To assess the diagnosis of asthma and find out the severity of a person's asthma, physicians give patients breathing tests. These tests normally involve having the patient blow through a tube to find out how open or tight the airways are. This can be done with a simple tube called a peak flow meter or a computerized device called a spirometer. The spirometer shows a value called the FEV1 (forced expiratory volume in 1 second). This value is compared to a chart that shows what's appropriate or predicted for a person's age, height, and weight. The patient's result is then expressed as a percentage of what's predicted. For example, if Michael's FEV1 is 80% of predicted, this means that he is breathing at only 80% of the capacity expected for his age, height, and weight.