This article is from the Asthma FAQ, by Patricia Wrean and Marie Goldenberg firstname.lastname@example.org with numerous contributions by others.
Asthma is diagnosed based on a physical examination, personal
history, and lung function tests. The physical examination looks
for typical asthma symptoms such as wheezing or coughing, and the
personal history provides additional clues such as allergies or a
familial tendency towards asthma. Although lung function tests
have not always been used for diagnosis in the past, the NHLBI
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 may be as simple as measuring peak flow with
a peak flow meter, or using a simple spirometer, or may involve
a battery of spirometry tests in a pulmonary function lab.