Spirometry

This is the main test for diagnosing asthma. The test device is called a spirometer. It measures how much air you breathe in, how much you breathe out and how quickly you breathe out. The results show how much your breathing is limited. The results are recorded as:

• Forced vital capacity (FVC). This is the largest amount of air that you can forcefully breathe out after breathing in as deeply as you can. An FVC reading that's lower than what's typical means your breathing is limited.
• Forced expiratory volume (FEV). This test shows how much air you can force out of your lungs. An FEV-1 result shows how much air you can force out in one second. Asthma makes it particularly difficult to breathe out. This reading helps your healthcare professional figure out how serious your breathing trouble is. Lower FEV-1 readings mean greater blockages in the airways.

The test may be repeated after you take an inhaled medicine that relaxes the muscles around the airways. Better spirometry results after taking the medicine help confirm an asthma diagnosis.