Assessing how well the lungs and respiratory system function is an important step in the diagnosis of asthma. The same tests may also be used to monitor the effectiveness of asthma treatments. Tests commonly used to assess lung function include:

• Pulmonary function test. One of the most commonly used methods to measure an individual’s breathing efficiency. This five-phase test makes frequent use of a spirometer, which features a tube and a recording device designed to gauge the volume of air inhaled and exhaled by an individual, and calculates the length of time between each breath. These tests also measure:
  
  
• • Maximum volume ventilation (MVV). Measures the complete volume an individual exhales in a 15 second period. This number is multiplied by four to find the value for one minute.
    
    
  • Forced vital capacity (FVC). Measures the volume of air exhaled over a continuous period up to six seconds. This helps provide an indication of how much air the lungs can hold.
    
    
  • Forced expiratory capacity (FEV1). Measures the speed of air exhaled in one second.
    
    
  Some phases of a pulmonary function test can be done in the home, while other parts must take place at a medical facility using specialized equipment.
  
  
• Gas diffusion test. A type of test that measures the amount of oxygen and other gases that cross the alveoli in one minute. This test may be performed if a physician suspects that another condition may also be present, such as chronic obstructive pulmonary disease. This type of test includes:
  
  
• • Arterial blood gas analysis (ABG). Used to assess lung efficiency by measuring the concentration of oxygen and carbon dioxide in the blood drawn from an artery (vessel that carries blood from the heart to the tissues). These are important measures, because they tell a physician how efficiently a patient is inhaling oxygen into the body, and exhaling carbon dioxide into the air. The body’s cells use oxygen to do their work, but in doing so they create carbon dioxide as a byproduct. Carbon dioxide is toxic at certain levels and must be expelled from the body. A poor ABG result might indicate an asthma diagnosis.
    
    
  • Carbon monoxide diffusing capacity. Measures how efficiently the lungs can transfer a small amount of carbon monoxide into the blood. A small amount of carbon monoxide is inhaled and tests evaluate how well the gas can move from the lungs into the blood.
    
    
• Peak flow meters can be used by patients to check their own lung function from the comfort of their own home. This device measures the force of breath during inhalation and exhalation. By using a peak flow meter each day, patients can detect changes in their breathing capacity before the condition manifests itself in asthma symptoms. Peak flow meters can also help an individual determine the severity of an asthma attack once it has begun.

  

• Body plethysmography. In this technique, a patient sits inside a clear, enclosed chamber and breathes on a mouthpiece. A computer measures how the chest wall alters the volume of gases inside the chamber as an individual inhales and exhales. This allows the computer to gauge the amount of gas left in the lungs at the end of a normal exhalation. Plethysmography often is used with a bronchoprovocation because it can detect very subtle changes in the airways.
  
  
• Pulse oximetry test. Measures the amount of oxygen in the blood. A computer reads information transmitted by a probe attached to a patient’s ear or finger.  A pulse oximetry test is faster and easier to obtain than an arterial blood gas analysis, but not as accurate.
  
  
• Bronchoscopy. A procedure that uses a bronchoscope to examine the airways leading to the lungs. A thin, lighted instrument is inserted through the nose and mouth and threaded to the airways. This test may be performed to rule out other conditions that may mimic the symptoms of asthma.

Most tests of the lungs present little or no risk to the individual being tested unless they have a serious heart or lung condition. Those who do should talk to their physician. Patients who have blood drawn during an arterial blood gas analysis may feel lightheaded, dizzy or nauseous and may have bruising around the puncture site. Continued bleeding can be a concern for those with bleeding disorders. Rarely, a needle can damage a nerve or artery, causing a blockage.

Since young patients often have difficulty following the instructions given during lung testing, children under the age of 5 cannot usually perform lung function tests. Instead, a physician will often base an asthma diagnosis on the patient’s clinical signs and symptoms, as well as the information collected from a medical history and physical examination. The physician may prescribe a bronchodilator for the child and confirm the diagnosis if the child’s signs and symptoms improve with its use.