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.