A diagnosis of asthma is based on a physical examination, personal history, and lung function tests. The physical examination looks for common asthma symptoms such as wheezing or coughing, and the personal history provides additional information, including allergies, exposure to second hand smoke, and/or a familial tendency towards asthma.
Lung Function Tests for Asthma
While lung function tests were not always used for diagnosis in the past, the NHLBI (National Heart, Lung, and Blood Institute) Guidelines for the Diagnosis and Management of Asthma state that "Pulmonary function studies are essential for diagnosing asthma and for assessing the severity of asthma in order to make appropriate therapeutic recommendations. The use of objective measures of lung function is particularly important, because subjective measures, such as patient symptom reports and physicians' physical examination findings, often do not correlate with the variability and severity of airflow obstruction."
Lung function tests include measuring peak flow with a peak flow meter, the use of a simple spirometer, or may involve a battery of spirometry tests in a pulmonary function lab.
More on Peak and Flow Meter
More on Spirometer
Lyn Frumkin, M.D., Ph.D. How is asthma diagnosed? (Online)http://www.radix.net/~mwg/howdiag.html accessed 01-31-10