Spirometry measures how much and how quickly air can be expelled following a deep breath. It is performed by having the patient breathe out forcefully into a device called a spirometer. At the same time, a machine makes a tracing of the rate at which the air leaves the lung. Diseases of airflow obstruction and of lung stiffening give characteristic tracings with spirometry.

Measures of the amount of air that can be expelled following a deep breath, forced vital capacity (FVC), and the amount of air that can be forcibly exhaled in one second, forced expiratory volume in one second (FEV1), are the most useful numbers derived from spirometry. The ratio of FEV1 to FVC is often used to assess patients for airflow obstruction. It is normally 75 to 85 percent, depending on the patient's age. The ratio is reduced in obstructive diseases, while it is preserved or even increased in restrictive disorders. A lower than normal FEV1 is a sign that a lung disease is present. A falling FEV1 is a sign that a person's lung disease is getting worse.

The "normal" values for FVC and FEV1 for a patient depend on the individual's age, gender, height, and race. They are higher for younger than for older people, higher for tall than for short individuals, higher for men than for women, and higher for whites than blacks or Asians. Therefore, the numbers are presented as percentages of the average expected in someone of the same age, height, sex, and race. This is called percent predicted. Any number smaller than 85 percent of predicted is considered abnormal.

If these numbers are abnormal, the patient is referred for additional pulmonary function tests to find out why. These may include checking the patient's response to bronchodilators, absolute lung volumes, and blood levels of oxygen and carbon dioxide, which tell how well gas exchange is occurring. Other important measures of lung function are arterial blood gas tensions (PaO2 and PaCO2) and the diffusing capacity of the lung for carbon monoxide (DLCO).

Some doctors recommend having spirometry before age 25 to get baseline numbers. However, if you are a smoker, are occupationally exposed to irritants, or have symptoms of cough, wheeze, or shortness of breath, you should be checked with a spirometer at intervals of 3 to 5 years or more frequently if your doctor recommends it.

Abnormal spirometry numbers at any age mean that you are at risk for early lung disease and even potentially fatal lung cancer, heart disease, or stroke. You should immediately stop smoking if you still smoke and talk to your doctor about other measures you may need to take depending on the reasons for your abnormal numbers.

Created by the National Heart, Lung, and Blood Institute.