Pulmonary function tests (PFT) measure how efficiently an individual’s lungs perform their essential tasks. Several separate techniques are used to determine the amount of air the lungs can hold, how well the lungs move air into and out of the body and how effectively the lungs exchange oxygen and carbon dioxide.

There are three primary phases of pulmonary function testing:

• Spirometry
• Lung volume measurement
• Diffusion capacity measurement

However, there are variations on these tests that also can provide physicians with insights into lung disorders. All of these tests provide measures of various lung functions, including tidal volume, carbon monoxide response, total lung capacity and many more. With this information, physicians can accurately diagnose both the nature of a lung disorder and its severity.

There are also a number of at–home tests, such as peak flow meters, that patients can use themselves to monitor lung function.

Pulmonary function tests (PFTs) help physicians to measure lung function and offer clues to the nature and severity of problems affecting an individual’s ability to breathe. The tests record both an individual’s capacity to breathe in (inspiration) and to breathe out (expiration).

PFTs measure several different aspects of lung function to diagnose lung conditions. Lung disorders fall into one of two main categories:

• Obstructive lung conditions. The airways are narrowed and clogged with mucus, increasing the time necessary for the lungs to empty themselves. Inflammation and muscle spasms around the bronchial tubes can also exacerbate the conditions.  Examples of obstructive lung conditions include asthma, bronchitis, chronic obstructive pulmonary disease (COPD) and infections that cause inflammation.
  
  
• Restrictive lung conditions. Total lung volume and gas-exchange efficiency are reduced. Examples of restrictive lung conditions include pneumonia, lung cancer, sarcoidosis, scleroderma and multiple sclerosis. Other factors that cause restrictive lung conditions include chest injuries, obesity and pregnancy. Restrictive lung conditions involve one or more of the following:
  
  
• • Lung tissue is lost
    
    
  • The lung’s ability to expand decreases
    
    
  • The lung loses the ability to efficiently transfer oxygen to the blood and/or carbon dioxide out of the blood

There is no single test that can determine if a lung condition is obstructive or restrictive in nature. It is, therefore, common for more than one type of pulmonary function test to be performed. The three primary types of pulmonary function testing are:

• Spirometry. The most widely used pulmonary function test, it records the amount of air breathed in and out, and the rate at which this process takes place. The device used in this testing is a spirometer, a long piece of tubing with a mouthpiece at one end, and a recording device at the other. Spirometry reveals any degree of narrowing or obstruction of the airways. An abnormally low reading usually indicates an obstructive disorder, such as asthma, bronchitis or emphysema.
  
  
• Lung volume measurement. Records the maximum amount of air the body can hold and helps determine the elasticity of the lungs and rib cage. The most accurate way to obtain this measure is using a body plethysmograph, a sealed, transparent box that looks like a phone booth and determines the volume of gas in the lungs. An abnormally low reading indicates a stiffness that is common in restrictive disorders, such as scarring of the lung tissue (interstitial lung disease) and kyphoscoliosis (curvature of the spine).
  
  
• Diffusing capacity measurement. Determines how efficiently the body is able to transfer oxygen from the lungs into the bloodstream. An abnormal reading indicates that oxygen is not being properly diffused between the lungs and the bloodstream. Poor results in this test might indicate a disorder that affects the blood vessels of the lungs, such as pulmonary fibrosis or emphysema.

There are also a number of at-home tests that patients can perform themselves to monitor lung function.

The results of a pulmonary function test can be used to evaluate a patient in several important ways. Some specific reasons for conducting a pulmonary function test include:

• Diagnosing the presence of an obstructive or restrictive disease (e.g., asthma, COPD)
• Evaluating a patient's ability to withstand surgery
• Evaluating the process of a respiratory disease
• Evaluating the effectiveness of a medicine of therapy

A physician also may order various tests to help rule out other conditions. These tests include:

• Chest x-ray. The presence of scarring and enlarged airway walls may indicate chronic bronchitis, often referred to as a “dirty chest.”  Chest x-rays can also reveal several other conditions, including a low-lying diaphragm, overly inflated lungs, large spaces in the lungs (bullae) and an abnormally small or enlarged heart.
  
  
• Arterial blood gas study. These tests measure how effectively the lungs get oxygen into the blood and carbon dioxide out of it. There are two types of tests:
  
  
• • Pulse oximetry. A small probe is placed on the ear or finger to measure the amount of oxygen in the blood. This painless procedure allows physicians to determine blood oxygen levels without drawing blood.
    
    
  • Arterial blood sample. Blood is drawn from the patient and analyzed. Using this procedure, a physician can also detect carbon dioxide concentrations in the blood and blood acidity.
    
    
• Exercise stress test. Helps determine the effect of exertion on the lungs. During this test, the patient walks on a treadmill while being monitored by a physician. The results can be analyzed to determine whether a patient might be suffering from coronary artery disease. Results also indicate how well the heart responds to inadequate blood supply (ischemia).
  
