Diagnosing Asthma

A diagnosis of asthma usually is based on the patient's symptoms, medical history, a physical examination, and laboratory tests that measure pulmonary (lung) function. Doctors typically look for signs that the patient's airflow is obstructed and that the obstruction is at least partially reversible. Factors that trigger symptoms may be evident, such as exercise, cold air, and exposure to an allergen; however, the precipitating factors may not be clearly identified.

Evidence of reversible airway obstruction is often detected in the physical examination or by physiologic testing. Physiologic testing generally is recommended to confirm the diagnosis. During an asthma attack, wheezing can be heard by listening to the chest with a stethoscope. The airway obstruction is considered reversible if the wheezing disappears in response to treatment, or when the suspected triggering factor is removed or resolved.

Diagnostic tests for asthma include


The most reliable way to determine reversible airway obstruction is with spirometry, a test that measures the amount of air entering and leaving the lungs. This simple test can be performed in the physician's office.

Spirometry uses a measuring device called a spirometer that is connected by a flexible tube to a disposable cardboard mouthpiece. The patient exhales and inhales deeply, then seals his or her lips around the mouthpiece and blows as forcefully and for as long as possible until all the air is exhaled from the lungs.

Ideally, the patient should exhale for at least 6 seconds. The spirometer measures the amount of air exhaled and the length of time it took to exhale it. The amount of air exhaled in the first second, expressed as "FEV1," is measured and compared to the total amount exhaled. If the amount exhaled in 1 second is disproportionately low to the total exhaled, the patient has an obstruction. To test for reversibility, the patient then inhales a bronchodilator (i.e., a drug that widens the airways in the lungs) and the spirometry is repeated. If the values of the test performed after administration of the bronchodilator are significantly better than the prebronchodilator values, the obstruction is considered reversible.

Sometimes a patient with asthma does not demonstrate reversibility after the inhalation of a bronchodilator. In this case, the patient may be treated for a few weeks with antiinflammatory medications and then returns for another spirometry test. If the posttreatment spirometry results are better than the initial results, the obstruction is considered reversible.

Peak Expiratory Flow

Because asthma symptoms vary, it is not unusual for a patient with chronic asthma to have normal spirometry. In such cases, peak expiratory flow (PEF) rate monitoring may be used to demonstrate reversible airway obstruction. A peak flow meter is a portable device that can be carried by the patient. It consists of a small tube with a gauge that measures the maximum force with which one can blow air through the tube.

The patient performs the peak flow meter test twice a day for about 2 weeks and records the results for review in a follow up appointment. The first test should be performed after waking in the morning, before taking bronchodilator medications. The patient should perform the peak expiratory flow maneuver 3 times and record the highest measurement. The second test should be done in the afternoon or early evening after taking a bronchodilator. Peak flows vary during the day and the early morning peak is lower than the evening peak. A variability greater than 20 percent indicates a reversible airway obstruction.

Bronchial Provocation

Occasionally, a patient with a suspected asthma-related airway obstruction does not demonstrate obstruction in spirometry or peak flow monitoring. In this circumstance, the diagnosis of airway obstruction may be provided by bronchial provocation.

Bronchial provocation, also known as bronchoprovocation and bronchial challenge, identifies and characterizes hyperresponsive airways by having the patient inhale an aerosolized chemical, called a broncho-spastic agonist, that triggers a hyperresponsive reaction. The chemicals most often used are histamine and methacholine.

Patients perform spirometry without inhaling the agent and then inhale increasingly higher doses of the agent. After each incremental dose inhalation, spirometry is performed. Patients who demonstrate a reduction in FEV1 of 20 percent with a low dose of methacholine or histamine have nonspecific hyperresponsiveness. Although some patients without asthma demonstrate hyperresponsiveness, most patients with a positive reaction have asthma.

The other common bronchoprovocation test is the exercise challenge test, which is used primarily with patients whose asthma is triggered by exercise. The patient performs spirometry and then exercises, usually on a treadmill or exercise cycle. The exercise test should resemble as closely as possible the conditions under which the symptoms are usually triggered. After the patient exercises, spirometry is repeated. This may be done several times, immediately after exercise and periodically, until there is a drop in the FEV1 greater than 20 percent or until 30 minutes have elapsed.

Other Asthma Tests

Tests may be employed to exclude other diseases and to evaluate conditions that worsen the asthmatic condition. These include the following:

  • Chest x-rays are often obtained initially to rule out other health conditions.
  • Allergy testing, either by skin testing or by measuring antibodies in the blood, sometimes is performed to determine if the asthma is allergy induced, and if so, what specific allergens are involved.
  • X-rays of the sinuses is often done to exclude sinusitis as a factor.
  • Evaluation for gastroesophageal reflux disease is often performed to evaluate its contribution to the control of asthma.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 01 Jun 2000

Last Modified: 09 Oct 2014