What is Pulmonary Function Testing?

Reading Time: 5 minutes
Picture of Stephen Biehl, RRT, RPFT
Stephen Biehl, RRT, RPFT

Respiratory therapist specializing in lung health investigations. If not I’m the unit, find me in the
PFT lab, bronch suite, rehab, or taking the stairs.

The Pre-brief

Not a day goes by where a patient doesn’t tell me how much they hate pulmonary function testing. They will say they can’t do it, they don’t have enough air, or that they fail the test. But what really are pulmonary function tests, and what must the patient do for the test? Here is where we will break down each part of the PFT, beginning with spirometry.

What is Spirometry?

Spirometry is the most common form of pulmonary function testing, and the basis for interpretation. It is a maximal forced effort. The test looks to see what amount of maximal air a patient can inspire and expire. The key wording here is maximal. With this effort, we can see the Forced Vital Capacity (FVC), the Forced Expiratory Volume in the first second (FEV1), and the Peak Expiratory Force (PEF). All three will reveal if the test is acceptable, reproducible, and how the patient did.

Figure 1. 1.) PEF is measured on the y-axis 2.) FEV1 is measured on the x axis after 1 second 3.) FVC measured on the x-axis through the entirety of the expiratory phase.

 The need to assess the respiratory status of patients is a must. For this reason, spirometry will help us do this. We can monitor disease progression, determine if and what interventions are needed, assess airway responsiveness to medications, measure disease severity, perform pre-op risk assessments, research, as well as assess prognosis, among many other reasons.

Predicted Values

Several key factors about each patient need to be recorded for predicted values. These include the patient’s height, weight, date of birth, birth sex, and ethnicity. And while the patient may identify as a different gender than their birth sex, it is important to record the birth sex, as this will determine the predicted size of the lungs. Likewise, ethnicity should be recorded according to the Global Lung Function Initiative, or the technician/therapist performing the test should make note for interpretation purposes.


When it comes to the actual test, it can easily be divided into five parts. The test will begin in a seated position, nose clips on, and a mouthpiece in the mouth, with normal tidal volume breathing. Then, after a few recorded breaths, there is the maximal inspiration to initiate the maneuver. This is the phase where the patient inhales deeply and quickly. The larger the breath, the better. Next is the forceful blow, or blast, to initiate exhalation. The faster this blow, the better. This fast blow transitions into a continued forced blow until the patient reaches a complete exhalation, at least six seconds and maximum of fifteen seconds. At that point, the patient will inhale deeply and quickly again and the maneuver will be complete.

Several factors can affect the test at different points. If the initial quick inspiration is slow, or if the patient pauses after inspiration, there can be a reduction in the PEF. The patient needs positive coaching throughout the maneuver to perform at their best during the test. If the patient begins to experience syncope, the test should be stopped. To help relieve this feeling, the patient may ease on the forcefulness of the blow after four seconds, but this in turn is not a true forced maneuver then.

Validity of Results

To verify the test was acceptable, the technologist should look for key factors. Are at least two FVCs within 150cc of each other? Are at least two FEV1s within 10%. And are at least two PEFs also within 10%. Did the patient exhale for long enough? The patient should exhale for at least six seconds, with either less than 0.025 L change in volume in the last second, or achieve a forced exhalation time of 15 seconds. Any deviation from these may result in the test not meeting ATS criteria and the therapist should have the patient perform the test again.

Reported Results

The values that do eventually get reported may not all be from the best test. While the patient must perform at least three acceptable maneuvers, they may perform more. The best values from all the tests should be on a separate report. These values include the largest FVC, as well as the largest FEV1. The best PEF performed should also be recorded. From here, you can calculate the FEV1/FVC.

Patient Preparation

There are some things that patients should avoid prior to testing. And since spirometry is often recorded in pre and post-bronchodilator, certain medications should be avoided to effectively look at bronchodilator response. Patients should refrain from short-acting beta-agonists (SABA) for 4-6 hours, short-acting muscarinic antagonists (SAMA) for at least 12 hours, long-acting beta-agonists (LABA) for 24 hours, and long-acting muscarinic antagonist (LAMA) and ultra-LABAs for at least 36 hours. Patients should also refrain from vigorous exercise at least one hour prior, smoking and/or vaping at least one hour, and using intoxicants at least 8 hours prior. It is also helpful if patients wear light fitting clothes for testing.


There are some relative contraindications for testing. Increases in myocardial demand, or if they have changes in blood pressure (ex: acute MI ~ 1 week, hypo/severe hypertension, significant arrhythmias, uncontrolled P-HTN, acute cor pulmonale, syncope, unstable pulmonary embolism). Increases in intraocular or intracranial pressure (ex: aneurysm, brain surgery ~ 4 weeks, eye surgery ~ 1 week). Increases in middle ear or sinus pressure (ex: sinus or middle ear surgery/infection ~1 week). If a patient cannot follow directions or is not willing to perform testing, results will be questionable. And while adverse results to testing are limited, they have been reported as 5 in every 10,000 routing PFTs by the ATS. Of these, the most common is syncope. It is advised if the patient has contraindications, they may benefit from having the test performed in a pulmonary function lab rather than a primary care office due to safety concerns.

Special Considerations

It is also important to note the ethnicity of patients performing testing when interpreting. While the Global Lung Initiative separates patients into four different classifications (Caucasians, African Americans, North Asian, and South Asian), the reference values for NHANES III only breaks patients down to Caucasian, African-American, and Hispanic Americans. And according to Paul Enright in the ATS Journals – June 2016, studies have found that Africans and Asians have a 20-30% smaller FVC than Caucasians if everything else remains constant, but after relocating to America, their descendants have increased lung capacities. This may possibly be due to socioeconomic conditions. These results can lead to a patient appearing healthier than they really are, which will lead to a delay in care and adverse results for the patient.

The Debrief

Although the spirometry portion of pulmonary function testing isn’t complicated, it can be a bit challenging to perform. It’s understandable how patients could feel like they don’t have enough air or that they failed the test. As I tell all of my patients, if you feel like you have air left when we finish, we did something wrong. And that the test will always feel like it wins. But the testing does provide crucial information that will dictate how they are treated and what interventions are to be done.


More Posts

Related Posts

Would love your thoughts, please comment.x