  
• Sputum analysis. A sample of a patient’s sputum – a substance expelled from the throat – can be analyzed to check for a respiratory infection.

Preparation is minimal for a pulmonary function test. Patients who are anxious about testing should be reassured that the procedures are painless, and that patients are allowed to rest between each phase of the testing if necessary. A physician may suggest the following preparatory steps:

• Do not eat a heavy meal before the test. A full stomach can keep the lungs from fully expanding.
  
  
• Do not smoke for four to six hours before testing. This can interfere with respiration during the test.
  
  
• Do not exercise for four to eight hours before the test. The can hamper breathing during the test.
  
  
• Patients should be advised not to use bronchodilators prior to testing. Such use can skew test results and mask potential problems.
  
  
• Tell the physician of any medications being taken prior to testing. Some medications, such as painkillers, can affect the results of the test.
  
  
• Use the bathroom just prior to testing.
  
  
• Wear loose clothing or loosen any tight-fitting clothing.

Those with dentures should wear them during testing. This will help form a seal around the mouthpiece used in the tests. 

If the patient being tested is a child, parents are encouraged to review the testing procedure with them in advance so they are not overly frightened by the testing itself.

Pulmonary function testing is usually performed at a physician’s office, hospital or clinic. The physician or technician will begin by explaining the test in detail and answering any questions the patient may have.

Patient cooperation is needed to obtain the most accurate results from pulmonary function testing. The tests require patients to inhale and exhale with a specific degree of effort and at various rates and repetitions. The information obtained from these tests is only as accurate as patient participation allows.

There are many different types of pulmonary function tests, and each measures a different aspect of lung function. In many of these tests, a clip is attached to the patient’s nose to ensure that air does not escape through the nostrils. Testing usually takes between five and 30 minutes, depending on which tests are being administered.

The most common side effects of pulmonary function testing are shortness of breath, cough or dizziness from breathing in and out forcefully. Much of this discomfort is quickly alleviated by taking short rest periods between each phase of testing. Those who feel any discomfort during testing should immediately tell the test administrator. For individuals who continue to display difficulty breathing or other lung-related symptoms, a bronchodilator or other medication may be administered.

The three primary pulmonary function tests are performed as follows:

• Spirometry. Patients are asked to sit up straight or stand, place their mouths on the spirometer’s mouthpiece and form a tight seal around it while breathing normally. The spirometer records lung functions during this normal breathing pattern.
  
  Patients are then asked to inhale deeply, then exhale vigorously and quickly. Typically, this is repeated twice more, and the best reading will be recorded and compared to norms based on age, gender, race and height. A reading that falls below 80 percent of the predicted norm is considered abnormal. 
  
  

  Some physicians will perform the test once under normal circumstances, and then administer a bronchodilator to the patient before repeating the test. This allows the physician to see whether the patient’s breathing can be improved with the use of bronchodilators.  
  
  

• Lung volume measurement. Patients sit in the body plethysmograph and seal their mouths around a mouthpiece. As they breathe in and out, changes in pressure inside the box reveal the volume of gas in the lungs.
  
  An alternative to the plethysmograph involves breathing in a gas mixture for three to seven minutes before slowly exhaling completely. Calculations determine the volume of gas in the lungs.
  
  

• Diffusing capacity measurement. Patients inhale a small amount of carbon monoxide, and hold their breath for 10 seconds before exhaling into a carbon monoxide detector. The amount of carbon monoxide remaining in the exhaled air indicates how rapidly the patient is able to exchange gases from the lungs into the bloodstream.

Most patients will feel fine following testing and can resume normal activities immediately after finishing the tests. However, in some cases patients develop symptoms after testing that require medical attention. Patients should contact their doctor if any of the following symptoms occur after pulmonary function testing:

• Shortness of breath
• Difficulty breathing
• Cough
• Chest pain
• Dizziness
• Headache
• Nausea
• Muscle aches
• Malaise (general feeling of illness)

The availability of test results varies depending on which types of tests were given. In some cases, a physician or healthcare provider will explain the test results to patients immediately after the test is complete. In other instances, a healthcare provider will contact the patient at a later time once the results have been analyzed and interpreted.

