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Lung function basics

The process of measuring lung function capacity — or the ability to inhale or exhale — is known as pulmonary function testing (PFT) or spirometry. These tests serve to help determine overall lung function, which can help diagnose chronic lung diseases, including asthma and Chronic Obstructive Pulmonary Disease (COPD).

Below, we've compiled this easy to follow and understand series of frequently asked questions (FAQs) about lung function basics.

Spirometry basics


What is a diagnostic spirometer?

A diagnostic spirometer is a medical device that is used to measure lung function. It can be a portable or stationary device that is operated by a medical professional who is specially trained in proper testing procedures and techniques to produce a more consistent and reliable result.


What is spirometry?

Spirometry is the primary method of assessing overall lung function. It works by measuring the volume of air that a patient can forcefully expel from the lungs after inhaling as much as possible. This non-invasive test is completed by medical professionals.


What is spirometry used for?

Spirometry is a pulmonary function test (PFT) used to diagnose lung diseases, including COPD, asthma, and other chronic and non-chronic respiratory conditions. Outside of pulmonary or cardiovascular health care, it is commonly used in occupational health to assess an employee's fitness to wear personal protective equipment (PPE) such as a respirator.

Spirometry is also used to monitor the employee’s lung health over time. Many industries utilize spirometry, such as coal mining, warehousing, manufacturing, and others, that introduce risk factors in the work environment.


What indications are used for spirometry?

There are multiple patient-symptoms or medical conditions that justify the use of spirometry. A few of the most common indicators include:

  • To manage asthma and COPD patients
  • Evaluate shortness of breath
  • Perform surveillance for occupational-related lung disease
  • Evaluate former or current smokers over the age of 45 for COPD
  • Classification of COPD. (COPD is an umbrella term that covers multiple respiratory diseases including chronic bronchitis, emphysema, and idiopathic pulmonary fibrosis (IPF))
  • Monitor disease progression
  • Measure response and progress due to treatment, medication, and respiratory therapy
  • Smoking cessation

How long does a spirometry test take?

A spirometry test is non-invasive and can take as little as 10-minutes to complete.


How do you perform spirometry?

There are multiple steps for completing a spirometry test. For visual reference, you can review a video on how to perform a spirometry test by clicking the video button below.

Watch the video

Take time to create a rapport with the patient—effective communication and participation between the patient and medical professional results in more consistent and robust testing results.


Accurately measure the patient’s height.


Carefully and correctly enter the patient data into the spirometer.


Demonstrate the correct techniques and maneuvers to the patient prior to completing the test.


Use body language to coach for a maximal inhalation.


Loudly prompt them to BLAST out the air.


For the next six seconds, quietly encourage the patient to continue BLASTING by saying, “keep going, keep going.”


Monitor the patient carefully during the maneuvers.


Obtain a good test session with a quality grade (A or B).


What are the important parameters in spirometry?

A spirometry test will produce a series of results or parameters that help medical professionals easily complete this exam. The three most important spirometry parameters are the FEV1, FVC, and FEV1/FVC ratio parameters.


What is FEV1 and what does it tell you?

FEV stands for Forced Expiratory Volume. The FEV1 parameter equates to the volume that is exhaled after one second. It is intended to measure the severity of an obstruction – with average FEV1 results measuring two to four liters. A lower FEV1 result equates to higher obstruction.


What is FVC and what does it tell you?

FVC stands for Forced Vital Capacity. FVC determines the total exhaled volume of air. FVC tells the spirometry tester how much air volume the patient can exhale. The FVC will fall when the patient can’t inhale deeply or can’t exhale completely. The average FVC result for an elderly man is about four liters (one gallon) of air. The intent of this parameter is to detect restriction.


What is the FEV1/FVC Ratio and what does it tell you?

The FEV1/FVC ratio is typically expressed as a percentage (such as 75%). Nearly three-fourths of lung volume can normally be exhaled during the first second of the spirometry test. As such, the normal ratio falls between 65 to 85 percent. The range of normal FEV1/FVC ratio is determined by the patient's age.


How to coach a patient to perform spirometry

The typical spirometry exam is a three-phased process – each with their own coaching techniques to help patients produce an effective test.


Phase 1
In the first phase, the goal is to instruct the patient on the proper techniques to apply their mouth to the spirometry mouthpiece. Best results are achieved when the patient takes a deep, deep breath, placing the mouthpiece between their teeth, and sealing the mouthpiece with their lips.


