptual level.

Spirometry: Measuring Airflow

Spirometry is the most common and most frequently performed component of pulmonary function testing. It measures how much air you can forcefully exhale after a full breath in, and how quickly you can exhale it. Together, those measurements help assess whether anything is narrowing or obstructing the airways as air moves out of the lungs.

Also a Standalone Test

Because spirometry is so central to lung function evaluation, it is also frequently ordered and performed as its own standalone test, separate from a full PFT battery.

Full Detail on Its Own Page

How the test is measured and how specific results are classified is covered on the dedicated spirometry page. This page focuses on how spirometry fits alongside the other PFT components as part of a broader evaluation.

Lung Volumes: Measuring Lung Capacity

Narrowing Problem

Airflow limitation on spirometry with normal overall lung capacity.

Capacity Problem

Reduced lung capacity with relatively normal airflow speed on spirometry.

Lung volume measurement is typically performed using specialized equipment, such as a sealed chamber called a body plethysmograph, or a gas-dilution technique: the specific method used depends on the equipment available and the clinical question being asked.

DLCO: Measuring Gas Exchange

Bronchodilator Testing

Bronchodilator testing repeats spirometry after you inhale a bronchodilator medication, a type of medication that temporarily relaxes and opens the airways.

1. 01→Baseline SpirometryThe technician performs the initial spirometry measurements.↓
2. 02Medication AdministeredYou inhale the bronchodilator medication.↓
3. 03→Wait PeriodThe technician waits a set period of time for the medication to take effect.↓
4. 04Repeat SpirometryThe spirometry measurements are repeated so they can be compared with the baseline.

Comparing your breathing measurements before and after the medication helps physicians see whether airflow limitation improves significantly with treatment: a pattern often described as reversibility. This comparison is particularly relevant when evaluating conditions like asthma and COPD, since the degree of reversibility can be one factor, among several, that helps distinguish between the two or informs treatment decisions.

Obstructive vs. Restrictive Patterns

PFT results are often described using two broad conceptual categories that describe the general shape of a breathing problem:

Obstructive Patterns

Generally reflect difficulty getting air out of the lungs. Commonly associated with conditions such as asthma and COPD, where the airways are narrowed, inflamed, or partially blocked, making it harder to exhale fully and efficiently.

Restrictive Patterns

Generally reflect reduced overall lung capacity, or difficulty getting air in. Commonly associated with conditions such as interstitial lung disease, where the lungs themselves (or sometimes the chest wall or respiratory muscles) are less able to expand fully.

Some patients show a mixed pattern with features of both, and some conditions can evolve or overlap over time. This is intentionally a simplified, patient-friendly distinction meant to convey the general idea rather than a diagnosis.

The detailed technical criteria a physician uses to classify a specific spirometry result (including the specific measurements and ratios involved), belong to a physician’s clinical interpretation and are discussed in more depth on the spirometry page.

Why Results Are Read as a Pattern, Not a Single Number

Because of this, it is not accurate or safe to try to self-diagnose a lung condition from a single value on a lab printout, or to compare your own results against a general reference range without clinical context. If you have questions about what your specific results mean, that conversation belongs with the physician who ordered your testing.

How PFT Supports Asthma and COPD Evaluation

Pulmonary function testing, and spirometry with bronchodilator testing in particular, plays an important role in evaluating suspected asthma and COPD, and in monitoring both conditions once diagnosed.

In general, PFT results are one important piece of evidence among several (alongside your reported symptoms, your history of triggers or exposures, findings on physical examination, and sometimes additional testing) rather than a standalone diagnostic answer on their own.

For monitoring, periodic PFT can help a physician see whether a condition is stable on current treatment, whether lung function is declining over time in a way that warrants a change in management, or whether symptoms reported between visits correspond to measurable changes in lung function.

How to Prepare

Preparation instructions vary somewhat depending on which PFT components are being performed and why, so always follow the specific guidance given to you ahead of your appointment. In general, your technician or physician may ask you to:

• Avoid certain inhaled medications, such as short-acting or long-acting bronchodilators, for a specified period before the test if bronchodilator response is being assessed, since taking them beforehand could mask the result the test is designed to capture.
• Avoid smoking and vigorous exercise for a period before testing, since both can temporarily affect breathing measurements.
• Avoid large meals shortly before testing, since a very full stomach can make deep breathing efforts more difficult.
• Wear loose, comfortable clothing that does not restrict deep breathing.

If you take daily inhaled medications for a chronic condition, do not stop them on your own: confirm with your physician’s office which medications, if any, should be held before your specific test.

What to Expect During the Test

During testing, a technician will guide you through each portion of the PFT:

1. 01→SpirometrySit upright, place a clip on your nose, seal your lips around a mouthpiece, take the deepest breath you can, then exhale as forcefully and completely as possible until instructed to stop.↓
2. 02→Lung Volume TestingSit inside an enclosed chamber or breathe a small amount of a harmless tracer gas, depending on the method used.↓
3. 03DLCOTake a single deep breath of a gas mixture, hold it briefly, and then exhale.

The effort required, especially for spirometry, can feel tiring or effortful (similar to a hard physical effort) but the test itself is not painful. A full PFT session combining multiple components typically takes somewhere between 30 and 60 minutes, depending on how many components are included and how many repeated efforts are needed to obtain quality results.

Test Quality and Reproducibility

Because results depend heavily on your effort and technique, you will typically be asked to repeat certain breathing efforts, particularly for spirometry, several times in a row. This is a normal and expected part of the test, not a sign that something went wrong.

The technician checks whether your repeated efforts are reasonably consistent with one another, a property referred to as reproducibility, before accepting the results as reliable enough to interpret.

If your early efforts are not reproducible, the technician may coach you further on technique or ask for additional attempts. This quality-control step matters because a poorly performed effort can produce a misleadingly low result that does not reflect your true lung function.

Pulmonary Function Testing in Children

Older children who are able to understand and follow the breathing instructions (including taking a maximal breath in and then exhaling forcefully and completely on cue), can often complete standard pulmonary function testing, including spirometry.

Younger children, or children who have difficulty cooperating with the specific breathing maneuvers required, may have lower-quality or less reliable results, and a physician may take that into account when interpreting them or may recommend alternative approaches to evaluation.

What Happens Next

After testing, your results are reviewed by a physician alongside your symptoms and medical history to help guide next steps, which may include starting or adjusting treatment, ordering additional testing such as imaging, or scheduling follow-up pulmonary function testing to monitor your condition over time.

How often testing is repeated for ongoing monitoring depends on the specific condition being tracked and your physician’s recommendation: ranging from routine periodic checks to more frequent testing during a change in symptoms or treatment.