Skip to Accessibility Tools Skip to Content Skip to Footer

What Are Pulmonary Function Tests (PFT)?

Pulmonary function tests (PFT) measure both the capacity (size) of the lungs and how well the air flows in and out of the lungs.

The medical team will follow the PFT numbers closely to track how well the lungs are working. Changes in PFT values may mean that an infection has taken hold and needs to be addressed. These numbers also measure long-term damage to the lungs and progression of CF.

Babies and preschool children with CF do not take PFT tests. Only children older than 5 to 6, teens, and adults are asked to perform these tests because the person with CF must be old enough to perform a specific breathing maneuver.1

Types of pulmonary function tests (PFTs)

The two most common types of pulmonary function tests that people with CF take are:

  • Spirometry
  • Diffusion capacity

A spirometer is a machine that measures how well the lungs are working. It records how much air the lungs can take in on a full breath or inspiration. Doctors use this number to monitor the progression of CF and the effectiveness of any treatments.

Diffusion capacity measures while the lungs move oxygen through the air sacs into the rest of the body.2

How a pulmonary function test (PFT) works

The PFT is performed by the respiratory therapist or medical team during visits to the care center. They will enter the person with CF’s current height and weight into the spirometer, as these numbers impact overall test results. The person with CF will also be asked about their breathing, coughing, and the amount of mucus (also called sputum or phlegm) that they are producing.

Then, the person with CF will put a clip on their nose to stop air from entering or leaving. They will put their mouth around a mouthpiece and blow hard into the machine.

What pulmonary function test (PFT) results mean

Each PFT captures how well the lungs are working on that day. It also helps the medical team understand if the lung disease is restrictive (meaning decreased airflow) or obstructive (clogging of the airways).1

The medical team will closely track the person with CF’s test results from one visit to the next, and year after year to watch for changes in their lungs. A sudden decrease in PFT results may mean they have acquired an infection even if there are few other signs yet.2-4

What is forced expiratory volume (FEV1)?

One part of these tests produce a result known as the FEV1 which stands for forced expiratory volume. FEV1 measures the amount of air a person can blow out in the first second of a strong exhalation. FEV1 is a good indication of how much mucus is blocking someone’s largest airways. A higher number is desirable. Lower numbers signal more advanced lung damage.1,4,5

The FEF 25/75 percent (forced expiratory flow) gauges how clear or blocked the small airways are and is especially useful in children. This measures the rate of airflow when 25 percent to 75 percent of the air in the lungs has been exhaled.

FVC, or forced vital capacity, measures the amount of air a person can forcefully and quickly exhale after taking a deep breath.4,5

Other PFT measurements

There are several other pulmonary function tests that a doctor may wish to do in order to understand how the lungs are functioning. Examples of other PFT measurements include:

  • Tidal volume (TV) – the amount of air inhaled and exhaled during normal breathing.
  • Minute volume (MV) – the total amount of air exhaled per minute.
  • Vital capacity (VC) – the total volume of air that can be exhaled after inhaling as much as possible.
  • Functional residual capacity (FRC) – the amount of air left in the lungs after a normal exhale.
  • Residual volume (RV) – the amount of air left in the lungs after exhaling as much as possible.
  • Total lung capacity (TLC) – the total volume of the lungs when filled with as much air as possible.
  • Peak expiratory flow rate (PEFR) – the fastest rate that air can be forced out of the lungs.4

These tests are used for people with all types of lung disease, including asthma, COPD, emphysema, chronic bronchitis, and pulmonary fibrosis.4

Written by: Jessica Johns Pool | Last reviewed: September 2019
  1. Beydon N, et al. Pulmonary Function Tests in Preschool Children with Cystic Fibrosis. Am J Respir Crit Care Med. 2002 Oct 15;166(8):1099-104. https://doi.org/10.1164/rccm.200205-421OC.
  2. Stanford Health Care. Pulmonary Function Test for Cystic Fibrosis. Available at: https://stanfordhealthcare.org/medical-conditions/chest-lungs-and-airways/adult-cystic-fibrosis/diagnosis/pulmonary-function-tests.html. Accessed 4/25/2019.
  3. Cystic Fibrosis Foundation. Basics of Lung Care. Available at: https://www.cff.org/Life-With-CF/Treatments-and-Therapies/Airway-Clearance/Basics-of-Lung-Care. Accessed 4/25/2019.
  4. Johns Hopkins Medicine. Pulmonary Function Tests. Available at: https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies/pulmonary-function-tests. Accessed 4/25/2019.
  5. Taylor-Robinson D, Whitehead M, Diderichsen F, et al. Understanding the natural progression in %FEV1 decline in patients with cystic fibrosis: a longitudinal study. Thorax 20