Phase 2
The second phase is the BLAST. This is when the patient will blow out as hard and as fast as possible. Coaching a patient in this phase is best achieved by developing a rapport with them prior to the exam, as the tester will learn which level of coaching is best for that individual patient.


Phase 3
The final phase is the remaining 5 to 6 seconds of exhalation. In this phase, you’ll quietly encourage the patient to “keep blowing” to complete the exhalation.

Pulmonary function basics


What is a pulmonary function test?

A pulmonary function test (PFTs) are a group of noninvasive tests that monitor the performance of the lungs. They measure lung volume, capacity, rates of flow, and gas exchange.


What is the difference between spirometry and a pulmonary function test (PFT)?

A spirometry test is a specific type of pulmonary function test. Spirometry measures the lungs volume (or how much) and flow (how quickly) the patient can move air into and out of their lungs. Spirometry will tell the tester if the patient has an obstruction, restriction, a mixed defect, or has normal lung flow.

Spirometry does NOT look at gas exchange and does not provide absolute lung volumes (RV, FRC, and TLC).

Diffusion capacity or transfer factor of the lung for carbon monoxide (CO) is known as a DLCO test. This measures the gas exchange and is used in conjunction with spirometry to provide a differential diagnosis. DLCO is also used to assess disease severity and is one of the best correlates of emphysema in COPD.

The final component that completes the full PFT exam is measuring absolute lung volume (RV, FRC, and TLC). This is completed by measuring body plethysmography, gas dilution, or nitrogen washout. Lung volumes are commonly used for the diagnosis of restriction. In obstructive lung disease, they are used to assess for hyperinflation. The changes in lung volumes can also be seen in a number of other clinical conditions.


What is a complete pulmonary function test?

A complete PFT includes spirometry (possibly with pre/post-bronchodilator), diffusion capacity (DLCO), and absolute lung volumes.

DLCO basics


What is Diffusion Capacity (DLCO) testing?

Diffusion capacity (DLCO) is a clinically useful test that provides a quantitative measure of gas transfer from the lungs to the blood. It complements spirometry in evaluating and managing patients with cardiac and/or respiratory disease.


When would you use DLCO testing?

Diffusion capacity testing (DLCO) is best used to pinpoint the type of respiratory disease. It is completed after spirometry when an obstruction or lung volume issues are predetermined.

  • Differentiating emphysema from obstructive bronchitis and chronic asthma
  • Assessment of COPD
  • Detection of pulmonary vascular disease
  • Assessment of shortness of breath (SOB)
  • Disability/Impairment evaluations for ILD or COPD

What are the disease states that are associated with low Diffusion Capacity (DLCO)?

  • Emphysema
  • Interstitial Lung Disease
  • Idiopathic Pulmonary Fibrosis
  • Cystic Fibrosis
  • Congestive Heart Failure
  • Primary Pulmonary Hypertension
  • Chronic Pulmonary Emboli
  • Anemia
  • Left-sided heart disease (systolic and diastolic heart failure, Mitral Valve Disease)

What are the disease states that are associated with high Diffusion Capacity (DLCO)

  • Asthma
  • Left-to-right intracardiac shunts
  • Polycythemia
  • Pulmonary hemorrhage

Lung volume basics


What will lung volumes show you?

Lung volumes are used to diagnose restrictive lung disease and to assess for hyperinflation in obstructive lung disease. With restrictive lung disease, TLC, VC, and RV may be decreased due to the inability of the lungs to expand properly.

The lungs are restricted from fully expanding and filling with air. In an obstructive disease such as COPD, patients may develop hyperinflation, which will result in a higher than normal TLC and RV. Conversely, IC will be decreased due to air trapping in the lungs.


What are examples of restrictive lung disease?

A restrictive lung disease is exactly that – one where a restriction occurs in the respiratory system.

A few examples of restrictive lung disease include:

  • Interstitial lung diseases such as idiopathic pulmonary fibrosis
  • Sarcoidosis
  • Obesity
  • Scoliosis
  • Neuromuscular diseases such as muscular dystrophy or ALS

What are examples of obstructive lung diseases?

A few examples of obstructive lung disease include:

  • Chronic Obstructive Pulmonary Disease (COPD)
  • Emphysema
  • Chronic Bronchitis
  • Asthma
  • Bronchiectasis
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