Some asthma symptoms are present in other conditions, such as coronary heart disease, bronchitis or emphysema. So breathing tests are used to pinpoint asthma and eliminate other diseases from the diagnosis. The two main devices for testing lung function are the peak flow meter and the spirometer. If you have asthma your airways become narrowed so the air will not flow freely.

It is impossible to measure the diameter of the air passages, but this can be estimated by finding out the speed at which air is expelled from the lungs. The peak flow meter, or peak expiratory flow meter, is a small instrument that measures the maximum rate at which you breathe out. You take a huge breath, inflating your lungs as far as you can, hold it for a couple of seconds, then force it out into the device as fast as possible.

As you breathe out the speed of the air quickly reaches its peak, then drops off. The instrument registers the fastest speed. There are inexpensive instruments and your family doctor will have one. You can also but one for yourself and use it to monitor your peak flow rate two or three times a day. The peak flow meter is invaluable for warning you that your condition may be deteriorating.

Using this device regularly could save your life. The volume of air expelled by a healthy lung is between 400 and 600 liters per minute. In asthma rates of between 200 and 400 liters are common and in severe attacks it could drop to 100 liters a minute or lower.