Some of the common aspects of lung function that are analyzed include:

• Tidal volume (VT) – Amount of air inhaled in a normal breath
  
  
• Vital capacity (VC) – Maximum amount of air exhaled after inhaling as deeply as possible
  
  
• Forced expiratory flow (FEF) – Average rate of flow during middle half of FVC
  
  
• Force expiratory volume (FEV) – Amount of air exhaled forcefully in a sustained breath at 1 second (FEV1) 2 seconds (FEV2), or 3 seconds (FEV3)
  
  
• Functional residual capacity (FRC) – Amount of air remaining in lungs after normal respiration
  
  
• Forced vital capacity (FVC) – Total amount of air exhaled during forced expiratory volume test
  
  
• Inspiratory capacity (IC) – Amount of air that can be inhaled after normal expiration
  
  
• Peak expiratory flow rate (PEFR) – Measure of how quickly a person can exhale
  
  
• Maximum ventilation volume (MVV) – Greatest amount of air a person can breathe in and out in one minute
  
  
• Residual volume (RV) – Amount of air left in the lungs after complete exhalation
  
  
• Total lung capacity (TLC) – Maximum amount of air in the lungs after complete inhalation
  
  
• Thoracic gas volume (TGV) – Total volume of gas in lungs from both ventilated and nonventilated airways
  
  
• Diffusing capacity for carbon monoxide (DLCO) – Estimate of lungs’ ability to transfer gas from lungs into the bloodstream.

Results can be interpreted as follows:

Potential risks are minimal in pulmonary function testing. The most common side effects are shortness of breath or dizziness from breathing in and out forcefully. Much of this discomfort is quickly alleviated by taking short rest periods between each phase of testing.

In rare cases, a person with lung disease can experience a collapsed lung brought on by testing. Those who feel any discomfort during testing should immediately tell the test administrator.

Pulmonary function testing can also exacerbate impaired heart function. Therefore testing may be avoided in patients who have recently experienced a heart attack and those who have certain forms of heart disease.

Pulmonary function tests are generally considered safe during pregnancy. However pregnancy may skew test results, because an enlarged stomach can prevent the lungs from fully expanding.

There are other types of pulmonary function tests that patients can perform themselves at home. Home testing may be recommended for people who need to monitor their condition between office visits. These tests are particularly useful for asthma patients, who can chart their test results and monitor them for signs of an impending asthma attack.

Home tests can measure the following aspects of lung function:

• Force of exhalation. Sometimes called a match test, patients attempt to blow out a match held approximately 6 inches away from the face. Results are normal if completed without any trouble.
  
  
• Forced expiratory time (FET). Patients time how quickly they can exhale a deep breath of air through the mouth. Results are normal if all air can be expelled within five seconds.
  
  
• Forced vital capacity (FVC) and forced expiratory volume (FEV). Patients quickly and forcefully blow into a hand-held spirometer and record the resulting measurements. The hand-held spirometer comes with tables that include predicted values to determine whether the patient’s measured values are normal. To calculate the measured values, patients divide the recorded numbers by the numbers that would be expected for someone of their height, weight, age and gender (the predicted values). This home test is normal if the calculated fractions are at least .80 (80 percent).
  
  
• Maximum ventilation volume (MVV). Patients repeatedly inhale and exhale as quickly as possible into a hand-held spirometer for 15 seconds, and record the value (recorded value). Multiplied by four, this value represents "VV" for a full 60 seconds. The hand-held spirometer comes with tables that include predicted values to determine whether the measured value is normal. To calculate the measured value, patients divide the recorded value by the number that would be expected for someone of his or her height, weight, age and gender (the predicted value). This home test is normal if the calculated fractions are at least .80 (80 percent).
  
  
• Peak expiratory flow rate (PEFR). Patients quickly and forcefully blow into a peak expiratory flow monitor and record the resulting measurement. The measurement can be compared to a “personal best” score, which is the highest peak–flow measurement recorded over a two–week period when a condition is under control. A score that is between 80 percent and 100 percent of a patient’s personal best score means the test is normal.
  
  Though it is best to establish a personal best score, peak expiratory flow can also be measured against predicted “normal” peak flow levels. Peak expiratory flow monitors come with tables that include predicted values to determine if the measured value is normal. To calculate the measured value, patients divide the recorded value by the number that would be expected for someone of his or her height, weight, age and gender (the predicted value). This home test is normal if the calculated fractions are at least .80 (80 percent).
  
  

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If abnormal results are obtained for the first time through home testing, or if results suddenly worsen, patients are encouraged to contact their physician.

It is also important to note that these tests are only valid if the effort is consistent between uses, and as long as the technique is good and reproducible.

Preparing questions in advance can help patients to have more meaningful discussions with their physicians. Patients may wish to ask their doctors the following questions related to pulmonary function tests:

1. Why are you recommending that I take this test?
   
   
2. Can you explain the testing procedure to me in detail?
   
   
3. Am I likely to experience pain during the test?
   
   
4. What risks do I face by taking this test?
   
   
5. How long will my test take?
   
   
6. Are there special preparations I need to make prior to the test?
   
   
7. Will I need to go to a hospital for the test?
   
   
8. When will my results be available?
   
   
9. What do my test results indicate?
   
   
10. Will you be prescribing me medication based on the results?
    
    
11. Would I benefit from measuring my own lung function at home?